Commodore 2001-8, PET 2001-8 User Manual

•

PET

PERSO

PERSONAL COMPUTER

OMPUTER

PERSONAL COMPUTER

PET 2001-8

PERSONAL COMPUTER

USER

MANUAL

OCTOBER 1978

The information in

no responsibility is assumed for inaccuracies. The material

this

manual has been reviewed and is believed tobeentirely reliable. However,

is

purposes only and

subjecttochange without notice.

first edition

"All

rights reserved"

in

this

manual is for information

Commodore Business Machines

901

California Avenue

Palo Alto, California 94303

TA61£OPCONTENTS

Chapter

Chapter4.

Chapter

1.

2.

3.

5.

6.

WelcometoyourPETcomputer

UnpackingyourPETandturning

Basickeyboardinput PET

keyboard

Screen

BeginningBASIC .

The PRINT statement Variables

Direct and program Literals Functions

Elementaryprogramming ' . Unconditional and Dataentry

Advancedprogrammingtechniques String variables and Subroutines

FORNEXT

Subscrptedvariables

editor

statements

conditional

functions

loops

.........................•....

iton

,

............•.

looping

1

.

3

11

19

•

I

32

38

Chapter 7.

ChapterS.

Chapter

9.

PETcommunicationwiththeoutsideworld PET

interfacesand Commands and operations for peripheral devices IEEE·488bus

Machinelanguageprogramming .

Allocationofmemory Commands from BASIC Machinelanguage

Errorsanddiagnostlcs. Debug techniques BAStCerrormessages

OS

errormessages

lines

.. .. .. ......... .. .. ..

monitor

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

ii

......

57

91

LIST OF FIGURES

2.1

2.2

2.3

2.4

2.5

2.6

3.1

6.1

6.2

7.1

Rear

viewofPET

PET

memory

Memory map

ASCII code

ASCII 6 bit code .

PET

graphic character codes .

PET

keyboard scan lines .

Functions expressed

Principal pointers into

Simplified

viewofPET

2001

bus

by

functional blocks .

in

main memory " .

in

terms of built-in BASIC functions .

PET

RAM

.

3

.

4

6

7

8

12

43

.

55

.

57

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

7.10

7.11

Edge

connectorsJ1and

PET

IEEE connector pinout .

Receptacles

IEEE

standard

Parallel user

6522 VIA addresses

Parallel user

Connector

Second cassette interface

PET

second cassette edge connector

for

the

connectors .

port

information .

port

example .

J3

contact identification .

J2

IEEE

interface .

in

PET

port

J3

.

57

58

59

.

61

62

.

62

.

63

7.12

Edge

connector J4 .

iii

63

LIST OF FIGURES(continued)

7.13

7.14

7.15

7.16

7.17

7.18

7.19

7.20

7.21

7.22

Memory expansion connector .

Multiple file structure .

for

write

from

OPEN

Status

Default parameters .

Examplesofdefault parameters .

IEEE

Transfer

Byte

for

readtoPET

word

errors .

bus

contact identification .

bus

handshake sequence .

transfer

from

PET

talkertolistener .

63

69

73

74

.

80

82

83

84

85

7.23

7.24

7.25

7.26

8.1

8.2

by

IEEE

bus

Signals described

Status codes for IEEE

IEEE-488registeraddresses

Code assignments for command mode operation. . . . . . . . . . . . . . 89

Example Floating Point

Machine Language Monitor Listing 100

bus..

Numbers.

groups

. . . . .

.. ..

. . .

.. ..

in

PET..........................

. . . . . . . . . . . . . . . . . . . . . . . . . . 94

. . . . . . .

.. .. ..

87

.

..

88

iv

LIST OF APPENDICES

A.

B.

c.

D.

E.

F.

G.

Detailed PET memory

BASIC

Expressions and operators

Space and speed hints

Main logic board assembly

Suggested reading

statements

commands

map

v

Chapter

1.

