6800 ROM MONITOR
VERSON 1.0
USERS GUIDE
Southwest Technical Products Corporation
219 W. Rhapsody San Antonio, Texas 78116
Copyright 1977, Southwest Technical Products Corporation
Scanned and edited by Michael Holley Sept. 17, 2000 Revised Aug. 6, 2002 Southwest Technical Products Corporation Document Circa 1977
SWTPC SWTBUG® (SWATBUG) MONITOR ROM
One of the features of the SWTPC 6800 Computer System is that the conventional programmer’s console has been replaced with a monitor ROM. The programmer’s console consists of all the pretty switches and lights often found on similar microcomputers that are used to bootstrap the system after power up. The programmer’s console not only raises the cost of the system, but more often than not is confusing and tedious to use for both be-ginning and experienced programmers. The monitor ROM on the other hand is a permanently stored program that gives the computer the intelligence required to communicate with the operator thru an interfaced terminal system immediately after power up without flipping switches for 10 minutes. This technique makes the computer do the work of simplifying communication between itself and the operator.
SWTBUG® is the name of the monitor program used in the SWTPC 6800 Computer System. It might be thought of as kind of a mini-operating system since it gives the operator command control over the computer system.
Features of the SWTBUG® ROM include:
Memory Examine and Change
Program loading from cassette or paper tape thru the control interface or thru I/O port # 0.
Program saving to cassette or paper tape
Go to user program
Display contents of registers
Erase SWTPC CT-1024 terminal system screen
SWTPC MF-68 floppy disk boot
Byte search
Breakpoint debugging
Vectored hardware and software interrupts to user defined addresses
SWTBUG® is a permanently stored program and cannot be erased or lost by either a loss of power or user program error. It is always resident in the computer while power is ON and need never be loaded into the machine. Subroutines within the ROM are documented and available to the user to simplify programming and conserve on the use of user RAM memory. Character input and output, string output and return to monitor are just a few subroutines available to the user.
SWTBUG® is a 1K byte program and is addressed high in memory, far above the amount of RAM memory required for most user programs. Since SWTBUG® does require a small amount of RAM memory for operation, a 128 byte scratchpad RAM has been implemented on the processor board so that no external user RAM memory is required for monitor operation. There is even enough extra room in this RAM so that short programs such as memory diagnostics can be loaded into and run from the scratchpad RAM without requiring any external user RAM memory. Extra care however must be exercised to avoid overstoring any memory locations required for proper monitor operation. Complete de-tails on this are given later in this writeup. The SWTBUG® ROM is located from memory addresses E000 thru E3FF. The scratchpad RAM is located from memory addresses A000 thru A07F. Both components are physically located on the processor board and are functional any time the system is powered up. Whenever computer control is transferred from SWTBUG® to the user program, there are only four ways to get back into the SWTBUG® command mode. The first is to put a jump to the CONTRL entry point address within SWTBUG® as the last step of your program. The second is to incorporate a command or action within your program which transfers program control to the CONTRL entry point address within SWTBUG® The third is to depress the front panel RESET button. The fourth is turn the computer OFF and then back ON again. The fourth method is rather drastic and wipes out all RAM memory data, it is only mentioned to let you know that the computer always powers up with the SWTBUG® monitor in the command mode.
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SWTBUG® INSTALLATION
SWTBUG® is a MOS device and MOS integrated circuits are susceptible to damage by static electricity. Although some degree of protection is provided internally within the integrated circuits, their cost demands the utmost in care. Before opening and/or installing SWTBUG® you should ground your body and all metallic tools coming into contact with the leads, thru a 1M ohm ¼ watt resistor. The ground must be an “earth” ground such as a water pipe, and not the circuit board ground. As for connection to your body, attach a clip lead to your watch or metal ID bracelet. Make absolutely sure that you have the resistor connected between you and the “earth” ground, otherwise you will be creating a dangerous shock hazard. Avoid touching the leads of the integrated circuits any more than necessary when installing it, even if you are grounded. Static electricity should be an important consideration in cold, dry environments. It is less of a problem when it is warm and humid.
When using SWTBUG® with an MP-A processor card, one board change is necessary since SWTBUG® is a full 1 K ROM and MIKBUG® was a 1/2K device, If this ROM is replacing MlKBUG, remove MIKBUG® from its socket. On the back side of the MP-A board you will notice that pin 15 of IC-2 (ROM) is grounded. The land coming from pin 15 should be broken and a wire added as shown below.
