Red Hat Enterprise Linux 4
Using as, the Gnu Assembler
Red Hat Enterprise Linux 4: Using as, the Gnu Assembler
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Table of Contents
1. Using as ............................................................................................................................................ 1
2. Overview .......................................................................................................................................... 3
2.1. Structure of this Manual...................................................................................................15
2.2. The GNU Assembler........................................................................................................15
2.3. Object File Formats.......................................................................................................... 15
2.4. Command Line................................................................................................................. 16
2.5. Input Files ........................................................................................................................ 16
2.5.1. Filenames and Line-numbers ............................................................................16
2.6. Output (Object) File ......................................................................................................... 17
2.7. Error and Warning Messages...........................................................................................17
3. Command-Line Options............................................................................................................... 19
3.1. Enable Listings: -a[cdhlns] .........................................................................................19
3.2. -alternate ....................................................................................................................19
3.3. -D .....................................................................................................................................19
3.4. Work Faster: -f................................................................................................................20
3.5. .includeSearch Path: -Ipath ......................................................................................20
3.6. Difference Tables: -K.......................................................................................................20
3.7. Include Local Labels: -L ................................................................................................. 20
3.8. Configuring listing output: -listing............................................................................. 20
3.9. Assemble in MRI Compatibility Mode: -M .....................................................................21
3.10. Dependency Tracking: -MD............................................................................................ 22
3.11. Name the Object File: -o ...............................................................................................23
3.12. Join Data and Text Sections: -R..................................................................................... 23
3.13. Display Assembly Statistics: -statistics ................................................................. 23
3.14. Compatible Output: -traditional-format..............................................................23
3.15. Announce Version: -v ...................................................................................................23
3.16. Control Warnings: -W, -warn, -no-warn, -fatal-warnings..................................23
3.17. Generate Object File in Spite of Errors: -Z ...................................................................24
4. Syntax............................................................................................................................................. 25
4.1. Preprocessing ...................................................................................................................25
4.2. Whitespace.......................................................................................................................25
4.3. Comments ........................................................................................................................ 25
4.4. Symbols ...........................................................................................................................26
4.5. Statements........................................................................................................................ 26
4.6. Constants..........................................................................................................................27
4.6.1. Character Constants ..........................................................................................27
4.6.2. Number Constants............................................................................................. 28
5. Sections and Relocation................................................................................................................31
5.1. Background...................................................................................................................... 31
5.2. Linker Sections ................................................................................................................ 32
5.3. Assembler Internal Sections ............................................................................................ 33
5.4. Sub-Sections .................................................................................................................... 33
5.5. bss Section .......................................................................................................................34
6. Symbols .......................................................................................................................................... 35
6.1. Labels............................................................................................................................... 35
6.2. Giving Symbols Other Values..........................................................................................35
6.3. Symbol Names................................................................................................................. 35
6.3.1. Local Symbol Names ........................................................................................35
6.3.2. Dollar Local Labels........................................................................................... 36
6.4. The Special Dot Symbol ..................................................................................................37
6.5. Symbol Attributes ............................................................................................................ 37
6.5.1. Value .................................................................................................................37
6.5.2. Type...................................................................................................................37
6.5.3. Symbol Attributes: a.out ................................................................................37
6.5.4. Symbol Attributes for COFF ............................................................................38
6.5.5. Symbol Attributes for SOM.............................................................................. 38
7. Expressions ....................................................................................................................................39
7.1. Empty Expressions...........................................................................................................39
7.2. Integer Expressions..........................................................................................................39
7.2.1. Arguments......................................................................................................... 39
7.2.2. Operators........................................................................................................... 39
7.2.3. Prefix Operator..................................................................................................39
7.2.4. Infix Operators ..................................................................................................40
8. Assembler Directives..................................................................................................................... 43
8.1. .abort............................................................................................................................. 43
8.2. .ABORT............................................................................................................................. 43
8.3. .align abs-expr, abs-expr, abs-expr ...................................................................43
8.4. .ascii "string". . .......................................................................................................43
8.5. .asciz "string". . .......................................................................................................44
8.6. .balign[wl] abs-expr, abs-expr, abs-expr ........................................................44
8.7. .byte expressions ....................................................................................................... 44
8.8. .comm symbol, length ................................................................................................. 44
8.9. .cfi_startproc ........................................................................................................... 45
8.10. .cfi_endproc.............................................................................................................. 45
8.11. .cfi_def_cfa register, offset ............................................................................45
8.12. .cfi_def_cfa_register register ........................................................................ 45
8.13. .cfi_def_cfa_offset offset................................................................................. 45
8.14. .cfi_adjust_cfa_offset offset .......................................................................... 45
8.15. .cfi_offset register, offset .............................................................................. 45
8.16. .cfi_rel_offset register, offset .....................................................................45
8.17. .cfi_window_save .....................................................................................................45
8.18. .cfi_escapeexpression[, .. . ].................................................................................46
8.19. .data subsection .......................................................................................................46
8.20. .def name .....................................................................................................................46
8.21. .desc symbol, abs-expression ...............................................................................46
8.22. .dim ...............................................................................................................................46
8.23. .double flonums......................................................................................................... 46
8.24. .eject...........................................................................................................................46
8.25. .else.............................................................................................................................46
8.26. .elseif ........................................................................................................................ 47
8.27. .end ...............................................................................................................................47
8.28. .endef...........................................................................................................................47
8.29. .endfunc ...................................................................................................................... 47
8.30. .endif...........................................................................................................................47
8.31. .equ symbol, expression .........................................................................................47
8.32. .equiv symbol, expression .....................................................................................47
8.33. .err ...............................................................................................................................48
8.34. .exitm...........................................................................................................................48
8.35. .extern ........................................................................................................................ 48
8.36. .fail expression .......................................................................................................48
8.37. .file string............................................................................................................... 48
8.38. .fill repeat, size, value .....................................................................................48
8.39. .float flonums........................................................................................................... 48
8.40. .func name[,label]................................................................................................... 49
8.41. .global symbol, .globl symbol.............................................................................. 49
8.42. .hidden names.............................................................................................................49
8.43. .hword expressions ...................................................................................................49
8.44. .ident...........................................................................................................................49
8.45. .if absolute expression .......................................................................................... 49
8.46. .incbin "file"[,skip[,count]]............................................................................ 51
8.47. .include "file" ........................................................................................................51
8.48. .int expressions .......................................................................................................51
8.49. .internal names ........................................................................................................ 51
8.50. .irp symbol,values. . . .............................................................................................. 51
8.51. .irpc symbol,values. . . ............................................................................................ 52
8.52. .lcomm symbol, length............................................................................................. 52
8.53. .lflags ........................................................................................................................ 52
8.54. .line line-number ..................................................................................................... 52
8.55. .linkonce [type]......................................................................................................53
8.56. .ln line-number .........................................................................................................53
8.57. .mri val.......................................................................................................................53
8.58. .list.............................................................................................................................53
8.59. .long expressions ..................................................................................................... 54
8.60. .macro...........................................................................................................................54
8.61. .altmacro .................................................................................................................... 55
8.62. .noaltmacro................................................................................................................56
8.63. .nolist ........................................................................................................................ 56
8.64. .octa bignums .............................................................................................................56
8.65. .org new-lc, fill ..................................................................................................... 56
8.66. .p2align[wl] abs-expr, abs-expr, abs-expr ....................................................56
8.67. .previous .................................................................................................................... 57
8.68. .popsection................................................................................................................57
8.69. .print string............................................................................................................. 57
8.70. .protected names ...................................................................................................... 57
8.71. .psize lines, columns............................................................................................. 57
8.72. .purgem name .............................................................................................................. 58
8.73. .pushsection name, subsection............................................................................58
8.74. .quad bignums .............................................................................................................58
8.75. .rept count................................................................................................................. 58
8.76. .sbttl "subheading".................................................................................................58
8.77. .scl class ................................................................................................................... 59
8.78. .section name ............................................................................................................ 59
8.78.1. COFF Version .................................................................................................59
8.78.2. ELF Version ....................................................................................................60
8.79. .set symbol, expression .........................................................................................62
8.80. .short expressions ...................................................................................................62
8.81. .single flonums......................................................................................................... 62
8.82. .size.............................................................................................................................63
8.82.1. COFF Version .................................................................................................63
8.82.2. ELF Version ....................................................................................................63
8.83. .sleb128 expressions............................................................................................... 63
8.84. .skip size, fill.......................................................................................................63
8.85. .space size, fill..................................................................................................... 63
8.86. .stabd, .stabn, .stabs........................................................................................ 64
8.87. .string"str"...............................................................................................................65
8.88. .struct expression...................................................................................................65
8.89. .subsection name......................................................................................................65
8.90. .symver ........................................................................................................................ 65
8.91. .tag structname ......................................................................................................... 66
8.92. .text subsection .......................................................................................................66
8.93. .title "heading" ......................................................................................................66
8.94. .type.............................................................................................................................66
8.94.1. COFF Version .................................................................................................66
8.94.2. ELF Version ....................................................................................................67
8.95. .uleb128 expressions............................................................................................... 67
8.96. .val addr .....................................................................................................................67
8.97. .version "string" .................................................................................................... 67
8.98. .vtable_entry table, offset................................................................................68
8.99. .vtable_inherit child, parent ...........................................................................68
8.100. .weak names............................................................................................................... 68
8.101. .word expressions ................................................................................................... 68
8.102. Deprecated Directives ..................................................................................................69
9. Machine Dependent Features ......................................................................................................71
10. AMD 29K Dependent Features..................................................................................................73
10.1. Options...........................................................................................................................73
10.2. Syntax ............................................................................................................................ 73
10.2.1. Macros.............................................................................................................73
10.2.2. Special Characters...........................................................................................73
10.2.3. Register Names ...............................................................................................73
10.3. Floating Point................................................................................................................. 74
10.4. AMD 29K Machine Directives......................................................................................74
10.5. Opcodes..........................................................................................................................74
11. Alpha Dependent Features.........................................................................................................75
11.1. Notes ..............................................................................................................................75
11.2. Options...........................................................................................................................75
11.3. Syntax ............................................................................................................................ 76
11.3.1. Special Characters...........................................................................................76
11.3.2. Register Names ...............................................................................................76
11.3.3. Relocations......................................................................................................76
11.4. Floating Point................................................................................................................. 78
11.5. Alpha Assembler Directives ..........................................................................................78
11.6. Opcodes..........................................................................................................................80
12. ARC Dependent Features...........................................................................................................83
12.1. Options...........................................................................................................................83
12.2. Syntax ............................................................................................................................ 83
12.2.1. Special Characters...........................................................................................83
12.2.2. Register Names ...............................................................................................83
12.3. Floating Point................................................................................................................. 84
12.4. ARC Machine Directives ............................................................................................... 84
12.5. Opcodes..........................................................................................................................85
13. ARM Dependent Features.......................................................................................................... 87
13.1. Options...........................................................................................................................87
13.2. Syntax ............................................................................................................................ 89
13.2.1. Special Characters...........................................................................................89
13.2.2. Register Names ...............................................................................................89
13.3. Floating Point................................................................................................................. 89
13.4. ARM Machine Directives .............................................................................................. 89
13.5. Opcodes..........................................................................................................................90
13.6. Mapping Symbols .......................................................................................................... 91
14. CRIS Dependent Features.......................................................................................................... 93
14.1. Command-line Options ..................................................................................................93
14.2. Instruction expansion .....................................................................................................93
14.3. Syntax ............................................................................................................................ 93
14.3.1. Special Characters...........................................................................................93
14.3.2. Symbols in position-independent code ........................................................... 94
14.3.3. Register names ................................................................................................95
14.3.4. Assembler Directives......................................................................................95
15. D10V Dependent Features..........................................................................................................97
15.1. D10V Options ................................................................................................................97
15.2. Syntax ............................................................................................................................ 97
15.2.1. Size Modifiers .................................................................................................97
15.2.2. Sub-Instructions .............................................................................................. 97
15.2.3. Special Characters...........................................................................................98
15.2.4. Register Names ...............................................................................................98
15.2.5. Addressing Modes ........................................................................................100
15.2.6. @WORD Modifier........................................................................................ 100
15.3. Floating Point............................................................................................................... 100
15.4. Opcodes........................................................................................................................101
16. D30V Dependent Features........................................................................................................103
16.1. D30V Options ..............................................................................................................103
16.2. Syntax ..........................................................................................................................103
16.2.1. Size Modifiers ............................................................................................... 103
16.2.2. Sub-Instructions ............................................................................................ 103
16.2.3. Special Characters.........................................................................................103
16.2.4. Guarded Execution........................................................................................105
16.2.5. Register Names .............................................................................................105
16.2.6. Addressing Modes ........................................................................................107
16.3. Floating Point............................................................................................................... 107
16.4. Opcodes........................................................................................................................107
17. H8/300 Dependent Features .....................................................................................................109
17.1. Options.........................................................................................................................109
17.2. Syntax ..........................................................................................................................109
17.2.1. Special Characters.........................................................................................109
17.2.2. Register Names .............................................................................................109
17.2.3. Addressing Modes ........................................................................................109
17.3. Floating Point............................................................................................................... 110
17.4. H8/300 Machine Directives .........................................................................................110
17.5. Opcodes........................................................................................................................111
18. H8/500 Dependent Features .....................................................................................................113
18.1. Options.........................................................................................................................113
18.2. Syntax ..........................................................................................................................113
18.2.1. Special Characters.........................................................................................113
18.2.2. Register Names .............................................................................................113
18.2.3. Addressing Modes ........................................................................................113
18.3. Floating Point............................................................................................................... 114
18.4. H8/500 Machine Directives .........................................................................................114
18.5. Opcodes........................................................................................................................114
19. HPPA Dependent Features.......................................................................................................117
19.1. Notes ............................................................................................................................117
19.2. Options.........................................................................................................................117
19.3. Syntax ..........................................................................................................................117
19.4. Floating Point............................................................................................................... 117
19.5. HPPA Assembler Directives ........................................................................................ 117
19.6. Opcodes........................................................................................................................121
20. ESA/390 Dependent Features .................................................................................................. 123
20.1. Notes ............................................................................................................................123
20.2. Options.........................................................................................................................123
20.3. Syntax ..........................................................................................................................123
20.4. Floating Point............................................................................................................... 124
20.5. ESA/390 Assembler Directives ................................................................................... 124
20.6. Opcodes........................................................................................................................125
21. 80386 Dependent Features .......................................................................................................127