WELCOMETOYOUR PET

COMPUTER

Congratulations and welcometothe exciting new worldofpersonal computers.Byselecting

the

PET

2001

you have

is now available for learning the

procedures

outlinedinthis

eliminated

manual,

the

problemsofgetting

functions

you

and

should

a personal

capabilitiesofyour

be abletoachieve

computer

system running. Your

PET.Infact,ifyou

initial

operationofyour Pet

follow

a few simple

a short period after unpacking the shipping container. The potentials

arise

that

this book has

to

Write

us at Commodore

with a newsletter we'll

are virtually limitless. This book,byits

not

coveredoreven anticipated.

with

your questions. We will answer many

be

sending out from timetotimetousers.

very nature,islimited.

that

you and other users

Questions

of

your

PET

Commodore Systems Divisions: 901

California Avenue

Palo Alto, California 94304

USA

360

Euston Road London NWI 381 England

3370

Pharmacy Avenue Agincourt Ofjario,

M1W 2k4

Canada

2001

will

time

within

will

pose

PET

is a Personal Electronic Transactor. Everything is complete in one steel cabinet. It contains a

board, keyboard, computer board, and a Commodore supplied cassette. There is a built-in black and

that

white television monitor, which displays characters in a format by

twenty-five lines.

of

your

PET

At the heart

2001isan

MCS 6502 microprocessor. This microprocessor

appeartoyoutobe forty characters

totally

controls operationofthe screen, keyboard, cassettes, and additional peripherals which can be addedtothe The product is so construed

be

cannot

destroyed because the computer software,oroperating instructions are contained in a fixed memory. (Called Read-Only·Memory) This allows both the first use the

In

PET

with impunity.

order to satisfy the needsofthe serious user as well as the

have used three formats in

Summary discussionsinthis type font are designed to answer the questions

programmer.

When

you are first using your

more detailed explanations which are in the type font

PET

a bit, the italicized summary sections will be useful when you want

that

you cannot damage the

this

manual.

PET

from the keyboard. The operating system

time

user and the sophisticated user

first

time userofa computer product, we

manual, read these sections

of

the preceding paragraph. Afteryou have used the

lightly

to

of

a professional

andspend time on the

review

howaparticular

instruction works.

The third typeofformat gives a detailed descriptionofhow

PET

sections are for people who use the sections

difficulttofollow

more familiar

with

the

and we recommend helshe use them

PET

operating systel\1.The languagewhich you

at the machine level. The

the

PET

Implements a section. These

first

time

reader may

only

on re

...

eading the material when

will

usetocommunicate

find

these

with

PETiscalled BASIC, an acronym for Beginners All-purpose Symbolic Instruction Code.

CRT

PET.

to

your

1

It shouldbenoted that there are of

usage in mind, which are availabletothe public. the appendix. Someofthese manuals maybe more' usefultosatisfy in

this

particular manual.

The ultimate teaching device for learning how the

PET

ask you to use the case,

this

documentation is

of

examples we

invite comments on

to

resolve any unclear statement.

Lastly, since

your own, any questions which

this

programming aspects

along with the text, rather than Just continuously reading whattodo.Inany

this

manual cannot presumetoprovide all technical informationofhardware or

of

the MCS 6502 microprocessor, we direct you to two other Commodore

other"'EY~~enCEt;

'1Ji:"teri~lwritten

We

have included a

PET

worksisthe

aRput BASIC, with different

listofsuch suggested reading

aspecffic

PET

need whichisnot covered

itself.Insome cases, we will

tYPlilS

in

sufficienttoallow youtoget started onanexample.Byconstructing similar

might

arise are answered by the machine itself. Of course,

material and particularly on examples which you asanindividual have used

publications: There is a manual available from your dealer called Hardware Manual which also includes descriptions

the auxilliary devices which generate the control signals necessarytoallow the

PET

to

of operate. Detailed specificationsofthe computer and the language in which it operates are availableina book called Programming Manual. This

is

available for purchase from your

PET

dealeroreither can

be

purchased from Commodore directly.

NOTES

2

Chapter

2.