SWTBUG® should now be installed in the socket for IC-2. Be sure to orient the ROM correctly when re-installing. The semicircle notch or dot should match with the MP-A board’s component layout drawing. When installing SWTBUG® in the MP-A2 processor board no board modifications are necessary. Follow the instructions supplied with the MP-A2 instruction set.
SWTBUG® OPERATION
The SWTBUG® firmware enables the computer to communicate with a terminal to perform various programming and debugging functions. SWTBUG® will communicate with a terminal via either a MP-C control interface or MP-S ACIA serial interface on I/O port 1. An optional MP-C interface can be installed on I/O port 0 for punch and load functions. Although SWTBUG® is essentially compatible with MIKBUG, be sure to read the COMPATIBILITY section before running any programs written for MIKBUG. Below is a detailed description of each SWTBUG® command.
RESET
Upon receiving a RESET command, as during power up, SWTBUG® will initialize the system to receive commands from a terminal. When the RESET button is pushed, control will transfer to location E0D0 of SWTBUG®. The RESET button should be used for exiting loops or malfunctioning programs. After resetting, the computer should respond with a carriage return, line feed and a $ sign. At this point, SWTBUG® is waiting for commands. If breakpoints are being used, the RESET function will not disable breakpoints. The BREAKPOINT function should be referenced for additional information.
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MEMORY EXAMINE AND CHANGE M (addr)
The Memory Examine and Change function can be used to enter machine code programs and to display and/or change the contents of memory. The Memory Examine and Change function should be used as follows:
1.) Type M. The computer should echo the M and output a space.
2.) Type in the four digit hexadecimal address that you wish to examine and/or change. The computer should respond with a carriage return, line feed, $, the address and the data that is stored at this address.
3.) At this point the user has the option of advancing, either forward or backward, to the next memory location, or changing the data stored at the displayed address and advancing to the next location or of exiting the M function.
a.) To display the next sequential address and data, a line feed or any character other than 0123456789ABCDEF:;^=>? or a space or a carriage return may be entered. Any leading spaces that are entered will be ignored by the memory change function.
b.) To display the next sequential address going backward from the present location, a ^ should be entered.
c.) To change the data stored at the displayed location, enter the new data, either with or without a leading space. If a non-hex value, such as a 3Q is entered the data will remain unchanged and the memory change function will be exited. If the data is unable to be changed (write protected memory, etc.) a ? will be output and the memory change function will be exited.
d.) To exit the Memory Examine and Change function, type a carriage return.
Below is an example. The underlined parts are what was entered by the user.
$M 0100 |
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MEMORY LOCATION 0100 OPENED |
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$0100 00 |
. |
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DISPLAY |
NEXT |
LOCATION |
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$0101 BD |
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. |
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DISPLAY |
NEXT |
LOCATION |
– SPACE IGNORED |
$0102 5D |
. |
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DISPLAY |
NEXT |
LOCATION |
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$0103 C1 |
01 |
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CHANGE CONTENTS TO 01 |
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$0104 C9 |
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23 CHANGE CONTENTS TO 23. SPACES IGNORED |
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$0105 15 |
^ |
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READ PREVIOUS LOCATION |
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$0104 23 |
. |
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DISPLAY |
NEXT |
LOCATION |
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$0105 15 |
3Q |
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ENTER NON-HEX VALUE |
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$M 0105 |
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SWTBUG CONTROL RESUMED. OPEN NEW LOCATION |
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$0105 15 |
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EXIT BY |
HITTING CARRIAGE RETURN |
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$M E000 |
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OPEN ANOTHER |
LOCATION |
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$E000 FE |
56? |
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ATTEMPTED TO |
CHANGE A |
WRITE PROTECTED MEMORY |
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$ |
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SWTBUG CONTROL RESUMED |
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REGISTER DUMP FUNCTION R
The R command will display the current contents of the MPU's pushdown stack. “Current” means the status of data stored on the stack immediately before SWTBUG® control is resumed. The following is a typical register dump:
$R |
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$FF |
16 |
23 |
17EC |
0103 |
A042 |
$ |
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CONDITION |
ACC |
ACC |
INDEX |
PGM. |
STACK |
CODES |
B |
A |
REG |
CTR. |
PTR |
3
The condition codes are as defined below:
BIT NO. |
LABEL |
CONDITION CODE |
0 |
C |
Carry-borrow |
1 |
V |
Overflow |
2 |
Z |
Zero |
3 |
N |
Negative |
4 |
I |
Interrupt mask |
5 |
H |
Half carry |
In the above example the condition code of "3C" can be interpreted as follows:
3C16 = |
0 |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
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BIT 7 |
BIT 6 |
BIT 5 |
BIT 4 |
BIT 3 |
BIT 2 |
BIT 1 |
BIT 0 |
Below are two examples of how the R command works. Assume that this small program was entered to change certain registers.