21.1. Options.........................................................................................................................127
21.2. AT&T Syntax versus Intel Syntax ............................................................................... 127
21.3. Instruction Naming ......................................................................................................128
21.4. Register Naming ..........................................................................................................128
21.5. Instruction Prefixes ......................................................................................................129
21.6. Memory References .....................................................................................................130
21.7. Handling of Jump Instructions..................................................................................... 131
21.8. Floating Point............................................................................................................... 131
21.9. Intel’s MMX and AMD’s 3DNow! SIMD Operations ................................................ 132
21.10. Writing 16-bit Code ...................................................................................................132
21.11. AT&T Syntax bugs ....................................................................................................132
21.12. Specifying CPU Architecture ....................................................................................133
21.13. Notes ..........................................................................................................................133
22. Intel i860 Dependent Features .................................................................................................135
22.1. i860 Notes ....................................................................................................................135
22.2. i860 Command-line Options........................................................................................ 135
22.2.1. SVR4 compatibility options..........................................................................135
22.2.2. Other options.................................................................................................135
22.3. i860 Machine Directives .............................................................................................. 136
22.4. i860 Opcodes ............................................................................................................... 136
22.4.1. Other instruction support (pseudo-instructions) ........................................... 136
23. Intel 80960 Dependent Features .............................................................................................. 139
23.1. i960 Command-line Options........................................................................................ 139
23.2. Floating Point............................................................................................................... 140
23.3. i960 Machine Directives .............................................................................................. 140
23.4. i960 Opcodes ............................................................................................................... 140
23.4.1. callj............................................................................................................141
23.4.2. Compare-and-Branch....................................................................................141
24. IP2K Dependent Features ........................................................................................................143
24.1. IP2K Options ............................................................................................................... 143
25. M32R Dependent Features....................................................................................................... 145
25.1. M32R Options.............................................................................................................. 145
25.2. M32R Directives..........................................................................................................147
25.3. M32R Warnings...........................................................................................................148
26. M680x0 Dependent Features.................................................................................................... 151
26.1. M680x0 Options .......................................................................................................... 151
26.2. Syntax ..........................................................................................................................153
26.3. Motorola Syntax........................................................................................................... 154
26.4. Floating Point............................................................................................................... 155
26.5. 680x0 Machine Directives ...........................................................................................156
26.6. Opcodes........................................................................................................................156
26.6.1. Branch Improvement..................................................................................... 156
26.6.2. Special Characters.........................................................................................158
27. M68HC11 and M68HC12 Dependent Features .....................................................................159
27.1. M68HC11 and M68HC12 Options ..............................................................................159
27.2. Syntax ..........................................................................................................................160
27.3. Symbolic Operand Modifiers ....................................................................................... 161
27.4. Assembler Directives...................................................................................................162
27.5. Floating Point............................................................................................................... 163
27.6. Opcodes........................................................................................................................163
27.6.1. Branch Improvement..................................................................................... 163
28. Motorola M88K Dependent Features...................................................................................... 165
28.1. M88K Machine Directives...........................................................................................165
29. MIPS Dependent Features ....................................................................................................... 167
29.1. Assembler options........................................................................................................167
29.2. MIPS ECOFF object code ...........................................................................................169
29.3. Directives for debugging information.......................................................................... 170
29.4. Directives to override the ISA level.............................................................................170
29.5. Directives for extending MIPS 16 bit instructions....................................................... 170
29.6. Directive to mark data as an instruction....................................................................... 170
29.7. Directives to save and restore options..........................................................................170
29.8. Directives to control generation of MIPS ASE instructions ........................................171
30. MMIX Dependent Features ..................................................................................................... 173
30.1. Command-line Options ................................................................................................173
30.2. Instruction expansion ...................................................................................................173
30.3. Syntax ..........................................................................................................................174
30.3.1. Special Characters.........................................................................................174
30.3.2. Symbols ........................................................................................................174
30.3.3. Register names ..............................................................................................175
30.3.4. Assembler Directives....................................................................................175
30.4. Differences to mmixal ................................................................................................. 177
31. MSP 430 Dependent Features.................................................................................................. 181
31.1. Options.........................................................................................................................181
31.2. Syntax ..........................................................................................................................181
31.2.1. Macros...........................................................................................................181
31.2.2. Special Characters.........................................................................................181
31.2.3. Register Names .............................................................................................181
31.2.4. Assembler Extensions................................................................................... 182
31.3. Floating Point............................................................................................................... 183
31.4. MSP 430 Machine Directives ......................................................................................183
31.5. Opcodes........................................................................................................................183
31.6. Profiling Capability......................................................................................................183
32. PDP-11 Dependent Features .................................................................................................... 187
32.1. Options.........................................................................................................................187
32.1.1. Code Generation Options..............................................................................187
32.1.2. Instruction Set Extension Options ................................................................187
32.1.3. CPU Model Options......................................................................................188
32.1.4. Machine Model Options ...............................................................................189
32.2. Assembler Directives...................................................................................................190
32.3. PDP-11 Assembly Language Syntax ........................................................................... 190
32.4. Instruction Naming ......................................................................................................191
32.5. Synthetic Instructions...................................................................................................191
33. picoJava Dependent Features ..................................................................................................193
33.1. Options.........................................................................................................................193
34. PowerPC Dependent Features .................................................................................................195
34.1. Options.........................................................................................................................195
34.2. PowerPC Assembler Directives...................................................................................196
35. Renesas / SuperH SH Dependent Features............................................................................. 197
35.1. Options.........................................................................................................................197
35.2. Syntax ..........................................................................................................................197
35.2.1. Special Characters.........................................................................................197
35.2.2. Register Names .............................................................................................197
35.2.3. Addressing Modes ........................................................................................198
35.3. Floating Point............................................................................................................... 199
35.4. SH Machine Directives ................................................................................................ 199
35.5. Opcodes........................................................................................................................199
36. SuperH SH64 Dependent Features.......................................................................................... 201
36.1. Options.........................................................................................................................201
36.2. Syntax ..........................................................................................................................201
36.2.1. Special Characters.........................................................................................202
36.2.2. Register Names .............................................................................................202
36.2.3. Addressing Modes ........................................................................................202
36.3. SH64 Machine Directives ............................................................................................ 202
36.4. Opcodes........................................................................................................................203
37. SPARC Dependent Features ....................................................................................................205
37.1. Options.........................................................................................................................205
37.2. Enforcing aligned data .................................................................................................205
37.3. Floating Point............................................................................................................... 206
37.4. Sparc Machine Directives ............................................................................................206
38. TIC54X Dependent Features ...................................................................................................209
38.1. Options.........................................................................................................................209
38.2. Blocking.......................................................................................................................209
38.3. Environment Settings................................................................................................... 209
38.4. Constants Syntax..........................................................................................................209
38.5. String Substitution ....................................................................................................... 209
38.6. Local Labels................................................................................................................. 210
38.7. Math Builtins ...............................................................................................................211
38.8. Extended Addressing ...................................................................................................213
38.9. Directives ..................................................................................................................... 213
38.10. Macros........................................................................................................................218
38.11. Memory-mapped Registers ........................................................................................ 219
39. Z8000 Dependent Features....................................................................................................... 221
39.1. Options.........................................................................................................................221
39.2. Syntax ..........................................................................................................................221
39.2.1. Special Characters.........................................................................................221
39.2.2. Register Names .............................................................................................221
39.2.3. Addressing Modes ........................................................................................221
39.3. Assembler Directives for the Z8000 ............................................................................ 222
39.4. Opcodes........................................................................................................................223
40. VAX Dependent Features .........................................................................................................225
40.1. VAX Command-Line Options .....................................................................................225
40.2. VAX Floating Point...................................................................................................... 226
40.3. Vax Machine Directives...............................................................................................226
40.4. VAX Opcodes ..............................................................................................................227
40.5. VAX Branch Improvement .......................................................................................... 227
40.6. VAX Operands.............................................................................................................229
40.7. Not Supported on VAX................................................................................................ 229
41. v850 Dependent Features ......................................................................................................... 231
41.1. Options.........................................................................................................................231
41.2. Syntax ..........................................................................................................................231
41.2.1. Special Characters.........................................................................................232
41.2.2. Register Names .............................................................................................232
41.3. Floating Point............................................................................................................... 234
41.4. V850 Machine Directives ............................................................................................234
41.5. Opcodes........................................................................................................................235
42. Xtensa Dependent Features...................................................................................................... 239
42.1. Command Line Options...............................................................................................239
42.2. Assembler Syntax ........................................................................................................239
42.2.1. Opcode Names ..............................................................................................240
42.2.2. Register Names .............................................................................................240
42.3. Xtensa Optimizations................................................................................................... 240
42.3.1. Using Density Instructions............................................................................240
42.3.2. Automatic Instruction Alignment .................................................................241
42.4. Xtensa Relaxation ........................................................................................................241
42.4.1. Conditional Branch Relaxation .....................................................................241
42.4.2. Function Call Relaxation .............................................................................. 242
42.4.3. Other Immediate Field Relaxation ................................................................ 242
42.5. Directives ..................................................................................................................... 243
42.5.1. density...........................................................................................................244
42.5.2. relax...............................................................................................................244
42.5.3. longcalls ........................................................................................................245
42.5.4. generics ......................................................................................................... 245
42.5.5. literal ............................................................................................................. 245
42.5.6. literal_position ..............................................................................................246
42.5.7. literal_prefix..................................................................................................246
42.5.8. freeregs.......................................................................................................... 246
42.5.9. frame .............................................................................................................247
43. Reporting Bugs.......................................................................................................................... 249
43.1. Have You Found a Bug? ..............................................................................................249
43.2. How to Report Bugs.....................................................................................................249
44. Acknowledgements ................................................................................................................... 253
A. GNU Free Documentation License ........................................................................................... 255
A.1. ADDENDUM: How to use this License for your documents.......................................259
Index.................................................................................................................................................261
Using as
Chapter 1.
Using as
This file is a user guide to the gnu assembler as version 2.15.92.0.2.
This document is distributed under the terms of the GNU Free Documentation License. A copy of the
license is included in the section entitled "GNU Free Documentation License".
2 Chapter 1. Using as
Chapter 2.
Overview
Here is a brief summary of how to invoke as. For details, Chapter 3 Command-Line Options.
as [-a[cdhlns][=file]] [--alternate] [-D]
[--defsym sym=val] [-f] [-g ] [--gstabs] [--gstabs+]
[--gdwarf-2] [--help] [-I dir] [-J] [-K ] [-L]
[--listing-lhs-width=NUM] [--listing-lhs-width2=NUM]
[--listing-rhs-width=NUM] [--listing-cont-lines=NUM]
[--keep-locals] [-o objfile] [-R] [--statistics] [-v]
[-version] [--version] [-W] [--warn] [--fatal-warnings]
[-w] [-x] [-Z] [--target-help] [target-options]
[--|files ...]
Target Alpha options:
[-mcpu]
[-mdebug | -no-mdebug]
[-relax] [-g] [-Gsize]
[-F] [-32addr]
Target ARC options:
[-marc[5|6|7|8]]
[-EB|-EL]
Target ARM options:
[-mcpu=processor[+extension...]]
[-march=architecture[+extension...]]
[-mfpu=floating-point-format]
[-mfloat-abi=abi]
[-meabi=ver]
[-mthumb]
[-EB|-EL]
[-mapcs-32|-mapcs-26|-mapcs-float|
-mapcs-reentrant]
[-mthumb-interwork] [-moabi] [-k]
Target CRIS options:
[--underscore | --no-underscore]
[--pic] [-N ]
[--emulation=criself | --emulation=crisaout]
Target D10V options:
[-O]
Target D30V options:
[-O|-n|-N ]
Target i386 options:
[--32|--64] [-n]
Target i960 options:
[-ACA|-ACA_A|-ACB|-ACC|-AKA|-AKB|
-AKC|-AMC]
[-b] [-no-relax]
Target IP2K options:
[-mip2022|-mip2022ext]
4 Chapter 2. Overview
Target M32R options:
[--m32rx|--[no-]warn-explicit-parallel-conflicts|
--W[n]p]
Target M680X0 options:
[-l] [-m68000|-m68010|-m68020|...]
Target M68HC11 options:
[-m68hc11|-m68hc12|-m68hcs12]
[-mshort|-mlong]
[-mshort-double|-mlong-double]
[--force-long-branchs] [--short-branchs]
[--strict-direct-mode] [--print-insn-syntax]
[--print-opcodes] [--generate-example]
Target MCORE options:
[-jsri2bsr] [-sifilter] [-relax]
[-mcpu=[210|340]]
Target MIPS options:
[-nocpp] [-EL] [-EB] [-O[optimization level]]
[-g[debug level]] [-G num] [-KPIC] [-call_shared]
[-non_shared] [-xgot]
[-mabi=ABI] [-32] [-n32] [-64] [-mfp32] [-mgp32]
[-march=CPU] [-mtune=CPU] [-mips1] [-mips2]
[-mips3] [-mips4] [-mips5] [-mips32] [-mips32r2]
[-mips64] [-mips64r2]
[-construct-floats] [-no-construct-floats]
[-trap] [-no-break] [-break] [-no-trap]
[-mfix7000] [-mno-fix7000]
[-mips16] [-no-mips16]
[-mips3d] [-no-mips3d]
[-mdmx] [-no-mdmx]
[-mdebug] [-no-mdebug]
[-mpdr] [-mno-pdr]
Target MMIX options:
[--fixed-special-register-names] [--globalize-symbols]
[--gnu-syntax] [--relax] [--no-predefined-symbols]
[--no-expand] [--no-merge-gregs] [-x]
[--linker-allocated-gregs]
Target PDP11 options:
[-mpic|-mno-pic] [-mall] [-mno-extensions]
[-mextension|-mno-extension]
[-mcpu] [-mmachine]
Target picoJava options:
[-mb|-me]
Target PowerPC options:
[-mpwrx|-mpwr2|-mpwr|-m601|-mppc|-mppc32|-m603|-m604|
-m403|-m405|-mppc64|-m620|-mppc64bridge|-mbooke|
-mbooke32|-mbooke64]
[-mcom|-many|-maltivec] [-memb]
[-mregnames|-mno-regnames]
[-mrelocatable|-mrelocatable-lib]
[-mlittle|-mlittle-endian|-mbig|-mbig-endian]
[-msolaris|-mno-solaris]
Target SPARC options:
Chapter 2. Overview 5
[-Av6|-Av7|-Av8|-Asparclet|-Asparclite
-Av8plus|-Av8plusa|-Av9|-Av9a]
[-xarch=v8plus|-xarch=v8plusa] [-bump]
[-32|-64]
Target TIC54X options:
[-mcpu=54[123589]|-mcpu=54[56]lp] [-mfar-mode|-mf]
[-merrors-to-file
Target Xtensa options:
[--[no-]density] [--[no-]relax] [--[no-]generics]
[--[no-]text-section-literals]
[--[no-]target-align] [--[no-]longcalls]
filename
|-me
filename
]
-a[cdhlmns]
Turn on listings, in any of a variety of ways:
-ac
omit false conditionals
-ad
omit debugging directives
-ah
include high-level source
-al
include assembly
-am
include macro expansions
-an
omit forms processing
-as
include symbols
=file
set the name of the listing file
You may combine these options; for example, use -aln for assembly listing without forms processing. The =file option, if used, must be the last one. By itself, -a defaults to -ahls.
-alternate
Begin in alternate macro mode, see Section 8.61 .altmacro.
-D
Ignored. This option is accepted for script compatibility with calls to other assemblers.
6 Chapter 2. Overview
-defsym sym=value
Define the symbol sym to be value before assembling the input file. value must be an integer
constant. As in C, a leading 0x indicates a hexadecimal value, and a leading 0 indicates an octal
value.
-f
"fast"--skip whitespace and comment preprocessing (assume source is compiler output).
-g
-gen-debug
Generate debugging information for each assembler source line using whichever debug format is
preferred by the target. This currently means either STABS, ECOFF or DWARF2.
-gstabs
Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
-gstabs+
Generate stabs debugging information for each assembler line, with GNU extensions that probably only gdb can handle, and that could make other debuggers crash or refuse to read your
program. This may help debugging assembler code. Currently the only GNU extension is the
location of the current working directory at assembling time.
-gdwarf-2
Generate DWARF2 debugging information for each assembler line. This may help debugging
assembler code, ifthe debugger can handle it. Note--this option is only supported by some targets,
not all of them.
-help
Print a summary of the command line options and exit.
-target-help
Print a summary of all target specific options and exit.
-I dir
Add directory dir to the search list for .include directives.
-J
Don’t warn about signed overflow.
-K
Issue warnings when difference tables altered for long displacements.
-L
-keep-locals
Keep (in the symbol table) local symbols. On traditional a.out systems these start with L, but
different systems have different local label prefixes.
-listing-lhs-width=number
Set the maximum width, in words, of the output data column for an assembler listing to number.
Chapter 2. Overview 7
-listing-lhs-width2=number
Set the maximum width, in words, of the output data column for continuation lines in an assembler listing to number.
-listing-rhs-width=number
Set the maximum width of an input source line, as displayed in a listing, to number bytes.
-listing-cont-lines=number
Set the maximum number of lines printed in a listing for a single line of input to number + 1.
-o objfile
Name the object-file output from as objfile.
-R
Fold the data section into the text section.
-statistics
Print the maximum space (in bytes) and total time (in seconds) used by assembly.
-strip-local-absolute
Remove local absolute symbols from the outgoing symbol table.
-v
-version
Print the as version.
-version
Print the as version and exit.
-W
-no-warn
Suppress warning messages.
-fatal-warnings
Treat warnings as errors.
-warn
Don’t suppress warning messages or treat them as errors.
-w
Ignored.
-x
Ignored.
-Z
Generate an object file even after errors.
8 Chapter 2. Overview
- | files ...
Standard input, or source files to assemble.
The following options are available when as is configured for an ARC processor.
-marc[5|6|7|8]
This option selects the core processor variant.
-EB | -EL
Select either big-endian (-EB) or little-endian (-EL) output.
The following options are available when as is configured for the ARM processor family.
-mcpu=processor[+extension...]
Specify which ARM processor variant is the target.
-march=architecture[+extension...]
Specify which ARM architecture variant is used by the target.
-mfpu=floating-point-format
Select which Floating Point architecture is the target.
-mfloat-abi=abi
Select which floating point ABI is in use.
-mthumb
Enable Thumb only instruction decoding.
-mapcs-32 | -mapcs-26 | -mapcs-float | -mapcs-reentrant | -moabi
Select which procedure calling convention is in use.
-EB | -EL
Select either big-endian (-EB) or little-endian (-EL) output.
-mthumb-interwork
Specify that the code has been generated with interworking between Thumb and ARM code in
mind.
-k
Specify that PIC code has been generated.
See the info pages for documentation of the CRIS-specific options.
The following options are available when as is configured for a D10V processor.
-O
Optimize output by parallelizing instructions.
The following options are available when as is configured for a D30V processor.
Chapter 2. Overview 9
-O
Optimize output by parallelizing instructions.
-n
Warn when nops are generated.
-N
Warn when a nop after a 32-bit multiply instruction is generated.
The following options are available when as is configured for the Intel 80960 processor.
-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC
Specify which variant of the 960 architecture is the target.
-b
Add code to collect statistics about branches taken.
-no-relax
Do not alter compare-and-branch instructions for long displacements; error if necessary.
The following options are available when as is configured for the Ubicom IP2K series.
-mip2022ext
Specifies that the extended IP2022 instructions are allowed.
-mip2022
Restores the default behaviour, which restricts the permitted instructions to just the basic IP2022
ones.
The following options are available when as is configured for the Renesas M32R (formerly Mitsubishi
M32R) series.
-m32rx
Specify which processor in the M32R family is the target. The default is normally the M32R, but
this option changes it to the M32RX.
-warn-explicit-parallel-conflicts or -Wp
Produce warning messages when questionable parallel constructs are encountered.
-no-warn-explicit-parallel-conflicts or -Wnp
Do not produce warning messages when questionable parallel constructs are encountered.
The following options are available when as is configured for the Motorola 68000 series.
-l
Shorten references to undefined symbols, to one word instead of two.
10 Chapter 2. Overview
-m68000 | -m68008 | -m68010 | -m68020 | -m68030
| -m68040 | -m68060 | -m68302 | -m68331 | -m68332
| -m68333 | -m68340 | -mcpu32 | -m5200
Specify what processor in the 68000 family is the target. The default is normally the 68020, but
this can be changed at configuration time.
-m68881 | -m68882 | -mno-68881 | -mno-68882
The target machine does (or does not) have a floating-point coprocessor. The default is to assume
a coprocessor for 68020, 68030, and cpu32. Although the basic 68000 is not compatible with
the 68881, a combination of the two can be specified, since it’s possible to do emulation of the
coprocessor instructions with the main processor.
-m68851 | -mno-68851
The target machine does (or does not) have a memory-management unit coprocessor. The default
is to assume an MMU for 68020 and up.
For details about the PDP-11 machine dependent features options, see Section 32.1 Options.
-mpic | -mno-pic
Generate position-independent (or position-dependent) code. The default is -mpic.
-mall
-mall-extensions
Enable all instruction set extensions. This is the default.
-mno-extensions
Disable all instruction set extensions.
-mextension | -mno-extension
Enable (or disable) a particular instruction set extension.
-mcpu
Enable the instruction set extensions supported by a particular CPU, and disable all other extensions.
-mmachine
Enable the instruction set extensions supported by a particular machine model, and disable all
other extensions.
The following options are available when as is configured for a picoJava processor.
-mb
Generate "big endian" format output.
-ml
Generate "little endian" format output.
The following options are available when as is configured for the Motorola 68HC11 or 68HC12 series.
Chapter 2. Overview 11
-m68hc11 | -m68hc12 | -m68hcs12
Specify what processor is the target. The default is defined by the configuration option when
building the assembler.
-mshort
Specify to use the 16-bit integer ABI.
-mlong
Specify to use the 32-bit integer ABI.
-mshort-double
Specify to use the 32-bit double ABI.
-mlong-double
Specify to use the 64-bit double ABI.
-force-long-branchs
Relative branches are turned into absolute ones. This concerns conditional branches, unconditional branches and branches to a sub routine.
-S | -short-branchs
Do not turn relative branchs into absolute ones when the offset is out of range.
-strict-direct-mode
Do not turn the direct addressing mode into extended addressing mode when the instruction does
not support direct addressing mode.
-print-insn-syntax
Print the syntax of instruction in case of error.
-print-opcodes
print the list of instructions with syntax and then exit.
-generate-example
print an example of instruction for each possible instruction and then exit. This option is only
useful for testing as.
The following options are available when as is configured for the SPARC architecture:
-Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a
Explicitly select a variant of the SPARC architecture.
-Av8plus and -Av8plusa select a 32 bit environment. -Av9 and -Av9a select a 64 bit envi-
ronment.
-Av8plusa and -Av9a enable the SPARC V9 instruction set with UltraSPARC extensions.
-xarch=v8plus | -xarch=v8plusa
For compatibility with the Solaris v9 assembler. These options are equivalent to -Av8plus and
-Av8plusa, respectively.
12 Chapter 2. Overview
-bump
Warn when the assembler switches to another architecture.
The following options are available when as is configured for the ’c54x architecture.
-mfar-mode
Enable extended addressing mode. All addresses and relocations will assume extended addressing (usually 23 bits).
-mcpu=CPU_VERSION
Sets the CPU version being compiled for.
-merrors-to-file FILENAME
Redirect error output to a file, for broken systems which don’t support such behaviour in the
shell.
The following options are available when as is configured for a mips processor.
-G num
This option sets the largest size of an object that can be referenced implicitly with the gp register.
It is only accepted for targets that use ECOFF format, such as a DECstation running Ultrix. The
default value is 8.
-EB
Generate "big endian" format output.
-EL
Generate "little endian" format output.
-mips1
-mips2
-mips3
-mips4
-mips5
-mips32
-mips32r2
-mips64
-mips64r2
Generate code for a particular mips Instruction Set Architecture level. -mips1 is an
alias for -march=r3000, -mips2 is an alias for -march=r6000, -mips3 is an alias for
-march=r4000 and -mips4 is an alias for -march=r8000. -mips5, -mips32, -mips32r2,
-mips64, and -mips64r2 correspond to generic MIPS V, MIPS32, MIPS32 Release 2,
MIPS64, and MIPS64 Release 2 ISA processors, respectively.
-march=CPU
Generate code for a particular mips cpu.
-mtune=cpu
Schedule and tune for a particular mips cpu.
Chapter 2. Overview 13
-mfix7000
-mno-fix7000
Cause nops to be inserted if the read of the destination register of an mfhi or mflo instruction
occurs in the following two instructions.
-mdebug
-no-mdebug
Cause stabs-style debugging output to go into an ECOFF-style .mdebug section instead of the
standard ELF .stabs sections.
-mpdr
-mno-pdr
Control generation of .pdr sections.
-mgp32
-mfp32
The register sizes are normally inferred from the ISA and ABI, but these flags force a certain
group of registers to be treated as 32 bits wide at all times. -mgp32 controls the size of generalpurpose registers and -mfp32 controls the size of floating-point registers.
-mips16
-no-mips16
Generate code for the MIPS 16 processor. This is equivalent to putting .set mips16 at the start
of the assembly file. -no-mips16 turns off this option.
-mips3d
-no-mips3d
Generate code for the MIPS-3D Application Specific Extension. This tells the assembler to accept MIPS-3D instructions. -no-mips3d turns off this option.
-mdmx
-no-mdmx
Generate code for the MDMX Application Specific Extension. This tells the assembler to accept
MDMX instructions. -no-mdmx turns off this option.
-construct-floats
-no-construct-floats
The -no-construct-floats option disables the construction of double width floating point
constants by loading the two halves of the value into the two single width floating point registers
that make up the double width register. By default -construct-floats is selected, allowing
construction of these floating point constants.
-emulation=name
This option causes as to emulate as configured for some other target, in all respects, including
output format (choosing between ELF and ECOFF only), handling of pseudo-opcodes which
may generate debugging information or store symbol table information, and default endianness.
The available configuration names are: mipsecoff, mipself, mipslecoff, mipsbecoff,
mipslelf, mipsbelf. The first two do not alter the default endianness from that of the pri-
mary target for which the assembler was configured; the others change the default to little- or
big-endian as indicated by the b or l in the name. Using -EB or -EL will override the endianness
selection in any case.
14 Chapter 2. Overview
This option is currently supported only when the primary target as is configured for is a
mips ELF or ECOFF target. Furthermore, the primary target or others specified with
-enable-targets=... at configuration time must include support for the other format, if
both are to be available. For example, the Irix 5 configuration includes support for both.
Eventually, this option will support more configurations, with more fine-grained control over the
assembler’s behavior, and will be supported for more processors.
-nocpp
as ignores this option. It is accepted for compatibility with the native tools.
-trap
-no-trap
-break
-no-break
Control how to deal with multiplication overflow and division by zero. -trap or -no-break
(which are synonyms) take a trap exception (and only work for Instruction Set Architecture level
2 and higher); -break or -no-trap (also synonyms, and the default) take a break exception.
-n
When this option is used, as will issue a warning every time it generates a nop instruction from
a macro.
The following options are available when as is configured for an MCore processor.
-jsri2bsr
-nojsri2bsr
Enable or disable the JSRI to BSR transformation. By default this is enabled. The command line
option -nojsri2bsr can be used to disable it.
-sifilter
-nosifilter
Enable or disable the silicon filter behaviour. By default this is disabled. The default can be
overridden by the -sifilter command line option.
-relax
Alter jump instructions for long displacements.
-mcpu=[210|340]
Select the cpu type on the target hardware. This controls which instructions can be assembled.
-EB
Assemble for a big endian target.
-EL
Assemble for a little endian target.
See the info pages for documentation of the MMIX-specific options.
The following options are available when as is configured for an Xtensa processor.
-density | -no-density
Enable or disable use of instructions from the Xtensa code density option. This is enabled by
default when the Xtensa processor supports the code density option.
Chapter 2. Overview 15
-relax | -no-relax
Enable or disable instruction relaxation. This is enabled by default. Note: In the current implementation, these options also control whether assembler optimizations are performed, making
these options equivalent to -generics and -no-generics.
-generics | -no-generics
Enable or disable all assembler transformations of Xtensa instructions. The default is
-generics; -no-generics should be used only in the rare cases when the instructions must
be exactly as specified in the assembly source.
-text-section-literals | -no-text-section-literals
With -text-section-literals, literal pools are interspersed in the text section. The default
is -no-text-section-literals, which places literals in a separate section in the output file.
-target-align | -no-target-align
Enable or disable automatic alignment to reduce branch penalties at the expense of some code
density. The default is -target-align.
-longcalls | -no-longcalls
Enable or disable transformation of call instructions to allow calls across a greater range of
addresses. The default is -no-longcalls.
2.1. Structure of this Manual
This manual is intended to describe what you need to know to use gnu as. We cover the syntax
expected in source files, including notation for symbols, constants, and expressions; the directives
that as understands; and of course how to invoke as.
This manual also describes some of the machine-dependent features of various flavors of the assembler.
On the other hand, this manual is not intended as an introduction to programming in assembly
language--let alone programming in general! In a similar vein, we make no attempt to introduce the
machine architecture; we do not describe the instruction set, standard mnemonics, registers or addressing modes that are standard to a particular architecture. You may want to consult the manufacturer’s
machine architecture manual for this information.
2.2. The GNU Assembler
gnu as is really a family of assemblers. If you use (or have used) the gnu assembler on one architecture, you should find a fairly similar environment when you use it on another architecture. Each
version has much in common with the others, including object file formats, most assembler directives
(often called pseudo-ops) and assembler syntax.
as is primarily intended to assemble the output of the gnu C compiler gcc for use by the linker ld.
Nevertheless, we’ve tried to make as assemble correctly everything that other assemblers for the same
machine would assemble. Any exceptions are documented explicitly (Chapter 9 Machine Dependent
Features). This doesn’t mean as always uses the same syntax as another assembler for the same
architecture; for example, we know of several incompatible versions of 680x0 assembly language
syntax.
Unlike older assemblers, as is designed to assemble a source program in one pass of the source file.
This has a subtle impact on the .org directive (Section 8.65 .org new-lc, fill).
16 Chapter 2. Overview
2.3. Object File Formats
The gnu assembler can be configured to produce several alternative object file formats. For the most
part, this does not affect how you write assembly language programs; but directives for debugging
symbols are typically different in different file formats. Section 6.5 Symbol Attributes.
2.4. Command Line
After the program name as, the command line may contain options and file names. Options may
appear in any order, and may be before, after, or between file names. The order of file names is
significant.
- (two hyphens) by itself names the standard input file explicitly, as one of the files for as to assemble.
Except for - any command line argument that begins with a hyphen (-) is an option. Each option
changes the behavior of as. No option changes the way another option works. An option is a followed by one or more letters; the case of the letter is important. All options are optional.
Some options expect exactly one file name to follow them. The file name may either immediately
follow the option’s letter (compatible with older assemblers) or it may be the next command argument
(gnu standard). These two command lines are equivalent:
as -o my-object-file.o mumble.s
as -omy-object-file.o mumble.s
2.5. Input Files
We use the phrase source program, abbreviated source, to describe the program input to one run of
as. The program may be in one or more files; how the source is partitioned into files doesn’t change
the meaning of the source.
The source program is a concatenation of the text in all the files, in the order specified.
Each time you run as it assembles exactly one source program. The source program is made up of
one or more files. (The standard input is also a file.)
You give as a command line that has zero or more input file names. The input files are read (from left
file name to right). A command line argument (in any position) that has no special meaning is taken
to be an input file name.
If you give as no file names it attempts to read one input file from the as standard input, which is
normally your terminal. You may have to type [ctl-D] to tell as there is no more program to assemble.
Use - if you need to explicitly name the standard input file in your command line.
If the source is empty, as produces a small, empty object file.
2.5.1. Filenames and Line-numbers
There are two ways of locating a line in the input file (or files) and either may be used in reporting
error messages. One way refers to a line number in a physical file; the other refers to a line number in
a "logical" file. Section 2.7 Error and Warning Messages.
Physical files are those files named in the command line given to as.
Logical files are simply names declared explicitly by assembler directives; they bear no relation to
physical files. Logical file names help error messages reflect the original source file, when as source
Chapter 2. Overview 17
is itself synthesized from other files. as understands the # directives emitted by the gcc preprocessor.
See also Section 8.37 .file string .
2.6. Output (Object) File
Every time you run as it produces an output file, which is your assembly language program translated
into numbers. This file is the object file. Its default name is a.out, or b.out when as is configured
for the Intel 80960. You can give it another name by using the -o option. Conventionally, object file
names end with .o. The default name is used for historical reasons: older assemblers were capable
of assembling self-contained programs directly into a runnable program. (For some formats, this isn’t
currently possible, but it can be done for the a.out format.)
The object file is meant for input to the linker ld. It contains assembled program code, information to
help ld integrate the assembled program into a runnable file, and (optionally) symbolic information
for the debugger.
2.7. Error and Warning Messages
as may write warnings and error messages to the standard error file (usually your terminal). This
should not happen when a compiler runs as automatically. Warnings report an assumption made so
that as could keep assembling a flawed program; errors report a grave problem that stops the assembly.
Warning messages have the format
file_name:NNN :Warning Message Text
(where NNN is a line number). If a logical file name has been given (Section 8.37 .file string) it is
used for the filename, otherwise the name of the current input file is used. If a logical line number was
given (Section 8.54 .line line-number) then it is used to calculate the number printed, otherwise
the actual line in the current source file is printed. The message text is intended to be self explanatory
(in the grand Unix tradition).
Error messages have the format
file_name:NNN :FATAL:Error Message Text
The file name and line number are derived as for warning messages. The actual message text may be
rather less explanatory because many of them aren’t supposed to happen.
18 Chapter 2. Overview
Chapter 3.
Command-Line Options
This chapter describes command-line options available in all versions of the gnu assembler; Chapter
9 Machine Dependent Features, for options specific to particular machine architectures.
If you are invoking as via the gnu C compiler, you can use the -Wa option to pass arguments through to
the assembler. The assembler arguments must be separated from each other (and the -Wa) by commas.
For example:
gcc -c -g -O -Wa,-alh,-L file.c
This passes two options to the assembler: -alh (emit a listing to standard output with high-level and
assembly source) and -L (retain local symbols in the symbol table).
Usually you do not need to use this -Wa mechanism, since many compiler command-line options are
automatically passed to the assembler by the compiler. (You can call the gnu compiler driver with the
-v option to see precisely what options it passes to each compilation pass, including the assembler.)
3.1. Enable Listings: -a[cdhlns]
These options enable listing output from the assembler. By itself, -a requests high-level, assembly,
and symbols listing. You can use other letters to select specific options for the list: -ah requests a highlevel language listing, -al requests an output-program assembly listing, and -as requests a symbol
table listing. High-level listings require that a compiler debugging option like -g be used, and that
assembly listings (-al) be requested also.
Use the -ac option to omit false conditionals from a listing. Any lines which are not assembled
because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else,
will be omitted from the listing.
Use the -ad option to omit debugging directives from the listing.
Once you have specified one of these options, you can further control listing output and its appearance
using the directives .list, .nolist, .psize, .eject, .title, and .sbttl. The -an option turns
off all forms processing. If you do not request listing output with one of the -a options, the listingcontrol directives have no effect.
The letters after -a may be combined into one option, e.g., -aln.
Note if the assembler source is coming from the standard input (eg because it is being created by gcc
and the -pipe command line switch is being used) then the listing will not contain any comments
or preprocessor directives. This is because the listing code buffers input source lines from stdin only
after they have been preprocessed by the assembler. This reduces memory usage and makes the code
more efficient.
3.2. -alternate
Begin in alternate macro mode, see Section 8.61 .altmacro.
20 Chapter 3. Command-Line Options
3.3. -D
This option has no effect whatsoever, but it is accepted to make it more likely that scripts written for
other assemblers also work with as.
3.4. Work Faster: -f
-f should only be used when assembling programs written by a (trusted) compiler. -f stops the
assembler from doing whitespace and comment preprocessing on the input file(s) before assembling
them. Section 4.1 Preprocessing.
Warning: if you use -f when the files actually need to be preprocessed (if they contain comments, for
example), as does not work correctly.
3.5. .includeSearch Path: -Ipath
Use this option to add a path to the list of directories as searches for files specified in .include
directives (Section 8.47 .include "file"). You may use -I as many times as necessary to include
a variety of paths. The current working directory is always searched first; after that, as searches any
-I directories in the same order as they were specified (left to right) on the command line.
3.6. Difference Tables: -K
as sometimes alters the code emitted for directives of the form .word sym1-sym2; Section 8.101
.word expressions. You can use the -K option if you want a warning issued when this is done.
3.7. Include Local Labels: -L
Labels beginning with L (upper case only) are called local labels. Section 6.3 Symbol Names. Normally you do not see such labels when debugging, because they are intended for the use of programs
(like compilers) that compose assembler programs, not for your notice. Normally both as and ld
discard such labels, so you do not normally debug with them.
This option tells as to retain those L... symbols in the object file. Usually if you do this you also tell
the linker ld to preserve symbols whose names begin with L.
By default, a local label is any label beginning with L, but each target is allowed to redefine the local
label prefix. On the HPPA local labels begin with L$.
3.8. Configuring listing output: -listing
The listing feature of the assembler can be enabled via the command line switch -a (Section 3.1
Enable Listings: -a[cdhlns]). This feature combines the input source file(s) with a hex dump of the
corresponding locations in the output object file, and displays them as a listing file. The format of this
listing can be controlled by pseudo ops inside the assembler source (Section 8.58 .list Section 8.93
.title "heading" Section 8.76 .sbttl "subheading" Section 8.71 .psize lines, columns
Section 8.24 .eject) and also by the following switches:
-listing-lhs-width=number
Sets the maximum width, in words, of the first line of the hex byte dump. This dump appears on
the left hand side of the listing output.
Chapter 3. Command-Line Options 21
-listing-lhs-width2=number
Sets the maximum width, in words, of any further lines of the hex byte dump for a given input
source line. If this value is not specified, it defaults to being the same as the value specified for
-listing-lhs-width. If neither switch is used the default is to one.
-listing-rhs-width=number
Sets the maximum width, in characters, of the source line that is displayed alongside the hex
dump. The default value for this parameter is 100. The source line is displayed on the right hand
side of the listing output.
-listing-cont-lines=number
Sets the maximum number of continuation lines of hex dump that will be displayed for a given
single line of source input. The default value is 4.
3.9. Assemble in MRI Compatibility Mode: -M
The -M or -mri option selects MRI compatibility mode. This changes the syntax and pseudo-op
handling of as to make it compatible with the ASM68K or the ASM960 (depending upon the configured
target) assembler from Microtec Research. The exact nature of the MRI syntax will not be documented
here; see the MRI manuals for more information. Note in particular that the handling of macros and
macro arguments is somewhat different. The purpose of this option is to permit assembling existing
MRI assembler code using as.
The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its
object file format, and can not be supported using other object file formats. Supporting these would
require enhancing each object file format individually. These are:
• global symbols in common section
The m68k MRI assembler supports common sections which are merged by the linker. Other object
file formats do not support this. as handles common sections by treating them as a single common
symbol. It permits local symbols to be defined within a common section, but it can not support
global symbols, since it has no way to describe them.
• complex relocations
The MRI assemblers support relocations against a negated section address, and relocations which
combine the start addresses of two or more sections. These are not support by other object file
formats.
• END pseudo-op specifying start address
The MRI END pseudo-op permits the specification of a start address. This is not supported by other
object file formats. The start address may instead be specified using the -e option to the linker, or
in a linker script.
• IDNT, .ident and NAME pseudo-ops
The MRI IDNT, .ident and NAME pseudo-ops assign a module name to the output file. This is not
supported by other object file formats.