$M 0100 |
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$0100 CE |
8E |
LOADS STACK POINTER TO 100 |
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$0101 12 |
10 |
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$0102 34 |
00 |
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$0103 86 |
CE |
LOAD INDEX REGISTER WITH 1234 |
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$0104 00 |
12 |
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$0105 C6 |
34 |
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$0106 FF |
86 |
LOAD ACCUMULATOR A WITH 00 |
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$0107 FD |
00 |
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$0108 08 |
C6 |
LOAD ACCUMULATOR B WITH FF |
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$0109 23 |
FF |
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$010A |
67 |
7E |
JUMP BACK TO SWTBUG CONTROL |
$010B |
F7 |
E0 |
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$010C |
60 |
E3 |
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$010D |
DD |
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AT THIS POINT THE STATUS WILL NOT BE PUSHED ON THE STACK
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$R |
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$FF DC FC |
6EFD 0100 A042 |
REGISTER DUMP BEFORE RUNNING PROGRAM |
$G |
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$R |
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$FF DC FC |
6EFD 0100 A042 |
NOTE R DUMP THE SAME AFTER RUNNING |
$J 0100 |
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$R |
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$FF DC FC |
6EFD 0100 A042 |
THE SAME AFTER A JUMP |
$M 010A |
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AT THIS POINT THE JUMP TO SWTBUG CONTROL |
$010A 7E |
3F |
IS REPLACED BY A SWI. NOTE THE VALUE |
$010B 50 |
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OF THE REGISTERS AFTER THE NEXT DUMP. |
$G |
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$F9 FF 00 |
1234 010A 0FF9 |
REGISTER DUMP GIVEN BY SWI |
$R |
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$F9 FF 00 |
1234 010A 0FF9 |
DUMP SHOWS PROGRAM CHANGES |
$ |
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$FF DC FC |
6EFD 0100 A042 |
R DUMP AFTER RESET |
In the above program you will notice that the register dump after running the program is the same as before even though the program contained statements that changed the processor’s registers. The register dump did not reflect these changes because the new conditions were not pushed on the computer’s stack. Note, however, the register dump did reflect the change when the last instruction was a software interrupt -- a SWI instruction will push the processor’s status on the stack and then display the contents of the registers.
CT-1024 CLEAR SCREEN COMMAND C
The C command outputs a home-up (1016) and an erase to end of frame (1616) control characters for the clearing of the screen on a SWTPC CT-1024 or equivalent terminal system.
GO TO USER’S PROGRAM FUNCTION G
Upon entering a G command, SWTBUG® will transfer control to the user’s program by executing a RTI (return from interrupt) instruction. This effectively causes the computer to jump to the memory address stored in memory locations A048 and A049 in the SWTBUG® RAM. A048 contains the most significant byte and A049 the least significant byte of the memory address. If, for example, you wish to execute a program starting at 0100, change A048 to a 01 using the M function and change A049 to 00. Typing a G will then cause the computer to execute the program whose starting address is 0100. Upon entering a program using the G function, the stack pointer will be set at A049. The G function is also used to restart a program after a breakpoint has been moved. In this case A048 and A049 should not be changed. See the Breakpoint section for further information.
JUMP TO USER’S PROGRAM J(addr)
The J command will cause the computer to execute the program whose starting address is given in the entered address. The J command does not look at locations A048 and A049. The stack pointer will be set at A042 upon entering a program with a jump command. Example: J 0100. If a non-hex character is entered, SWTBUG® control will resume.
ASCII TAPE PUNCH COMMAND P
The P command provides a means for storing the contents of specified memory on either cassette or paper tape. Normal output from the computer during a punch is thru either an MP-C or MP-S serial interface on I/O port 1, but a MP-C interface on I/O 0 can be selected. (See the description of the O command.) To use the P command, the upper and lower limits of the range to be punched must first be stored in locations A002-A005 of the SWTBUG® RAM. If you wanted to punch from addresses 0123 to 4567 (inclusive) you would use the memory examine and change functions to set computer memory as follows.
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