• ORG pseudo-op
The m68k MRI ORG pseudo-op begins an absolute section at a given address. This differs from the
usual as .org pseudo-op, which changes the location within the current section. Absolute sections
are not supported by other object file formats. The address of a section may be assigned within a
linker script.
22 Chapter 3. Command-Line Options
There are some other features of the MRI assembler which are not supported by as, typically either
because they are difficult or because they seem of little consequence. Some of these may be supported
in future releases.
• EBCDIC strings
EBCDIC strings are not supported.
• packed binary coded decimal
Packed binary coded decimal is not supported. This means that the DC.P and DCB.P pseudo-ops
are not supported.
• FEQU pseudo-op
The m68k FEQU pseudo-op is not supported.
• NOOBJ pseudo-op
The m68k NOOBJ pseudo-op is not supported.
• OPT branch control options
The m68k OPT branch control options--B, BRS, BRB, BRL, and BRW--are ignored. as automatically
relaxes all branches, whether forward or backward, to an appropriate size, so these options serve no
purpose.
• OPT list control options
The following m68k OPT list control options are ignored: C, CEX, CL, CRE, E, G, I, M, MEX, MC, MD,
X.
• other OPT options
The following m68k OPT options are ignored: NEST, O, OLD, OP, P, PCO, PCR, PCS, R.
• OPT D option is default
The m68k OPT D option is the default, unlike the MRI assembler. OPT NOD may be used to turn it
off.
• XREF pseudo-op.
The m68k XREF pseudo-op is ignored.
• .debug pseudo-op
The i960 .debug pseudo-op is not supported.
• .extended pseudo-op
The i960 .extended pseudo-op is not supported.
• .list pseudo-op.
The various options of the i960 .list pseudo-op are not supported.
• .optimize pseudo-op
The i960 .optimize pseudo-op is not supported.
• .output pseudo-op
The i960 .output pseudo-op is not supported.
• .setreal pseudo-op
The i960 .setreal pseudo-op is not supported.
Chapter 3. Command-Line Options 23
3.10. Dependency Tracking: -MD
as can generate a dependency file for the file it creates. This file consists of a single rule suitable for
make describing the dependencies of the main source file.
The rule is written to the file named in its argument.
This feature is used in the automatic updating of makefiles.
3.11. Name the Object File: -o
There is always one object file output when you run as. By default it has the name a.out (or b.out,
for Intel 960 targets only). You use this option (which takes exactly one filename) to give the object
file a different name.
Whatever the object file is called, as overwrites any existing file of the same name.
3.12. Join Data and Text Sections: -R
-R tells as to write the object file as if all data-section data lives in the text section. This is only done
at the very last moment: your binary data are the same, but data section parts are relocated differently.
The data section part of your object file is zero bytes long because all its bytes are appended to the
text section. (Chapter 5 Sections and Relocation.)
When you specify -R it would be possible to generate shorter address displacements (because we do
not have to cross between text and data section). We refrain from doing this simply for compatibility
with older versions of as. In future, -R may work this way.
When as is configured for COFF or ELF output, this option is only useful if you use sections named
.text and .data.
-R is not supported for any of the HPPA targets. Using -R generates a warning from as.
3.13. Display Assembly Statistics: -statistics
Use -statistics to display two statistics about the resources used by as: the maximum amount of
space allocated during the assembly (in bytes), and the total execution time taken for the assembly (in
cpu seconds).
3.14. Compatible Output: -traditional-format
For some targets, the output of as is different in some ways from the output of some existing assembler. This switch requests as to use the traditional format instead.
For example, it disables the exception frame optimizations which as normally does by default on gcc
output.
3.15. Announce Version: -v
You can find out what version of as is running by including the option -v (which you can also spell
as -version) on the command line.
24 Chapter 3. Command-Line Options
3.16. Control Warnings: -W, -warn, -no-warn, -fatal-warnings
as should never give a warning or error message when assembling compiler output. But programs
written by people often cause as to give a warning that a particular assumption was made. All such
warnings are directed to the standard error file.
If you use the -W and -no-warn options, no warnings are issued. This only affects the warning
messages: it does not change any particular of how as assembles your file. Errors, which stop the
assembly, are still reported.
If you use the -fatal-warnings option, as considers files that generate warnings to be in error.
You can switch these options off again by specifying -warn, which causes warnings to be output as
usual.
3.17. Generate Object File in Spite of Errors: -Z
After an error message, as normally produces no output. If for some reason you are interested in
object file output even after as gives an error message on your program, use the -Z option. If there
are any errors, as continues anyways, and writes an object file after a final warning message of the
form n errors, m warnings, generating bad object file.
Chapter 4.
Syntax
This chapter describes the machine-independent syntax allowed in a source file. as syntax is similar
to what many other assemblers use; it is inspired by the BSD 4.2 assembler, except that as does not
assemble Vax bit-fields.
4.1. Preprocessing
The as internal preprocessor:
• adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and
turns any other whitespace on the line into a single space.
• removes all comments, replacing them with a single space, or an appropriate number of newlines.
• converts character constants into the appropriate numeric values.
It does not do macro processing, include file handling, or anything else you may get from your C
compiler’s preprocessor. You can do include file processing with the .include directive (Section 8.47
.include "file"). You can use the gnu C compiler driver to get other "CPP" style preprocessing
by giving the input file a .S suffix. .
Excess whitespace, comments, and character constants cannot be used in the portions of the input text
that are not preprocessed.
If the first line of an input file is #NO_APP or if you use the -f option, whitespace and comments are
not removed from the input file. Within an input file,you can ask for whitespace and comment removal
in specific portions of the by putting a line that says #APP before the text that may contain whitespace
or comments, and putting a line that says #NO_APP after this text. This feature is mainly intend to
support asm statements in compilers whose output is otherwise free of comments and whitespace.
4.2. Whitespace
Whitespace is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to
make programs neater for people to read. Unless within character constants (Section 4.6.1 Character
Constants), any whitespace means the same as exactly one space.
4.3. Comments
There are two ways of rendering comments to as. In both cases the comment is equivalent to one
space.
Anything from /* through the next */ is a comment. This means you may not nest these comments.
/*
The only way to include a newline (’\n’) in a comment
is to use this sort of comment.
*/
/* This sort of comment does not nest. */
26 Chapter 4. Syntax
Anything from the line comment character to the next newline is considered a comment and is ignored.
The line comment character is ; for the AMD 29K family; ; on the ARC; @ on the ARM; ; for the
H8/300 family; ! for the H8/500 family; ; for the HPPA; # on the i386 and x86-64; # on the i960;
; for the PDP-11; ; for picoJava; # for Motorola PowerPC; ! for the Renesas / SuperH SH; ! on
the SPARC; # on the ip2k; # on the m32r; | on the 680x0; # on the 68HC11 and 68HC12; ; on the
M880x0; # on the Vax; ! for the Z8000; # on the V850; # for Xtensa systems; see Chapter 9 Machine
Dependent Features.
On some machines there are two different line comment characters. One character only begins a
comment if it is the first non-whitespace character on a line, while the other always begins a comment.
The V850 assembler also supports a double dash as starting a comment that extends to the end of the
line.
-;
To be compatible with past assemblers, lines that begin with # have a special interpretation. Following
the # should be an absolute expression (Chapter 7 Expressions): the logical line number of the next
line. Then a string (Section 4.6.1.1 Strings) is allowed: if present it is a new logical file name. The rest
of the line, if any, should be whitespace.
If the first non-whitespace characters on the line are not numeric, the line is ignored. (Just like a
comment.)
# 42-6 "new_file_name" # New logical file name
# This is an ordinary comment.
# This is logical line # 36.
This feature is deprecated, and may disappear from future versions of as.
4.4. Symbols
A symbol is one or more characters chosen from the set of all letters (both upper and lower case), digits
and the three characters _.$. On most machines, you can also use $ in symbol names; exceptions
are noted in Chapter 9 Machine Dependent Features. No symbol may begin with a digit. Case is
significant. There is no length limit: all characters are significant. Symbols are delimited by characters
not in that set, or by the beginning of a file (since the source program must end with a newline, the
end of a file is not a possible symbol delimiter). Chapter 6 Symbols.
4.5. Statements
A statement ends at a newline character (\n) or line separator character. (The line separator is usually
;, unless this conflicts with the comment character; Chapter 9 Machine Dependent Features.) The
newline or separator character is considered part of the preceding statement. Newlines and separators
within character constants are an exception: they do not end statements.
It is an error to end any statement with end-of-file: the last character of any input file should be a
newline.
An empty statement is allowed, and may include whitespace. It is ignored.
A statement begins with zero or more labels, optionally followed by a key symbol which determines
what kind of statement it is. The key symbol determines the syntax of the rest of the statement.
If the symbol begins with a dot . then the statement is an assembler directive: typically valid for
any computer. If the symbol begins with a letter the statement is an assembly language instruction:
it assembles into a machine language instruction. Different versions of as for different computers
recognize different instructions. In fact, the same symbol may represent a different instruction in a
different computer’s assembly language.
Chapter 4. Syntax 27
A label is a symbol immediately followed by a colon (:). Whitespace before a label or after a colon
is permitted, but you may not have whitespace between a label’s symbol and its colon. Section 6.1
Labels.
For HPPA targets, labels need not be immediately followed by a colon, but the definition of a label
must begin in column zero. This also implies that only one label may be defined on each line.
label: .directive followed by something
another_label: # This is an empty statement.
instruction operand_1, operand_2, ...
4.6. Constants
A constant is a number, written so that its value is known by inspection, without knowing any context.
Like this:
.byte 74, 0112, 092, 0x4A, 0X4a, ’J, ’\J # All the same value.
.ascii "Ring the bell\7" # A string constant.
.octa 0x123456789abcdef0123456789ABCDEF0 # A bignum.
.float 0f-314159265358979323846264338327\
95028841971.693993751E-40 # - pi, a flonum.
4.6.1. Character Constants
There are two kinds of character constants. A character stands for one character in one byte and
its value may be used in numeric expressions. String constants (properly called string literals) are
potentially many bytes and their values may not be used in arithmetic expressions.
4.6.1.1. Strings
A string is written between double-quotes. It may contain double-quotes or null characters. The way
to get special characters into a string is to escape these characters: precede them with a backslash \
character. For example \\ represents one backslash: the first \ is an escape which tells as to interpret
the second character literally as a backslash (which prevents as from recognizing the second \ as an
escape character). The complete list of escapes follows.
\b
Mnemonic for backspace; for ASCII this is octal code 010.
\f
Mnemonic for FormFeed; for ASCII this is octal code 014.
\n
Mnemonic for newline; for ASCII this is octal code 012.
28 Chapter 4. Syntax
\r
Mnemonic for carriage-Return; for ASCII this is octal code 015.
\t
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
\ digit digit digit
An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix
systems, 8 and 9 are accepted as digits: for example, \008 has the value 010, and \009 the value
011.
\x hex-digits...
A hex character code. All trailing hex digits are combined. Either upper or lower case x works.
\\
Represents one \ character.
\"
Represents one " character. Needed in strings to represent this character, because an unescaped
" would end the string.
\ anything-else
Any other character when escaped by \ gives a warning, but assembles as if the \ was not present.
The idea is that if you used an escape sequence you clearly didn’t want the literal interpretation
of the following character. However as has no other interpretation, so as knows it is giving you
the wrong code and warns you of the fact.
Which characters are escapable, and what those escapes represent, varies widely among assemblers.
The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C
compilers recognize. If you are in doubt, do not use an escape sequence.
4.6.1.2. Characters
A single character may be written as a single quote immediately followed by that character. The same
escapes apply to characters as to strings. So if you want to write the character backslash, you must
write ’\\ where the first \ escapes the second \. As you can see, the quote is an acute accent, not a
grave accent. A newline immediately following an acute accent is taken as a literal character and does
not count as the end of a statement. The value of a character constant in a numeric expression is the
machine’s byte-wide code for that character. as assumes your character code is ASCII: ’A means 65,
’B means 66, and so on.
4.6.2. Number Constants
as distinguishes three kinds of numbers according to how they are stored in the target machine.
Integers are numbers that would fit into an int in the C language. Bignums are integers, but they are
stored in more than 32 bits. Flonums are floating point numbers, described below.
Chapter 4. Syntax 29
4.6.2.1. Integers
A binary integer is 0b or 0B followed by zero or more of the binary digits 01.
An octal integer is 0 followed by zero or more of the octal digits (01234567).
A decimal integer starts with a non-zero digit followed by zero or more digits (0123456789).
A hexadecimal integer is 0x or 0X followed by one or more hexadecimal digits chosen from
0123456789abcdefABCDEF.
Integers have the usual values. To denote a negative integer, use the prefix operator - discussed under
expressions (Section 7.2.3 Prefix Operator).
4.6.2.2. Bignums
A bignum has the same syntax and semantics as an integer except that the number (or its negative)
takes more than 32 bits to represent in binary. The distinction is made because in some places integers
are permitted while bignums are not.
4.6.2.3. Flonums
A flonum represents a floating point number. The translation is indirect: a decimal floating point number from the text is converted by as to a generic binary floating point number of more than sufficient
precision. This generic floating point number is converted to a particular computer’s floating point
format (or formats) by a portion of as specialized to that computer.
A flonum is written by writing (in order)
• The digit 0. (0 is optional on the HPPA.)
• A letter, to tell as the rest of the number is a flonum. e is recommended. Case is not important.
On the H8/300, H8/500, Renesas / SuperH SH, and AMD 29K architectures, the letter must be one
of the letters DFPRSX (in upper or lower case).
On the ARC, the letter must be one of the letters DFRS (in upper or lower case).
On the Intel 960 architecture, the letter must be one of the letters DFT (in upper or lower case).
On the HPPA architecture, the letter must be E (upper case only).
• An optional sign: either + or -.
• An optional integer part: zero or more decimal digits.
• An optional fractional part: . followed by zero or more decimal digits.
• An optional exponent, consisting of:
• An E or e.
• Optional sign: either + or -.
• One or more decimal digits.
At least one of the integer part or the fractional part must be present. The floating point number has
the usual base-10 value.
as does all processing using integers. Flonums are computed independently of any floating point
hardware in the computer running as.
30 Chapter 4. Syntax
Chapter 5.
Sections and Relocation
5.1. Background
Roughly, a section is a range of addresses, with no gaps; all data "in" those addresses is treated the
same for some particular purpose. For example there may be a "read only" section.
The linker ld reads many object files (partial programs) and combines their contents to form a
runnable program. When as emits an object file, the partial program is assumed to start at address
0. ld assigns the final addresses for the partial program, so that different partial programs do not
overlap. This is actually an oversimplification, but it suffices to explain how as uses sections.
ld moves blocks of bytes of your program to their run-time addresses. These blocks slide to their
run-time addresses as rigid units; their length does not change and neither does the order of bytes
within them. Such a rigid unit is called a section. Assigning run-time addresses to sections is called
relocation. It includes the task of adjusting mentions of object-file addresses so they refer to the proper
run-time addresses. For the H8/300 and H8/500, and for the Renesas / SuperH SH, as pads sections
if needed to ensure they end on a word (sixteen bit) boundary.
An object file written by as has at least three sections, any of which may be empty. These are named
text, data and bss sections.
When it generates COFF or ELF output, as can also generate whatever other named sections you
specify using the .section directive (Section 8.78 .section name). If you do not use any directives
that place output in the .text or .data sections, these sections still exist, but are empty.
When as generates SOM or ELF output for the HPPA, as can also generate whatever other named
sections you specify using the .space and .subspace directives. See [HP9000 Series 800 Assembly
Language Reference Manual] (HP 92432-90001) for details on the .space and .subspace assembler directives.
Additionally, as uses different names for the standard text, data, and bss sections when generating
SOM output. Program text is placed into the $CODE$ section, data into $DATA$, and BSS into $BSS$.
Within the object file, the text section starts at address 0, the data section follows, and the bss section
follows the data section.
When generating either SOM or ELF output files on the HPPA, the text section starts at address 0, the
data section at address 0x4000000, and the bss section follows the data section.
To let ld know which data changes when the sections are relocated, and how to change that data, as
also writes to the object file details of the relocation needed. To perform relocation ld must know,
each time an address in the object file is mentioned:
• Where in the object file is the beginning of this reference to an address?
• How long (in bytes) is this reference?
• Which section does the address refer to? What is the numeric value of
(address) - (start-address of section)?
• Is the reference to an address "Program-Counter relative"?
In fact, every address as ever uses is expressed as
(section) + (offset into section)
32 Chapter 5. Sections and Relocation
Further, most expressions as computes have this section-relative nature. (For some object formats,
such as SOM for the HPPA, some expressions are symbol-relative instead.)
In this manual we use the notation {secname N} to mean "offset N into section secname."
Apart from text, data and bss sections you need to know about the absolute section. When ld
mixes partial programs, addresses in the absolute section remain unchanged. For example, address
{absolute 0} is "relocated" to run-time address 0 by ld. Although the linker never arranges two
partial programs’ data sections with overlapping addresses after linking, by definition their absolute
sections must overlap. Address {absolute 239} in one part of a program is always the same
address when the program is running as address {absolute 239} in any other part of the program.
The idea of sections is extended to the undefined section. Any address whose section is unknown at
assembly time is by definition rendered {undefined U}--where U is filled in later. Since numbers are
always defined, the only way to generate an undefined address is to mention an undefined symbol. A
reference to a named common block would be such a symbol: its value is unknown at assembly time
so it has section undefined.
By analogy the word section is used to describe groups of sections in the linked program. ld puts
all partial programs’ text sections in contiguous addresses in the linked program. It is customary to
refer to the text section of a program, meaning all the addresses of all partial programs’ text sections.
Likewise for data and bss sections.
Some sections are manipulated by ld; others are invented for use of as and have no meaning except
during assembly.
5.2. Linker Sections
ld deals with just four kinds of sections, summarized below.
named sections
text section
data section
These sections hold your program. as and ld treat them as separate but equal sections. Anything
you can say of one section is true of another. When the program is running, however, it is customary for the text section to be unalterable. The text section is often shared among processes:
it contains instructions, constants and the like. The data section of a running program is usually
alterable: for example, C variables would be stored in the data section.
bss section
This section contains zeroed bytes when your program begins running. It is used to hold uninitialized variables or common storage. The length of each partial program’s bss section is important,
but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in
the object file. The bss section was invented to eliminate those explicit zeros from object files.
absolute section
Address 0 of this section is always "relocated" to runtime address 0. This is useful if you want to
refer to an address that ld must not change when relocating. In this sense we speak of absolute
addresses being "unrelocatable": they do not change during relocation.
Chapter 5. Sections and Relocation 33
undefined section
This "section" is a catch-all for address references to objects not in the preceding sections.
An idealized example of three relocatable sections follows. The example uses the traditional section
names .text and .data. Memory addresses are on the horizontal axis.
partial program # 1: |ttttt|dddd|00|
partial program # 2: |TTT|DDD|000|
linked program: | |TTT|ttttt| |dddd|DDD|00000|
addresses: 0 ...
+-----+----+--+
+-----+----+--+
text data bss
seg. seg. seg.
+---+---+---+
+---+---+---+
+--+---+-----+--+----+---+-----+~~
+--+---+-----+--+----+---+-----+~~
5.3. Assembler Internal Sections
These sections are meant only for the internal use of as. They have no meaning at run-time. You do not
really need to know about these sections for most purposes; but they can be mentioned in as warning
messages, so it might be helpful to have an idea of their meanings to as. These sections are used
to permit the value of every expression in your assembly language program to be a section-relative
address.
ASSEMBLER-INTERNAL-LOGIC-ERROR!
An internal assembler logic error has been found. This means there is a bug in the assembler.
expr section
The assembler stores complex expression internally as combinations of symbols. When it needs
to represent an expression as a symbol, it puts it in the expr section.
5.4. Sub-Sections
Assembled bytes conventionally fall into two sections: text and data. You may have separate groups of
data in named sections that you want to end up near to each other in the object file, even though they
are not contiguous in the assembler source. as allows you to use subsections for this purpose. Within
each section, there can be numbered subsections with values from 0 to 8192. Objects assembled into
the same subsection go into the object file together with other objects in the same subsection. For
example, a compiler might want to store constants in the text section, but might not want to have
them interspersed with the program being assembled. In this case, the compiler could issue a .text
0 before each section of code being output, and a .text 1 before each group of constants being
output.
Subsections are optional. If you do not use subsections, everything goes in subsection number zero.
34 Chapter 5. Sections and Relocation
Each subsection is zero-padded up to a multiple of four bytes. (Subsections may be padded a different
amount on different flavors of as.)
Subsections appear in your object file in numeric order, lowest numbered to highest. (All this to be
compatible with other people’s assemblers.) The object file contains no representation of subsections;
ld and other programs that manipulate object files see no trace of them. They just see all your text
subsections as a text section, and all your data subsections as a data section.
To specify which subsection you want subsequent statements assembled into, use a numeric argument
to specify it, in a .text expression or a .data expression statement. When generating COFF
output, you can also use an extra subsection argument with arbitrary named sections: .section
name, expression. When generating ELF output, you can also use the .subsection directive (Sec-
tion 8.89 .subsection name) to specify a subsection: .subsection expression. Expression
should be an absolute expression. (Chapter 7 Expressions.) If you just say .text then .text 0 is
assumed. Likewise .data means .data 0. Assembly begins in text 0. For instance:
.text 0 # The default subsection is text 0 anyway.
.ascii "This lives in the first text subsection. *"
.text 1
.ascii "But this lives in the second text subsection."
.data 0
.ascii "This lives in the data section,"
.ascii "in the first data subsection."
.text 0
.ascii "This lives in the first text section,"
.ascii "immediately following the asterisk (*)."
Each section has a location counter incremented by one for every byte assembled into that section.
Because subsections are merely a convenience restricted to as there is no concept of a subsection
location counter. There is no way to directly manipulate a location counter--but the .align directive
changes it, and any label definition captures its current value. The location counter of the section
where statements are being assembled is said to be the active location counter.
5.5. bss Section
The bss section is used for local common variable storage. You may allocate address space in the bss
section, but you may not dictate data to load into it before your program executes. When your program
starts running, all the contents of the bss section are zeroed bytes.
The .lcomm pseudo-op defines a symbol in the bss section; see Section 8.52 .lcomm symbol,
length.
The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol; see Section 8.8 .comm symbol, length.
When assembling for a target which supports multiple sections, such as ELF or COFF, you may switch
into the .bss section and define symbols as usual; see Section 8.78 .section name. You may only
assemble zero values into the section. Typically the section will only contain symbol definitions and
.skip directives (Section 8.84 .skip size, fill).
Chapter 6.
Symbols
Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols
to link, and the debugger uses symbols to debug.
Warning: as does not place symbols in the object file in the same order they were declared. This may break
some debuggers.
6.1. Labels
A label is written as a symbol immediately followed by a colon :. The symbol then represents the
current value of the active location counter, and is, for example, a suitable instruction operand. You are
warned if you use the same symbol to represent two different locations: the first definition overrides
any other definitions.
On the HPPA, the usual form for a label need not be immediately followed by a colon, but instead
must start in column zero. Only one label may be defined on a single line. To work around this, the
HPPA version of as also provides a special directive .label for defining labels more flexibly.
6.2. Giving Symbols Other Values
A symbol can be given an arbitrary value by writing a symbol, followed by an equals sign =, followed
by an expression (Chapter 7 Expressions). This is equivalent to using the .set directive. Section 8.79
.set symbol, expression.
6.3. Symbol Names
Symbol names begin with a letter or with one of ._. On most machines, you can also use $ in symbol names; exceptions are noted in Chapter 9 Machine Dependent Features. That character may be
followed by any string of digits, letters, dollar signs (unless otherwise noted in Chapter 9 Machine
Dependent Features), and underscores. For the AMD 29K family, ? is also allowed in the body of a
symbol name, though not at its beginning.
Case of letters is significant: foo is a different symbol name than Foo.
Each symbol has exactly one name. Each name in an assembly language program refers to exactly
one symbol. You may use that symbol name any number of times in a program.
6.3.1. Local Symbol Names
Local symbols help compilers and programmers use names temporarily. They create symbols which
are guaranteed to be unique over the entire scope of the input source code and which can be referred
to by a simple notation. To define a local symbol, write a label of the form N: (where N represents
any positive integer). To refer to the most recent previous definition of that symbol write Nb, using the
same number as when you defined the label. To refer to the next definition of a local label, write Nf-The b stands for"backwards" and the f stands for "forwards".
There is no restriction on how you can use these labels, and you can reuse them too. So that it is
possible to repeatedly define the same local label (using the same number N), although you can only
refer to the most recently defined local label of that number (for a backwards reference) or the next
36 Chapter 6. Symbols
definition of a specific local label for a forward reference. It is also worth noting that the first 10 local
labels (0:. .. 9:) are implemented in a slightly more efficient manner than the others.
Here is an example:
1: branch 1f
2: branch 1b
1: branch 2f
2: branch 1b
Which is the equivalent of:
label_1: branch label_3
label_2: branch label_1
label_3: branch label_4
label_4: branch label_3
Local symbol names are only a notational device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names stored in the symbol
table, appearing in error messages and optionally emitted to the object file. The names are constructed
using these parts:
L
All local labels begin with L. Normally both as and ld forget symbols that start with L. These
labels are used for symbols you are never intended to see. If you use the -L option then as retains
these symbols in the object file. If you also instruct ld to retain these symbols, you may use them
in debugging.
number
This is the number that was used in the local label definition. So if the label is written 55: then
the number is 55.
C-B
This unusual character is included so you do not accidentally invent a symbol of the same name.
The character has ASCII value of \002 (control-B).
ordinal number
This is a serial number to keep the labels distinct. The first definition of 0: gets the number 1.
The 15th definition of 0: gets the number 15, and so on. Likewise the first definition of 1: gets
the number 1 and its 15th defintion gets 15 as well.
So for example, the first 1: is named L1C-B1, the 44th 3: is named L3C-B44.
6.3.2. Dollar Local Labels
as also supports an even more local form of local labels called dollar labels. These labels go out of
scope (ie they become undefined) as soon as a non-local label is defined. Thus they remain valid for
only a small region of the input source code. Normal local labels, by contrast, remain in scope for the
entire file, or until they are redefined by another occurrence of the same local label.
Dollar labels are defined in exactly the same way as ordinary local labels, except that instead of being
terminated by a colon, they are terminated by a dollar sign. eg 55$.
Chapter 6. Symbols 37
They can also be distinguished from ordinary local labels by their transformed name which uses
ASCII character \001 (control-A) as the magic character to distinguish them from ordinary labels.
Thus the 5th defintion of 6$ is named L6C-A5.
6.4. The Special Dot Symbol
The special symbol . refers to the current address that as is assembling into. Thus, the expression
melvin: .long . defines melvin to contain its own address. Assigning a value to . is treated the
same as a .org directive. Thus, the expression .=.+4 is the same as saying .space 4.
6.5. Symbol Attributes
Every symbol has, as well as its name, the attributes "Value" and "Type". Depending on output format,
symbols can also have auxiliary attributes.
If you use a symbol without defining it, as assumes zero for all these attributes, and probably won’t
warn you. This makes the symbol an externally defined symbol, which is generally what you would
want.
6.5.1. Value
The value of a symbol is (usually) 32 bits. For a symbol which labels a location in the text, data, bss
or absolute sections the value is the number of addresses from the start of that section to the label.
Naturally for text, data and bss sections the value of a symbol changes as ld changes section base
addresses during linking. Absolute symbols’ values do not change during linking: that is why they are
called absolute.
The value of an undefined symbol is treated in a special way. If it is 0 then the symbol is not defined
in this assembler source file, and ld tries to determine its value from other files linked into the same
program. You make this kind of symbol simply by mentioning a symbol name without defining it. A
non-zero value represents a .comm common declaration. The value is how much common storage to
reserve, in bytes (addresses). The symbol refers to the first address of the allocated storage.
6.5.2. Type
The type attribute of a symbol contains relocation (section) information, any flag settings indicating
that a symbol is external, and (optionally), other information for linkers and debuggers. The exact
format depends on the object-code output format in use.
6.5.3. Symbol Attributes: a.out
6.5.3.1. Descriptor
This is an arbitrary 16-bit value. You may establish a symbol’s descriptor value by using a .desc
statement (Section 8.21 .desc symbol, abs-expression). A descriptor value means nothing to as.
38 Chapter 6. Symbols
6.5.3.2. Other
This is an arbitrary 8-bit value. It means nothing to as.
6.5.4. Symbol Attributes for COFF
The COFF format supports a multitude of auxiliary symbol attributes; like the primary symbol attributes, they are set between .def and .endef directives.
6.5.4.1. Primary Attributes
The symbol name is set with .def; the value and type, respectively, with .val and .type.
6.5.4.2. Auxiliary Attributes
The as directives .dim, .line, .scl, .size, .tag, and .weak can generate auxiliary symbol table
information for COFF.
6.5.5. Symbol Attributes for SOM
The SOM format for the HPPA supports a multitude of symbol attributes set with the .EXPORT and
.IMPORT directives.
The attributes are described in [HP9000 Series 800 Assembly Language Reference Manual] (HP
92432-90001) under the IMPORT and EXPORT assembler directive documentation.
Chapter 7.
Expressions
An expression specifies an address or numeric value. Whitespace may precede and/or follow an expression.
The result of an expression must be an absolute number, or else an offset into a particular section. If an
expression is not absolute, and there is not enough information when as sees the expression to know
its section, a second pass over the source program might be necessary to interpret the expression--but
the second pass is currently not implemented. as aborts with an error message in this situation.
7.1. Empty Expressions
An empty expression has no value: it is just whitespace or null. Wherever an absolute expression is
required, you may omit the expression, and as assumes a value of (absolute) 0. This is compatible
with other assemblers.
7.2. Integer Expressions
An integer expression is one or more arguments delimited by operators.
7.2.1. Arguments
Arguments are symbols, numbers or subexpressions. In other contexts arguments are sometimes called
"arithmetic operands". In this manual, to avoid confusing them with the "instruction operands" of the
machine language, we use the term "argument" to refer to parts of expressions only, reserving the
word "operand" to refer only to machine instruction operands.
Symbols are evaluated to yield {section NNN} where section is one of text, data, bss, absolute, or
undefined. NNN is a signed, 2’s complement 32 bit integer.
Numbers are usually integers.
A number can be a flonum or bignum. In this case, you are warned that only the low order 32 bits are
used, and as pretends these 32 bits are an integer. You may write integer-manipulating instructions
that act on exotic constants, compatible with other assemblers.
Subexpressions are a left parenthesis ( followed by an integer expression, followed by a right parenthesis ); or a prefix operator followed by an argument.
7.2.2. Operators
Operators are arithmetic functions, like + or %. Prefix operators are followed by an argument. Infix
operators appear between their arguments. Operators may be preceded and/or followed by whitespace.
7.2.3. Prefix Operator
as has the following prefix operators. They each take one argument, which must be absolute.
-
Negation. Two’s complement negation.
40 Chapter 7. Expressions
~
Complementation. Bitwise not.
7.2.4. Infix Operators
Infix operators take two arguments, one on either side. Operators have precedence, but operations with
equal precedence are performed left to right. Apart from + or -, both arguments must be absolute, and
the result is absolute.
1. Highest Precedence
*
Multiplication.
/
Division. Truncation is the same as the C operator /
%
Remainder.
Shift Left. Same as the C operator
Shift Right. Same as the C operator
.
.
2. Intermediate precedence
|
Bitwise Inclusive Or.
&
Bitwise And.
^
Bitwise Exclusive Or.
!
Bitwise Or Not.
3. Low Precedence
Chapter 7. Expressions 41
+
Addition. If either argument is absolute, the result has the section of the other argument.
You may not add together arguments from different sections.
-
Subtraction. If the right argument is absolute, the result has the section of the left argument.
If both arguments are in the same section, the result is absolute. You may not subtract
arguments from different sections.
==
Is Equal To
Is Not Equal To
Is Less Than
Is Greater Than
=
Is Greater Than Or Equal To
=
Is Less Than Or Equal To
The comparison operators can be used as infix operators. A true results has a value of -1
whereas a false result has a value of 0. Note, these operators perform signed comparisons.
4. Lowest Precedence
&&
Logical And.
||
Logical Or.
These two logical operations can be used to combine the results of sub expressions. Note,
unlike the comparison operators a true result returns a value of 1 but a false results does
still return 0. Also note that the logical or operator has a slightly lower precedence than
logical and.
In short, it’s only meaningful to add or subtract the offsets in an address; you can only have a defined
section in one of the two arguments.
42 Chapter 7. Expressions
Chapter 8.
Assembler Directives
All assembler directives have names that begin with a period (.). The rest of the name is letters,
usually in lower case.
This chapter discusses directives that are available regardless of the target machine configuration for
the gnu assembler. Some machine configurations provide additional directives. Chapter 9 Machine
Dependent Features.
8.1. .abort
This directive stops the assembly immediately. It is for compatibility with other assemblers. The
original idea was that the assembly language source would be piped into the assembler. If the sender of
the source quit, it could use this directive tells as to quit also. One day .abort will not be supported.
8.2. .ABORT
When producing COFF output, as accepts this directive as a synonym for .abort.
When producing b.out output, as accepts this directive, but ignores it.
8.3. .align abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment required, as described below.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and
the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some
systems, if the section is marked as containing code and the fill value is omitted, the space is filled
with no-op instructions.
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of
bytes that should be skipped by this alignment directive. If doing the alignment would require skipping
more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill
value (the second argument) entirely by simply using two commas after the required alignment; this
can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The way the required alignment is specified varies from systemto system. For the a29k, arc, hppa, i386
using ELF, i860, iq2000, m68k, m88k, or32, s390, sparc, tic4x, tic80 and xtensa, the first expression
is the alignment request in bytes. For example .align 8 advances the location counter until it is a
multiple of 8. If the location counter is already a multiple of 8, no change is needed. For the tic54x,
the first expression is the alignment request in words.
For other systems, including the i386 using a.out format, and the arm and strongarm, it is the number
of low-order zero bits the location counter must have after advancement. For example .align 3
advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8,
no change is needed.
This inconsistency is due to the different behaviors of the various native assemblers for these systems
which GAS must emulate. GAS also provides .balign and .p2align directives, described later,
which have a consistent behavior across all architectures (but are specific to GAS).
44 Chapter 8. Assembler Directives
8.4. .ascii "string". . .
.ascii expects zero or more string literals (Section 4.6.1.1 Strings) separated by commas. It assem-
bles each string (with no automatic trailing zero byte) into consecutive addresses.
8.5. .asciz "string". . .
.asciz is just like .ascii, but each string is followed by a zero byte. The "z" in .asciz stands for
"zero".
8.6. .balign[wl] abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment request in bytes. For example .balign 8 advances
the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no
change is needed.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and
the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some
systems, if the section is marked as containing code and the fill value is omitted, the space is filled
with no-op instructions.
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of
bytes that should be skipped by this alignment directive. If doing the alignment would require skipping
more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill
value (the second argument) entirely by simply using two commas after the required alignment; this
can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The .balignw and .balignl directives are variants of the .balign directive. The .balignw directive treats the fill pattern as a two byte word value. The .balignl directives treats the fill pattern
as a four byte longword value. For example, .balignw 4,0x368d will align to a multiple of 4. If it
skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends
upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.
8.7. .byte expressions
.byte expects zero or more expressions, separated by commas. Each expression is assembled into
the next byte.
8.8. .comm symbol, length
.comm declares a common symbol named symbol. When linking, a common symbol in one object
file may be merged with a defined or common symbol of the same name in another object file. If
ld does not see a definition for the symbol-just one or more common symbols-then it will allocate
length bytes of uninitialized memory. length must be an absolute expression. If ld sees multiple
common symbols with the same name, and they do not all have the same size, it will allocate space
using the largest size.
When using ELF, the .comm directive takes an optional third argument. This is the desired alignment
of the symbol, specified as a byte boundary (for example, an alignment of 16 means that the least
significant 4 bits of the address should be zero). The alignment must be an absolute expression, and
it must be a power of two. If ld allocates uninitialized memory for the common symbol, it will use
the alignment when placing the symbol. If no alignment is specified, as will set the alignment to the
largest power of two less than or equal to the size of the symbol, up to a maximum of 16.
Chapter 8. Assembler Directives 45
The syntax for .comm differs slightly on the HPPA. The syntax is symbol .comm, length; symbol
is optional.
8.9. .cfi_startproc
.cfi_startproc is used at the beginning of each function that should have an entry in .eh_frame.
It initializes some internal data structures and emits architecture dependent initial CFI instructions.
Don’t forget to close the function by .cfi_endproc.
8.10. .cfi_endproc
.cfi_endproc is used at the end of a function where it closes its unwind entry previously opened by
.cfi_startproc. and emits it to .eh_frame.
8.11. .cfi_def_cfa register, offset
.cfi_def_cfa defines a rule for computing CFA as: take address from register and add offset to it.
8.12. .cfi_def_cfa_register register
.cfi_def_cfa_register modifies a rule for computing CFA. From now on register will be
used instead of the old one. Offset remains the same.
8.13. .cfi_def_cfa_offset offset
.cfi_def_cfa_offset modifies a rule for computing CFA. Register remains the same, but offset
is new. Note that it is the absolute offset that will be added to a defined register to compute CFA
address.
8.14. .cfi_adjust_cfa_offset offset
Same as .cfi_def_cfa_offset but offset is a relative value that is added/substracted from the
previous offset.
8.15. .cfi_offset register, offset
Previous value of register is saved at offset offset from CFA.
8.16. .cfi_rel_offset register, offset
Previous value of register is saved at offset offset from the current CFA register. This is transformed to .cfi_offset using the known displacement of the CFA register from the CFA. This is
often easier to use, because the number will match the code it’s annotating.
46 Chapter 8. Assembler Directives
8.17. .cfi_window_save
SPARC register window has been saved.
8.18. .cfi_escapeexpression[, . .. ]
Allows the user to add arbitrary bytes to the unwind info. One might use this to add OS-specific CFI
opcodes, or generic CFI opcodes that GAS does not yet support.
8.19. .data subsection
.data tells as to assemble the following statements onto the end of the data subsection numbered
subsection (which is an absolute expression). If subsection is omitted, it defaults to zero.
8.20. .def name
Begin defining debugging information for a symbol name; the definition extends until the .endef
directive is encountered.
This directive is only observed when as is configured for COFF format output; when producing
b.out, .def is recognized, but ignored.
8.21. .desc symbol, abs-expression
This directive sets the descriptor of the symbol (Section 6.5 Symbol Attributes) to the low 16 bits of
an absolute expression.
The .desc directive is not available when as is configured for COFF output; it is only for a.out
or b.out object format. For the sake of compatibility, as accepts it, but produces no output, when
configured for COFF.
8.22. .dim
This directive is generated by compilers to include auxiliary debugging information in the symbol
table. It is only permitted inside .def/.endef pairs.
.dim is only meaningful when generating COFF format output; when as is generating b.out, it
accepts this directive but ignores it.
8.23. .double flonums
.double expects zero or more flonums, separated by commas. It assembles floating point numbers.
The exact kind of floating point numbers emitted depends on how as is configured. Chapter 9 Machine
Dependent Features.
8.24. .eject
Force a page break at this point, when generating assembly listings.
Chapter 8. Assembler Directives 47
8.25. .else
.else is part of the as support for conditional assembly; Section 8.45 .if absolute expression.
It marks the beginning of a section of code to be assembled if the condition for the preceding .if was
false.
8.26. .elseif
.elseif is part of the as support for conditional assembly; Section 8.45 .if absolute
expression. It is shorthand for beginning a new .if block that would otherwise fill the entire
.else section.
8.27. .end
.end marks the end of the assembly file. as does not process anything in the file past the .end
directive.
8.28. .endef
This directive flags the end of a symbol definition begun with .def.
.endef is only meaningful when generating COFF format output; if as is configured to generate
b.out, it accepts this directive but ignores it.
8.29. .endfunc
.endfunc marks the end of a function specified with .func.
8.30. .endif
.endif is part of the as support for conditional assembly; it marks the end of a block of code that is
only assembled conditionally. Section 8.45 .if absolute expression.
8.31. .equ symbol, expression
This directive sets the value of symbol to expression. It is synonymous with .set; Section 8.79
.set symbol, expression.
The syntax for equ on the HPPA is symbol .equ expression.
8.32. .equiv symbol, expression
The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol
is already defined. Note a symbol which has been referenced but not actually defined is considered to
be undefined.
Except for the contents of the error message, this is roughly equivalent to
.ifdef SYM
.err
.endif
48 Chapter 8. Assembler Directives
.equ SYM,VAL
8.33. .err
If as assembles a .err directive, it will print an error message and, unless the -Z option was used, it
will not generate an object file. This can be used to signal error an conditionally compiled code.
8.34. .exitm
Exit early from the current macro definition. Section 8.60 .macro.
8.35. .extern
.extern is accepted in the source program--for compatibility with other assemblers--but it is ignored.
as treats all undefined symbols as external.
8.36. .fail expression
Generates an error or a warning. If the value of the expression is 500 or more, as will print a
warning message. If the value is less than 500, as will print an error message. The message will
include the value of expression. This can occasionally be useful inside complex nested macros or
conditional assembly.
8.37. .file string
.file tells as that we are about to start a new logical file. string is the new file name. In general,
the filename is recognized whether or not it is surrounded by quotes "; but if you wish to specify
an empty file name, you must give the quotes-"". This statement may go away in future: it is only
recognized to be compatible with old as programs. In some configurations of as, .file has already
been removed to avoid conflicts with other assemblers. Chapter 9 Machine Dependent Features.
8.38. .fill repeat, size, value
repeat, size and value are absolute expressions. This emits repeat copies of size bytes. Repeat
may be zero or more. Size may be zero or more, but if it is more than 8, then it is deemed to have the
value 8, compatible with other people’s assemblers. The contents of each repeat bytes is taken from
an 8-byte number. The highest order 4 bytes are zero. The lowest order 4 bytes are value rendered
in the byte-order of an integer on the computer as is assembling for. Each size bytes in a repetition
is taken from the lowest order size bytes of this number. Again, this bizarre behavior is compatible
with other people’s assemblers.
size and value are optional. If the second comma and value are absent, value is assumed zero. If
the first comma and following tokens are absent, size is assumed to be 1.
Chapter 8. Assembler Directives 49
8.39. .float flonums
This directive assembles zero or more flonums, separated by commas. It has the same effect as
.single. The exact kind of floating point numbers emitted depends on how as is configured. Chapter
9 Machine Dependent Features.
8.40. .func name[,label]
.func emits debugging information to denote function name, and is ignored unless the file is as-
sembled with debugging enabled. Only -gstabs[+] is currently supported. label is the entry point
of the function and if omitted name prepended with the leading char is used. leading char is
usually _ or nothing, depending on the target. All functions are currently defined to have void return
type. The function must be terminated with .endfunc.
8.41. .global symbol, .globl symbol
.global makes the symbol visible to ld. If you define symbol in your partial program, its value is
made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes
from a symbol of the same name from another file linked into the same program.
Both spellings (.globl and .global) are accepted, for compatibility with other assemblers.
On the HPPA, .global is not always enough to make it accessible to other partial programs. You
may need the HPPA-only .EXPORT directive as well. Section 19.5 HPPA Assembler Directives.
8.42. .hidden names
This is one of the ELF visibility directives. The other two are .internal (Section 8.49 .internal
names) and .protected (Section 8.70 .protected names).
This directive overrides the named symbols default visibility (which is set by their binding: local,
global or weak). The directive sets the visibility to hidden which means that the symbols are not
visible to other components. Such symbols are always considered to be protected as well.
8.43. .hword expressions
This expects zero or more expressions, and emits a 16 bit number for each.
This directive is a synonym for .short; depending on the target architecture, it may also be a synonym for .word.
8.44. .ident
This directive is used by some assemblers to place tags in object files. as simply accepts the directive
for source-file compatibility with such assemblers, but does not actually emit anything for it.
8.45. .if absolute expression
.if marks the beginning of a section of code which is only considered part of the source program
being assembled if the argument (which must be an absolute expression) is non-zero. The end
of the conditional section of code must be marked by .endif (Section 8.30 .endif ); optionally,
you may include code for the alternative condition, flagged by .else (Section 8.25 .else). If you
50 Chapter 8. Assembler Directives
have several conditions to check, .elseif may be used to avoid nesting blocks if/else within each
subsequent .else block.
The following variants of .if are also supported:
.ifdef symbol
Assembles the following section of code if the specified symbol has been defined. Note a symbol
which has been referenced but not yet defined is considered to be undefined.
.ifc string1,string2
Assembles the following section of code if the two strings are the same. The strings may be
optionally quoted with single quotes. If they are not quoted, the first string stops at the first
comma, and the second string stops at the end of the line. Strings which contain whitespace
should be quoted. The string comparison is case sensitive.
.ifeq absolute expression
Assembles the following section of code if the argument is zero.
.ifeqs string1,string2
Another form of .ifc. The strings must be quoted using double quotes.
.ifge absolute expression
Assembles the following section of code if the argument is greater than or equal to zero.
.ifgt absolute expression
Assembles the following section of code if the argument is greater than zero.
.ifle absolute expression
Assembles the following section of code if the argument is less than or equal to zero.
.iflt absolute expression
Assembles the following section of code if the argument is less than zero.
.ifnc string1,string2.
Like .ifc, but the sense of the test is reversed: this assembles the following section of code if
the two strings are not the same.
.ifndef symbol
.ifnotdef symbol
Assembles the following section of code if the specified symbol has not been defined. Both
spelling variants are equivalent. Note a symbol which has been referenced but not yet defined is
considered to be undefined.
.ifne absolute expression
Assembles the following section of code if the argument is not equal to zero (in other words, this
is equivalent to .if).
Chapter 8. Assembler Directives 51
.ifnes string1,string2
Like .ifeqs, but the sense of the test is reversed: this assembles the following section of code
if the two strings are not the same.
8.46. .incbin "file"[,skip[,count]]
The incbin directive includes file verbatim at the current location. You can control the search
paths used with the -I command-line option (Chapter 3 Command-Line Options). Quotation marks
are required around file.
The skip argument skips a number of bytes from the start of the file. The count argument indicates
the maximum number of bytes to read. Note that the data is not aligned in any way, so it is the
user’s responsibility to make sure that proper alignment is provided both before and after the incbin
directive.
8.47. .include "file"
This directive provides a way to include supporting files at specified points in your source program.
The code from file is assembled as if it followed the point of the .include; when the end of the
included file is reached, assembly of the original file continues. You can control the search paths used
with the -I command-line option (Chapter 3 Command-Line Options). Quotation marks are required
around file.
8.48. .int expressions
Expect zero or more expressions, of any section, separated by commas. For each expression, emit
a number that, at run time, is the value of that expression. The byte order and bit size of the number
depends on what kind of target the assembly is for.
8.49. .internal names
This is one of the ELF visibility directives. The other two are .hidden (Section 8.42 .hidden
names) and .protected (Section 8.70 .protected names).
This directive overrides the named symbols default visibility (which is set by their binding: local,
global or weak). The directive sets the visibility to internal which means that the symbols are
considered to be hidden (i.e., not visible to other components), and that some extra, processor specific
processing must also be performed upon the symbols as well.
8.50. .irp symbol,values. . .
Evaluate a sequence of statements assigning different values to symbol. The sequence of statements
starts at the .irp directive, and is terminated by an .endr directive. For each value, symbol is set to
value, and the sequence of statements is assembled. If no value is listed, the sequence of statements
is assembled once, with symbol set to the null string. To refer to symbol within the sequence of
statements, use \symbol.
For example, assembling
.irp param,1,2,3
move d\param,sp@.endr
52 Chapter 8. Assembler Directives
is equivalent to assembling
move d1,sp@move d2,sp@move d3,sp@-
8.51. .irpc symbol,values. . .
Evaluate a sequence of statements assigning different values to symbol. The sequence of statements
starts at the .irpc directive, and is terminated by an .endr directive. For each character in value,
symbol is set to the character, and the sequence of statements is assembled. If no value is listed,
the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol
within the sequence of statements, use \symbol.
For example, assembling
.irpc param,123
move d\param,sp@.endr
is equivalent to assembling
move d1,sp@move d2,sp@move d3,sp@-
8.52. .lcomm symbol, length
Reserve length (an absolute expression) bytes for a local common denoted by symbol. The section
and value of symbol are those of the new local common. The addresses are allocated in the bss
section, so that at run-time the bytes start off zeroed. Symbol is not declared global (Section 8.41
.global symbol, .globl symbol), so is normally not visible to ld.
Some targets permit a third argument to be used with .lcomm. This argument specifies the desired
alignment of the symbol in the bss section.
The syntax for .lcomm differs slightly on the HPPA. The syntax is symbol .lcomm, length;
symbol is optional.
8.53. .lflags
as accepts this directive, for compatibility with other assemblers, but ignores it.
Chapter 8. Assembler Directives 53
8.54. .line line-number
Change the logical line number. line-number must be an absolute expression. The next line has that
logical line number. Therefore any other statements on the current line (after a statement separator
character) are reported as onlogical line number line-number - 1. One day as will no longer support
this directive: it is recognized only for compatibility with existing assembler programs.
Warning: In the AMD29K configuration of as, this command is not available; use the synonym .ln
in that context.
Even though this is a directive associated with the a.out or b.out object-code formats, as still
recognizes it when producing COFF output, and treats .line as though it were the COFF .ln if it is
found outside a .def/.endef pair.
Inside a .def, .line is, instead, one of the directives used by compilers to generate auxiliary symbol
information for debugging.
8.55. .linkonce [type]
Mark the current section so that the linker only includes a single copy of it. This may be used to
include the same section in several different object files, but ensure that the linker will only include it
once in the final output file. The .linkonce pseudo-op must be used for each instance of the section.
Duplicate sections are detected based on the section name, so it should be unique.
This directive is only supported by a few object file formats; as of this writing, the only object file
format which supports it is the Portable Executable format used on Windows NT.
The type argument is optional. If specified, it must be one of the following strings. For example:
.linkonce same_size
Not all types may be supported on all object file formats.
discard
Silently discard duplicate sections. This is the default.
one_only
Warn if there are duplicate sections, but still keep only one copy.
same_size
Warn if any of the duplicates have different sizes.
same_contents
Warn if any of the duplicates do not have exactly the same contents.
8.56. .ln line-number
.ln is a synonym for .line.
8.57. .mri val
If val is non-zero, this tells as to enter MRI mode. If val is zero, this tells as to exit MRI mode. This
change affects code assembled until the next .mri directive, or until the end of the file. MRI mode.
54 Chapter 8. Assembler Directives
8.58. .list
Control (in conjunction with the .nolist directive) whether or not assembly listings are generated.
These two directives maintain an internal counter (which is zero initially). .list increments the
counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater
than zero.
By default, listings are disabled. When you enable them (with the -a command line option; Chapter
3 Command-Line Options), the initial value of the listing counter is one.
8.59. .long expressions
.long is the same as .int, Section 8.48 .int expressions.
8.60. .macro
The commands .macro and .endm allow you to define macros that generate assembly output. For
example, this definition specifies a macro sum that puts a sequence of numbers into memory:
.macro sum from=0, to=5
.long \from
.if \to-\from
sum "(\from+1)",\to
.endif
.endm
With that definition, SUM 0,5 is equivalent to this assembly input:
.long 0
.long 1
.long 2
.long 3
.long 4
.long 5
.macro macname
.macro macname macargs ...
Begin the definition of a macro called macname. If your macro definition requires arguments,
specify their names after the macro name, separated by commas or spaces. You can supply a
default value for any macro argument by following the name with =deflt. For example, these
are all valid .macro statements:
.macro comm
Begin the definition of a macro called comm, which takes no arguments.
.macro plus1 p, p1
.macro plus1 p p1
Either statement begins the definition of a macro called plus1, which takes two arguments;
within the macro definition, write \p or \p1 to evaluate the arguments.
Chapter 8. Assembler Directives 55
.macro reserve_str p1=0 p2
Begin the definition of a macro called reserve_str, with two arguments. The first argument has a default value, but not the second. After the definition is complete, you can call
the macro either as reserve_str a,b (with \p1 evaluating to a and \p2 evaluating to
b), or as reserve_str ,b (with \p1 evaluating as the default, in this case 0, and \p2
evaluating to b).
When you call a macro, you can specify the argument values either by position, or by keyword.
For example, sum 9,17 is equivalent to sum to=17, from=9.
.endm
Mark the end of a macro definition.
.exitm
Exit early from the current macro definition.
\@
as maintains a counter of how many macros it has executed in this pseudo-variable; you can
copy that number to your output with \@, but only within a macro definition.
LOCAL name [ , ... ]
Warning: LOCAL is only available if you select "alternate macro syntax" with -alternate or
.altmacro. Section 8.61 .altmacro.
8.61. .altmacro
Enable alternate macro mode, enabling:
LOCAL name [ , ... ]
One additional directive, LOCAL, is available. It is used to generate a string replacement for
each of the name arguments, and replace any instances of name in each macro expansion. The
replacement string is unique in the assembly, and different for each separate macro expansion.
LOCAL allows you to write macros that define symbols, without fear of conflict between separate
macro expansions.
String delimiters
You can write strings delimited in these other ways besides "string":
’string’
You can delimit strings with single-quote charaters.
string
You can delimit strings with matching angle brackets.
single-character string escape
To include any single character literally in a string (even if the character would otherwise have
some special meaning), you can prefix the character with ! (an exclamation mark). For example,
you can write
4.3 !5.4!!
to get the literal text 4.3
5.4!.
56 Chapter 8. Assembler Directives
Expression results as strings
You can write %expr to evaluate the expression expr and use the result as a string.
8.62. .noaltmacro
Disable alternate macro mode. Section 8.61 .altmacro
8.63. .nolist
Control (in conjunction with the .list directive) whether or not assembly listings are generated.
These two directives maintain an internal counter (which is zero initially). .list increments the
counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater
than zero.
8.64. .octa bignums
This directive expects zero or more bignums, separated by commas. For each bignum, it emits a 16byte integer.
The term "octa" comes from contexts in which a "word" is two bytes; hence octa-word for 16 bytes.
8.65. .org new-lc, fill
Advance the location counter of the current section to new-lc. new-lc is either an absolute expression or an expression with the same section as the current subsection. That is, you can’t use .org to
cross sections: if new-lc has the wrong section, the .org directive is ignored. To be compatible with
former assemblers, if the section of new-lc is absolute, as issues a warning, then pretends the section
of new-lc is the same as the current subsection.
.org may only increase the location counter, or leave it unchanged; you cannot use .org to move the
location counter backwards.
Because as tries to assemble programs in one pass, new-lc may not be undefined. If you really detest
this restriction we eagerly await a chance to share your improved assembler.
Beware that the origin is relative to the start of the section, not to the start of the subsection. This is
compatible with other people’s assemblers.
When the location counter (of the current subsection) is advanced, the intervening bytes are filled with
fill which should be an absolute expression. If the comma and fill are omitted, fill defaults to
zero.
8.66. .p2align[wl] abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the number of low-order zero bits the location counter must have
after advancement. For example .p2align 3 advances the location counter until it a multiple of 8.
If the location counter is already a multiple of 8, no change is needed.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and
the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some
systems, if the section is marked as containing code and the fill value is omitted, the space is filled
with no-op instructions.
Chapter 8. Assembler Directives 57
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of
bytes that should be skipped by this alignment directive. If doing the alignment would require skipping
more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill
value (the second argument) entirely by simply using two commas after the required alignment; this
can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The .p2alignw and .p2alignl directives are variants of the .p2align directive. The .p2alignw
directive treats the fill pattern as a two byte word value. The .p2alignl directives treats the fill
pattern as a four byte longword value. For example, .p2alignw 2,0x368d will align to a multiple
of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes
depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.
8.67. .previous
This is one of the ELF section stack manipulation directives. The others are .section (Section 8.78
.section name), .subsection (Section 8.89 .subsection name), .pushsection (Section 8.73
.pushsection name, subsection), and .popsection (Section 8.68 .popsection).
This directive swaps the current section (and subsection) with most recently referenced section (and
subsection) prior to this one. Multiple .previous directives in a row will flip between two sections
(and their subsections).
In terms of the section stack, this directive swaps the current section with the top section on the section
stack.
8.68. .popsection
This is one of the ELF section stack manipulation directives. The others are .section (Section 8.78
.section name), .subsection (Section 8.89 .subsection name), .pushsection (Section 8.73
.pushsection name, subsection), and .previous (Section 8.67 .previous).
This directive replaces the current section (and subsection) with the top section (and subsection) on
the section stack. This section is popped off the stack.
8.69. .print string
as will print string on the standard output during assembly. You must put string in double quotes.
8.70. .protected names
This is one of the ELF visibility directives. The other two are .hidden (Section 8.42 .hidden
names) and .internal (Section 8.49 .internal names).
This directive overrides the named symbols default visibility (which is set by their binding: local,
global or weak). The directive sets the visibility to protected which means that any references to
the symbols from within the components that defines them must be resolved to the definition in that
component, even if a definition in another component would normally preempt this.
8.71. .psize lines, columns
Use this directive to declare the number of lines--and, optionally, the number of columns--to use for
each page, when generating listings.
58 Chapter 8. Assembler Directives
If you do not use .psize, listings use a default line-count of 60. You may omit the comma and
columns specification; the default width is 200 columns.
as generates formfeeds whenever the specified number of lines is exceeded (or whenever you explic-
itly request one, using .eject).
If you specify lines as 0, no formfeeds are generated save those explicitly specified with .eject.
8.72. .purgem name
Undefine the macro name, so that later uses of the string will not be expanded. Section 8.60 .macro.
8.73. .pushsection name, subsection
This is one of the ELF section stack manipulation directives. The others are .section (Section 8.78
.section name), .subsection (Section 8.89 .subsection name), .popsection (Section 8.68
.popsection), and .previous (Section 8.67 .previous).
This directive pushes the current section (and subsection) onto the top of the section stack, and then
replaces the current section and subsection with name and subsection.
8.74. .quad bignums
.quad expects zero or more bignums, separated by commas. For each bignum, it emits an 8-byte
integer. If the bignum won’t fit in 8 bytes, it prints a warning message; and just takes the lowest order
8 bytes of the bignum. The term "quad" comes from contexts in which a "word" is two bytes; hence
quad-word for 8 bytes.
8.75. .rept count
Repeat the sequence of lines between the .rept directive and the next .endr directive count times.
For example, assembling
.rept 3
.long 0
.endr
is equivalent to assembling
.long 0
.long 0
.long 0
Chapter 8. Assembler Directives 59
8.76. .sbttl "subheading"
Use subheading as the title (third line, immediately after the title line) when generating assembly
listings.
This directive affects subsequent pages, as well as the current page if it appears within ten lines of the
top of a page.
8.77. .scl class
Set the storage-class value for a symbol. This directive may only be used inside a .def/.endef
pair. Storage class may flag whether a symbol is static or external, or it may record further symbolic
debugging information.
The .scl directive is primarily associated with COFF output; when configured to generate b.out
output format, as accepts this directive but ignores it.
8.78. .section name
Use the .section directive to assemble the following code into a section named name.
This directive is only supported for targets that actually support arbitrarily named sections; on a.out
targets, for example, it is not accepted, even with a standard a.out section name.
8.78.1. COFF Version
For COFF targets, the .section directive is used in one of the following ways:
.section name[, "flags"]
.section name[, subsegment]
If the optional argument is quoted, it is taken as flags to use for the section. Each flag is a single
character. The following flags are recognized:
b
bss section (uninitialized data)
n
section is not loaded
w
writable section
d
data section
r
read-only section
x
executable section
60 Chapter 8. Assembler Directives
s
shared section (meaningful for PE targets)
a
ignored. (For compatibility with the ELF version)
If no flags are specified, the default flags depend upon the section name. If the section name is not
recognized, the default will be for the section to be loaded and writable. Note the n and w flags remove
attributes from the section, rather than adding them, so if they are used on their own it will be as if no
flags had been specified at all.
If the optional argument to the .section directive is not quoted, it is taken as a subsegment number
(Section 5.4 Sub-Sections).
8.78.2. ELF Version
This is one of the ELF section stack manipulation directives. The others are .subsection (Section
8.89 .subsection name), .pushsection (Section 8.73 .pushsection name, subsection),
.popsection (Section 8.68 .popsection), and .previous (Section 8.67 .previous).
For ELF targets, the .section directive is used like this:
.section name [, "flags"[, @type[,flag_specific_arguments]]
The optional flags argument is a quoted string which may contain any combination of the following
characters:
a
section is allocatable
w
section is writable
x
section is executable
M
section is mergeable
S
section contains zero terminated strings
G
section is a member of a section group
T
section is used for thread-local-storage
The optional type argument may contain one of the following constants:
Chapter 8. Assembler Directives 61
@progbits
section contains data
@nobits
section does not contain data (i.e., section only occupies space)
@note
section contains data which is used by things other than the program
@init_array
section contains an array of pointers to init functions
@fini_array
section contains an array of pointers to finish functions
@preinit_array
section contains an array of pointers to pre-init functions
Many targets only support the first three section types.
Note on targets where the @ character is the start of a comment (eg ARM) then another character is
used instead. For example the ARM port uses the % character.
If flags contains the M symbol then the type argument must be specified as well as an extra argument
- entsize - like this:
.section name , "flags"M, @type, entsize
Sections with the M flag but not S flag must contain fixed size constants, each entsize octets long.
Sections with both M and S must contain zero terminated strings where each character is entsize
bytes long. The linker may remove duplicates within sections with the same name, same entity size
and same flags. entsize must be an absolute expression.
If flags contains the G symbol then the type argument must be present along with an additional
field like this:
.section name , "flags"G, @type, GroupName[, linkage]
The GroupName field specifies the name of the section group to which this particular section belongs.
The optional linkage field can contain:
comdat
indicates that only one copy of this section should be retained
.gnu.linkonce
an alias for comdat
Note - if both the M and G flags are present then the fields for the Merge flag should come first, like
this:
.section name , "flags"MG, @type, entsize, GroupName[, linkage]
62 Chapter 8. Assembler Directives
If no flags are specified, the default flags depend upon the section name. If the section name is not
recognized, the default will be for the section to have none of the above flags: it will not be allocated
in memory, nor writable, nor executable. The section will contain data.
For ELF targets, the assembler supports another type of .section directive for compatibility with
the Solaris assembler:
.section "name"[, flags...]
Note that the section name is quoted. There may be a sequence of comma separated flags:
#alloc
section is allocatable
#write
section is writable
#execinstr
section is executable
#tls
section is used for thread local storage
This directive replaces the current section and subsection. See the contents of the gas testsuite directory gas/testsuite/gas/elf for some examples of how this directive and the other section stack
directives work.
8.79. .set symbol, expression
Set the value of symbol to expression. This changes symbol’s value and type to conform to
expression. If symbol was flagged as external, it remains flagged (Section 6.5 Symbol Attributes).
You may .set a symbol many times in the same assembly.
If you .set a global symbol, the value stored in the object file is the last value stored into it.
The syntax for set on the HPPA is symbol .set expression.
8.80. .short expressions
.short is normally the same as .word. Section 8.101 .word expressions.
In some configurations, however, .short and .word generate numbers of different lengths; Chapter
9 Machine Dependent Features.
Chapter 8. Assembler Directives 63
8.81. .single flonums
This directive assembles zero or more flonums, separated by commas. It has the same effect as
.float. The exact kind of floating point numbers emitted depends on how as is configured. Chapter
9 Machine Dependent Features.
8.82. .size
This directive is used to set the size associated with a symbol.
8.82.1. COFF Version
For COFF targets, the .size directive is only permitted inside .def/.endef pairs. It is used like
this:
.size expression
.size is only meaningful when generating COFF format output; when as is generating b.out, it
accepts this directive but ignores it.
8.82.2. ELF Version
For ELF targets, the .size directive is used like this:
.size name , expression
This directive sets the size associated with a symbol name. The size in bytes is computed from
expression which can make use of label arithmetic. This directive is typically used to set the size
of function symbols.
8.83. .sleb128 expressions
sleb128 stands for "signed little endian base 128." This is a compact, variable length representation
of numbers used by the DWARF symbolic debugging format. .uleb128.
8.84. .skip size, fill
This directive emits size bytes, each of value fill. Both size and fill are absolute expressions.
If the comma and fill are omitted, fill is assumed to be zero. This is the same as .space.
8.85. .space size, fill
This directive emits size bytes, each of value fill. Both size and fill are absolute expressions.
If the comma and fill are omitted, fill is assumed to be zero. This is the same as .skip.
64 Chapter 8. Assembler Directives
Warning: .space has a completely different meaning for HPPA targets; use .block as a substitute. See
[HP9000 Series 800 Assembly Language Reference Manual] (HP 92432-90001) for the meaning of the
.space directive. Section 19.5 HPPA Assembler Directives, for a summary.
On the AMD 29K, this directive is ignored; it is accepted for compatibility with other AMD 29K
assemblers.
Warning: In most versions of the gnu assembler, the directive .space has the effect of .block Chapter 9
Machine Dependent Features.
8.86. .stabd, .stabn, .stabs
There are three directives that begin .stab. All emit symbols (Chapter 6 Symbols), for use by symbolic debuggers. Thesymbols are not entered in the as hash table: they cannot be referenced elsewhere
in the source file. Up to five fields are required:
string
This is the symbol’s name. It may contain any character except \000, so is more general than
ordinary symbol names. Some debuggers used to code arbitrarily complex structures into symbol
names using this field.
type
An absolute expression. The symbol’s type is set to the low 8 bits of this expression. Any bit
pattern is permitted, but ld and debuggers choke on silly bit patterns.
other
An absolute expression. The symbol’s "other" attribute is set to the low 8 bits of this expression.
desc
An absolute expression. The symbol’s descriptor is set to the low 16 bits of this expression.
value
An absolute expression which becomes the symbol’s value.
If a warning is detected while reading a .stabd, .stabn, or .stabs statement, the symbol has
probably already been created; you get a half-formed symbol in your object file. This is compatible
with earlier assemblers!
.stabd type , other , desc
The "name" of the symbol generated is not even an empty string. It is a null pointer, for compatibility. Older assemblers used a null pointer so they didn’t waste space in object files with empty
strings.
The symbol’s value is set to the location counter, relocatably. When your program is linked, the
value of this symbol is the address of the location counter when the .stabd was assembled.
.stabn type , other , desc , value
The name of the symbol is set to the empty string "".
Chapter 8. Assembler Directives 65
.stabs string , type , other , desc , value
All five fields are specified.
8.87. .string"str"
Copy the characters in str to the object file. You may specify more than one string to copy, separated
by commas. Unless otherwise specified for a particular machine, the assembler marks the end of each
string with a 0 byte. You can use any of the escape sequences described in Section 4.6.1.1 Strings.
8.88. .struct expression
Switch to the absolute section, and set the section offset to expression, which must be an absolute
expression. You might use this as follows:
field1:
field2:
field3:
.struct 0
.struct field1 + 4
.struct field2 + 4
This would define the symbol field1 to have the value 0, the symbol field2 to have the value 4,
and the symbol field3 to have the value 8. Assembly would be left in the absolute section, and you
would need to use a .section directive of some sort to change to some other section before further
assembly.
8.89. .subsection name
This is one of the ELF section stack manipulation directives. The others are .section (Section
8.78 .section name), .pushsection (Section 8.73 .pushsection name, subsection),
.popsection (Section 8.68 .popsection), and .previous (Section 8.67 .previous).
This directive replaces the current subsection with name. The current section is not changed. The
replaced subsection is put onto the section stack in place of the then current top of stack subsection.
8.90. .symver
Use the .symver directive to bind symbols to specific version nodes within a source file. This is only
supported on ELF platforms, and is typically used when assembling files to be linked into a shared
library. There are cases where it may make sense to use this in objects to be bound into an application
itself so as to override a versioned symbol from a shared library.
For ELF targets, the .symver directive can be used like this:
.symver name, name2@nodename
If the symbol name is defined within the file being assembled, the .symver directive effectively
creates a symbol alias with the name name2@nodename, and in fact the main reason that we just
don’t try and create a regular alias is that the @ character isn’t permitted in symbol names. The name2
part of the name is the actual name of the symbol by which it will be externally referenced. The name
name itself is merely a name of convenience that is used so that it is possible to have definitions for
multiple versions of a function within a single source file, and so that the compiler can unambiguously
know which version of a function is being mentioned. The nodename portion of the alias should be
66 Chapter 8. Assembler Directives
the name of a node specified in the version script supplied to the linker when building a shared library.
If you are attempting to override a versioned symbol from a shared library, then nodename should
correspond to the nodename of the symbol you are trying to override.
If the symbol name is not defined within the file being assembled, all references to name will be
changed to name2@nodename. If no reference to name is made, name2@nodename will be removed
from the symbol table.
Another usage of the .symver directive is:
.symver name, name2@@nodename
In this case, the symbol name must exist and be defined within the file being assembled. It is similar
to name2@nodename. The difference is name2@@nodename will also be used to resolve references to
name2 by the linker.
The third usage of the .symver directive is:
.symver name, name2@@@nodename
When name is not defined within the file being assembled, it is treated as name2@nodename.
When name is defined within the file being assembled, the symbol name, name, will be changed to
name2@@nodename.
8.91. .tag structname
This directive is generated by compilers to include auxiliary debugging information in the symbol
table. It is only permitted inside .def/.endef pairs. Tags are used to link structure definitions in the
symbol table with instances of those structures.
.tag is only used when generating COFF format output; when as is generating b.out, it accepts this
directive but ignores it.
8.92. .text subsection
Tells as to assemble the following statements onto the end of the text subsection numbered
subsection, which is an absolute expression. If subsection is omitted, subsection number zero is
used.
8.93. .title "heading"
Use heading as the title (second line, immediately after the source file name and pagenumber) when
generating assembly listings.
This directive affects subsequent pages, as well as the current page if it appears within ten lines of the
top of a page.
8.94. .type
This directive is used to set the type of a symbol.
Chapter 8. Assembler Directives 67
8.94.1. COFF Version
For COFF targets, this directive is permitted only within .def/.endef pairs. It is used like this:
.type int
This records the integer int as the type attribute of a symbol table entry.
.type is associated only with COFF format output; when as is configured for b.out output, it
accepts this directive but ignores it.
8.94.2. ELF Version
For ELF targets, the .type directive is used like this:
.type name , type description
This sets the type of symbol name to be either a function symbol or an object symbol. There are five
different syntaxes supported for the type description field, in order to provide compatibility with
various other assemblers. The syntaxes supported are:
.type
.typename,#object
.typename,@function
.typename,@object
.typename,%function
.typename,%object
.typename,"function"
.type
.typenameSTT_FUNCTION
.typenameSTT_OBJECT
name,#function
name,"object"
8.95. .uleb128 expressions
uleb128 stands for "unsigned little endian base 128." This is a compact, variable length representa-
tion of numbers used by the DWARF symbolic debugging format. .sleb128.
8.96. .val addr
This directive, permitted only within .def/.endef pairs, records the address addr as the value attribute of a symbol table entry.
.val is used only for COFF output; when as is configured for b.out, it accepts this directive but
ignores it.
68 Chapter 8. Assembler Directives
8.97. .version "string"
This directive creates a .note section and places into it an ELF formatted note of type NT_VERSION.
The note’s name is set to string.
8.98. .vtable_entry table, offset
This directive finds or creates a symbol table and creates a VTABLE_ENTRY relocation for it with an
addend of offset.
8.99. .vtable_inherit child, parent
This directive finds the symbol child and finds or creates the symbol parent and then creates a
VTABLE_INHERIT relocation for the parent whose addend is the value of the child symbol. As a
special case the parent name of 0 is treated as refering the *ABS* section.
8.100. .weak names
This directive sets the weak attribute on the comma separated list of symbol names. If the symbols do
not already exist, they will be created.
Weak symbols are supported in COFF as a GNU extension. This directive sets the weak attribute on
the comma separated list of symbol names. If the symbols do not already exist, they will be created.
.weak name [
= | ==
alternate] [, ...]
On the PE target, weak aliases are supported natively. Weak aliases (usually called "weak externals"
in PE) are created when an alternate name is specified. When a weak symbol is linked and the symbol
is not defined, the weak symbol becomes an alias for the alternate symbol. If one equal sign is used,
the linker searches for defined symbols within other objects and libraries. This is the usual mode,
historically called "lazy externals." Otherwise, when two equal signs are used, the linker searches for
defined symbols only within other objects.
Non-alias weak symbols are supported on PE as a GNU extension.
8.101. .word expressions
This directive expects zero or more expressions, of any section, separated by commas.
The size of the number emitted, and its byte order, depend on what target computer the assembly is
for.
Warning: Special Treatment to support Compilers
Machines with a 32-bit address space, but that do less than 32-bit addressing, require the following special treatment. If the machine of interest to you does 32-bit addressing (or doesn’t require it;
Chapter 9 Machine Dependent Features), you can ignore this issue.
In order to assemble compiler output into something that works, as occasionally does strange things
to .word directives. Directives of the form .word sym1-sym2 are often emitted by compilers as
part of jump tables. Therefore, when as assembles a directive of the form .word sym1-sym2, and
the difference between sym1 and sym2 does not fit in 16 bits, as creates a secondary jump table,
Chapter 8. Assembler Directives 69
immediately before the next label. This secondary jump table is preceded by a short-jump to the first
byte after the secondary table. This short-jump prevents the flow of control from accidentally falling
into the new table. Inside the table is a long-jump to sym2. The original .word contains sym1 minus
the address of the long-jump to sym2.
If there were several occurrences of .word sym1-sym2 before the secondary jump table, all of them
are adjusted. If there was a .word sym3-sym4, that also did not fit in sixteen bits, a long-jump to
sym4 is included in the secondary jump table, and the .word directives are adjusted to contain sym3
minus the address of the long-jump to sym4; and so on, for as many entries in the original jump table
as necessary.
8.102. Deprecated Directives
.abort
.line
One day these directives won’t work. They are included for compatibility with older assemblers.
70 Chapter 8. Assembler Directives
Chapter 9.
Machine Dependent Features
The machine instruction sets are (almost by definition) different on each machine where as runs.
Floating point representations vary as well, and as often supports a few additional directives or
command-line options for compatibility with other assemblers on a particular platform. Finally, some
versions of as support special pseudo-instructions for branch optimization.
This chapter discusses most of these differences, though it does not include details on any machine’s
instruction set. For details on that subject, see the hardware manufacturer’s manual.
72 Chapter 9. Machine Dependent Features
Chapter 10.
AMD 29K Dependent Features
10.1. Options
as has no additional command-line options for the AMD 29K family.
10.2. Syntax
10.2.1. Macros
The macro syntax used on the AMD 29K is like that described in the AMD 29K Family Macro
Assembler Specification. Normal as macros should still work.
10.2.2. Special Characters
; is the line comment character.
The character ? is permitted in identifiers (but may not begin an identifier).
10.2.3. Register Names
General-purpose registers are represented by predefined symbols of the form GRnnn (for global registers) or LRnnn (for local registers), where nnn represents a number between 0 and 127, written with
no leading zeros. The leading letters may be in either upper or lower case; for example, gr13 and LR7
are both valid register names.
You may also refer to general-purpose registers by specifying the register number as the result of an
expression (prefixed with %% to flag the expression as a register number):
%%expression
--where expression must be an absolute expression evaluating to a number between 0 and 255. The
range [0, 127] refers to global registers, and the range [128, 255] to local registers.
In addition, as understands the following protected special-purpose register names for the AMD 29K
family:
vab chd pc0
ops chc pc1
cps rbp pc2
cfg tmc mmu
cha tmr lru
These unprotected special-purpose register names are also recognized:
ipc alu fpe
ipa bp inte
74 Chapter 10. AMD 29K Dependent Features
ipb fc fps
q cr exop
10.3. Floating Point
The AMD 29K family uses ieee floating-point numbers.
10.4. AMD 29K Machine Directives
.block size , fill
This directive emits size bytes, each of value fill. Both size and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero.
In other versions of the gnu assembler, this directive is called .space.
.cputype
This directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.
.file
This directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.
Warning: in other versions of the gnu assembler, .file is used for the directive called .app-file in
the AMD 29K support.
.line
This directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.
.sect
This directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.
.use section name
Establishes the section and subsection for the following code; section name may be one of
.text, .data, .data1, or .lit. With one of the first three section name options, .use is
equivalent to the machine directive section name; the remaining case, .use .lit,is the same
as .data 200.
10.5. Opcodes
as implements all the standard AMD 29K opcodes. No additional pseudo-instructions are needed on
this family.
For information on the 29K machine instruction set, see [Am29000 User’s Manual], Advanced Micro
Devices, Inc.
Chapter 11.
Alpha Dependent Features
11.1. Notes
The documentation here is primarily for the ELF object format. as also supports the ECOFF and
EVAX formats, but features specific to these formats are not yet documented.
11.2. Options
-mcpu
This option specifies the target processor. If an attempt is made to assemble an instruction which
will not execute on the target processor, the assembler may either expand the instruction as a
macro or issue an error message. This option is equivalent to the .arch directive.
The following processor names are recognized: 21064, 21064a, 21066, 21068, 21164,
21164a, 21164pc, 21264, 21264a, 21264b, ev4, ev5, lca45, ev5, ev56, pca56, ev6,
ev67, ev68. The special name all may be used to allow the assembler to accept instructions
valid for any Alpha processor.
In order to support existing practice in OSF/1 with respect to .arch, and existing practice
within MILO (the Linux ARC bootloader), the numbered processor names (e.g. 21064) enable
the processor-specific PALcode instructions, while the "electro-vlasic" names (e.g. ev4) do not.
-mdebug
-no-mdebug
Enables or disables the generation of .mdebug encapsulation for stabs directives and procedure
descriptors. The default is to automatically enable .mdebug when the first stabs directive is seen.
-relax
This option forces all relocations to be put into the object file, instead of saving space and resolving some relocations at assembly time. Note that this option does not propagate all symbol
arithmetic into the object file, because not all symbol arithmetic can be represented. However,
the option can still be useful in specific applications.
-g
This option is used when the compiler generates debug information. When gcc is using
mips-tfile to generate debug information for ECOFF, local labels must be passed through to
the object file. Otherwise this option has no effect.
-Gsize
A local common symbol larger than size is placed in .bss, while smaller symbols are placed
in .sbss.
76 Chapter 11. Alpha Dependent Features
-F
-32addr
These options are ignored for backward compatibility.
11.3. Syntax
The assembler syntax closely follow the Alpha Reference Manual; assembler directives and general
syntax closely follow the OSF/1 and OpenVMS syntax, with a few differences for ELF.
11.3.1. Special Characters
# is the line comment character.
; can be used instead of a newline to separate statements.
11.3.2. Register Names
The 32 integer registers are referred to as $n or $rn. In addition, registers 15, 28, 29, and 30 may be
referred to by the symbols $fp, $at, $gp, and $sp respectively.
The 32 floating-point registers are referred to as $fn.
11.3.3. Relocations
Some of these relocations are available for ECOFF, but mostly only for ELF. They are modeled after
the relocation format introduced in Digital Unix 4.0, but there are additions.
The format is !tag or !tag!number where tag is the name of the relocation. In some cases number
is used to relate specific instructions.
The relocation is placed at the end of the instruction like so:
ldah $0,a($29) !gprelhigh
lda $0,a($0) !gprellow
ldq $1,b($29) !literal!100
ldl $2,0($1) !lituse_base!100
!literal
!literal!N
Used with an ldq instruction to load the address of a symbol from the GOT.
A sequence number N is optional, and if present is used to pair lituse relocations with this
literal relocation. The lituse relocations are used by the linker to optimize the code based
on the final location of the symbol.
Note that these optimizations are dependent on the data flow of the program. Therefore, if any
lituse is paired with a literal relocation, then all uses of the register set by the literal
instruction must also be marked with lituse relocations. This is because the original literal
instruction may be deleted or transformed into another instruction.
Also note that there may be a one-to-many relationship between literal and lituse, but not
a many-to-one. That is, if there are two code paths that load up the same address and feed the
value to a single use, then the use may not use a lituse relocation.
Chapter 11. Alpha Dependent Features 77
!lituse_base!N
Used with any memory format instruction (e.g. ldl) to indicate that the literal is used for an
address load. The offset field of the instruction must be zero. During relaxation, the code may be
altered to use a gp-relative load.
!lituse_jsr!N
Used with a register branch format instruction (e.g. jsr) to indicate that the literal is used for a
call. During relaxation, the code may be altered to use a direct branch (e.g. bsr).
!lituse_bytoff!N
Used with a byte mask instruction (e.g. extbl) to indicate that only the low 3 bits of the address
are relevant. During relaxation, the code may be altered to use an immediate instead of a register
shift.
!lituse_addr!N
Used with any other instruction to indicate that the original address is in fact used, andthe original
ldq instruction may not be altered or deleted. This is useful in conjunction with lituse_jsr to
test whether a weak symbol is defined.
ldq $27,foo($29) !literal!1
beq $27,is_undef !lituse_addr!1
jsr $26,($27),foo !lituse_jsr!1
!lituse_tlsgd!N
Used with a register branch format instruction to indicate that the literal is the call to
__tls_get_addr used to compute the address of the thread-local storage variable whose
descriptor was loaded with !tlsgd!N.
!lituse_tlsldm!N
Used with a register branch format instruction to indicate that the literal is the call to
__tls_get_addr used to compute the address of the base of the thread-local storage block for
the current module. The descriptor for the module must have been loaded with !tlsldm!N.
!gpdisp!N
Used with ldah and lda to load the GP from the current address, a-la the ldgp macro. The
source register for the ldah instruction must contain the address of the ldah instruction. There
must be exactly one lda instruction paired with the ldah instruction, though it may appear
anywhere in the instruction stream. The immediate operands must be zero.
bsr $26,foo
ldah $29,0($26) !gpdisp!1
lda $29,0($29) !gpdisp!1
!gprelhigh
Used with an ldah instruction to add the high 16 bits of a 32-bit displacement from the GP.
!gprellow
Used with any memory format instruction to add the low 16 bits of a 32-bit displacement from
the GP.
!gprel
Used with any memory format instruction to add a 16-bit displacement from the GP.
78 Chapter 11. Alpha Dependent Features
!samegp
Used with any branch format instruction to skip the GP load at the target address. The referenced
symbol must have the same GP as the source object file, and it must be declared to either not use
$27 or perform a standard GP load in the first two instructions via the .prologue directive.
!tlsgd
!tlsgd!N
Used with an lda instruction to load the address of a TLS descriptor for a symbol in the GOT.
The sequence number N is optional, and if present it used to pair the descriptor load with both
the literal loading the address of the __tls_get_addr function and the lituse_tlsgd
marking the call to that function.
For proper relaxation, both the tlsgd, literal and lituse relocations must be in the same
extended basic block. That is, the relocation with the lowest address must be executed first at
runtime.
!tlsldm
!tlsldm!N
Used with an lda instruction to load the address of a TLS descriptor for the current module in
the GOT.
Similar in other respects to tlsgd.
!gotdtprel
Used with an ldq instruction to load the offset of the TLS symbol within its module’s threadlocal storage block. Also known as the dynamic thread pointer offset or dtp-relative offset.
!dtprelhi
!dtprello
!dtprel
Like gprel relocations except they compute dtp-relative offsets.
!gottprel
Used with an ldq instruction to load the offset of the TLS symbol from the thread pointer. Also
known as the tp-relative offset.
!tprelhi
!tprello
!tprel
Like gprel relocations except they compute tp-relative offsets.
11.4. Floating Point
The Alpha family uses both ieee and VAX floating-point numbers.
11.5. Alpha Assembler Directives
as for the Alpha supports many additional directives for compatibility with the native assembler. This
section describes them only briefly.
These are the additional directives in as for the Alpha:
Chapter 11. Alpha Dependent Features 79
.arch cpu
Specifies the target processor. This is equivalent to the -mcpu command-line option. Options, for
a list of values for cpu.
.ent function[, n]
Mark the beginning of function. An optional number may follow for compatibility with the
OSF/1 assembler, but is ignored. When generating .mdebug information, this will create a procedure descriptor for the function. In ELF, it will mark the symbol as a function a-la the generic
.type directive.
.end function
Mark the end of function. In ELF, it will set the size of the symbol a-la the generic .size
directive.
.mask mask, offset
Indicate which of the integer registers are saved in the current function’s stack frame. mask is
interpreted a bit mask in which bit n set indicates that register n is saved. The registers are saved
in a block located offset bytes from the canonical frame address (CFA) which is the value of
the stack pointer on entry to the function. The registers are saved sequentially, except that the
return address register (normally $26) is saved first.
This and the other directives that describe the stack frame are currently only used when generating .mdebug information. They may in the future be used to generate DWARF2 .debug_frame
unwind information for hand written assembly.
.fmask mask, offset
Indicate which of the floating-point registers are saved in the current stack frame. The mask and
offset parameters are interpreted as with .mask.
.frame framereg, frameoffset, retreg[, argoffset]
Describes the shape of the stack frame. The frame pointer in use is framereg; normally this is
either $fp or $sp. The frame pointer is frameoffset bytes below the CFA. The return address
is initially located in retreg until it is saved as indicated in .mask. For compatibility with
OSF/1 an optional argoffset parameter is accepted and ignored. It is believed to indicate the
offset from the CFA to the saved argument registers.
.prologue n
Indicate that the stack frame is set up and all registers have been spilled. Theargument n indicates
whether and how the function uses the incoming procedure vector (the address of the called
function) in $27. 0 indicates that $27 is not used; 1 indicates that the first two instructions of the
function use $27 to perform a load of the GP register; 2 indicates that $27 is used in some nonstandard way and so the linker cannot elide the load of the procedure vector during relaxation.
.usepv function, which
Used to indicate the use of the $27 register, similar to .prologue, but without the other semantics of needing to be inside an open .ent/.end block.
The which argument should be either no, indicating that $27 is not used, or std, indicating that
the first two instructions of the function perform a GP load.
One might use this directive instead of .prologue if you are also using dwarf2 CFI directives.
80 Chapter 11. Alpha Dependent Features
.gprel32 expression
Computes the difference between the address in expression and the GP for the current object
file, and stores it in 4 bytes. In addition to being smaller than a full 8 byte address, this also does
not require a dynamic relocation when used in a shared library.
.t_floating expression
Stores expression as an ieee double precision value.
.s_floating expression
Stores expression as an ieee single precision value.
.f_floating expression
Stores expression as a VAX F format value.
.g_floating expression
Stores expression as a VAX G format value.
.d_floating expression
Stores expression as a VAX D format value.
.set feature
Enables or disables various assembler features. Using the positive name of the feature enables
while using nofeature disables.
at
Indicates that macro expansions may clobber the assembler temporary ($at or $28) register. Some macros may not be expanded without this and will generate an error message if
noat is in effect. When at is in effect, a warning will be generated if $at is used by the
programmer.
macro
Enables the expansion of macro instructions. Note that variants of real instructions, such as
br label vs br $31,label are considered alternate forms and not macros.
move
reorder
volatile
These control whether and how the assembler may re-order instructions. Accepted for compatibility with the OSF/1 assembler, but as does not do instruction scheduling, so these
features are ignored.
The following directives are recognized for compatibility with the OSF/1 assembler but are ignored.
.proc .aproc
.reguse .livereg
.option .aent
.ugen .eflag
.alias .noalias
Chapter 11. Alpha Dependent Features 81
11.6. Opcodes
For detailed information on the Alpha machine instruction set, see the
ftp://ftp.digital.com/pub/Digital/info/semiconductor/literature/alphaahb.pdfAlpha Architecture
Handbook.
82 Chapter 11. Alpha Dependent Features
Chapter 12.
ARC Dependent Features
12.1. Options
-marc[5|6|7|8]
This option selects the core processor variant. Using -marc is the same as -marc6, which is also
the default.
arc5
Base instruction set.
arc6
Jump-and-link (jl) instruction. No requirement of an instruction between setting flags and
conditional jump. For example:
mov.f r0,r1
beq foo
arc7
Break (brk) and sleep (sleep) instructions.
arc8
Software interrupt (swi) instruction.
Note: the .option directive can to be used to select a core variant from within assembly code.
-EB
This option specifies that the output generated by the assembler should be marked as being
encoded for a big-endian processor.
-EL
This option specifies that the output generated by the assembler should be marked as being
encoded for a little-endian processor - this is the default.
12.2. Syntax
12.2.1. Special Characters
*TODO*
84 Chapter 12. ARC Dependent Features
12.2.2. Register Names
*TODO*
12.3. Floating Point
The ARC core does not currently have hardware floating point support. Software floating point support
is provided by GCC and uses ieee floating-point numbers.
12.4. ARC Machine Directives
The ARC version of as supports the following additional machine directives:
.2byte expressions
*TODO*
.3byte expressions
*TODO*
.4byte expressions
*TODO*
.extAuxRegister name,address,mode
*TODO*
.extAuxRegister mulhi,0x12,w
.extCondCode suffix,value
*TODO*
.extCondCode is_busy,0x14
.extCoreRegister name,regnum,mode,shortcut
*TODO*
.extCoreRegister mlo,57,r,can_shortcut
.extInstruction name,opcode,subopcode,suffixclass,syntaxclass
*TODO*
.extInstruction mul64,0x14,0x0,SUFFIX_COND,SYNTAX_3OP|OP1_MUST_BE_IMM
.half expressions
*TODO*
.long expressions
*TODO*
Chapter 12. ARC Dependent Features 85
.option arc|arc5|arc6|arc7|arc8
The .option directive must be followed by the desired core version. Again arc is an alias for
arc6.
Note: the .option directive overrides the command line option -marc; a warning is emitted
when the version is not consistent between the two - even for the implicit default core version
(arc6).
.short expressions
*TODO*
.word expressions
*TODO*
12.5. Opcodes
For information on the ARC instruction set, see [ARC Programmers Reference Manual], ARC Cores
Ltd.
86 Chapter 12. ARC Dependent Features
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