UNPACKtNGYOUA

PET

AN'lfTURNINGITON

Please check the carton for any special unpacking instnl-cti9ns and carefully examine your

is

concealed damage. If anything

amiss, reportly this immediatelytoboth the placeofpurchase and the

shipping agent.

PET

from

its

Remove your surface, then plug

it

into

protective shipping carton and placeiton the counter, desk,orother suitable

any standard, grounded electrical

Tv

BRIGHTNESS

ADJUSTMENT

outlet.

(In

some countries no plug is provided.)

PET

for any

MEMORY EXPANSION

J4 CONNECTOR

Figure

2.1.

Rear

fuse, line cord and interfacing connectors

The power closing position,

Immediately, when the power time-out

switchislocated in the

it

to the right turnsitoff.

oranON/OFF label.)

circuit

in a special condition (reset condition) which initializes them into a known state. If the

left

rearofthe

(Thereisa white

switchisturned on, power is suppliedtothe internal circuits. There is a

screen has had power immediately prior characters which reflect the current contents

to

The screen memory transfer

the screen is done

,3:~~,~,

INTERFACE

view of

to

PET

PET. dot

this

time, you will see on

of

the computer memory which is controlling the screen.

microprocessor, and so, even when the computer is

of

current contents

the screen memory.

1.6A

FUSE -.,..:=/

ISlO

.d

I

PARAllEL

USER PORT J2

2001·8 showing switch,

SERIAL NUMBER

ELECTRICAL SPEC.

'""

Closing the switchtothe

on the power

with

circuitry outside controlofthe main

not

operational,

switchtoindicateitis

BW)

3-WIRE

POWER

CoRO

left

the

screen a varietyofstrange

the

screen always displays the

AC

turns the

PET

on and

in the power-on

3

At the endofthe

, . .,

.....

temporarily, and then displa)'s on the

power~n

,."j...

, , • .

cycje,

the

.•

, , "':.16,,"

computer initializes the internal membrv, biank's the

sc.r13~n

",'

',.-,','

a m13ssage.llke the followir:tg:

.-'

s'c~reen

,

:'.

u*COMMODOREBASIC***

7167

BYTES

FREE

READY,

o

The 7167 referstoavailable users'programmable memory. A byteIsthe fundamental data elementofthe PET

computer and corresponds roughlytoone letter or

theory·

In

"S192

bytes".

But a few hundred are used by the

net available bytes.

If

you failtoget the power·updisplay the

To get the display, four different types

first

of

memory

time,

(Input/Output), and Screen Memory. The relationship between these memories Is shown In figure

microprocessor

digitofInformation.TheSKmodel should show-

PET

internally. The balance shown "7167" is

try

turning the power switch slowly

are

used:

ROM,

User ReadlWrite,

off,

then back on.

1/0

2.2.

6502

r-

ROM

14k

RAM

Bk

address and

TV RAM

1k

data-bus

/

keyboard

110

2k

\ I

,

/

IEEE·4BB

I

[\

\

II

user port

Figure 2.2.

PET

memory bus

4

ROM

(READ ONLY MEMORY)

ROM

causes the

programs writtenbyCommodore which scan the keyboard, print the display, control inputfoutput, count the real the operation

keyboard,

powered on, but also

1/0

The second typeofmemory is devices called PIA* and

computer. Except when special

interfere in any way with these areas. The operating system automatically handles these locations in

order

time

lowest cost memory for storing

of or

MEMORY

to

perform legitimate Input/Output operations.

PETtoperform

clock, and execute commands that the user types in.

his

machine. This is because the operating system memory Is indestructible from the

from the user's program. Notonly is the machine availabletorun basic from the moment it is

the

user program cannot damage

VIA**

mostofits

this

that

whichisdevotedtoInput/outputoperations.This memory contains

which

allow

110

operations are desired, the user should not allow his program

operations.Ineach

data, but also give the user the

the

PETtoindividually control the

PET,

Read

basic operating system.

14K

of

ROM

contains a series

Only memories are not only

most

protection and

bits

that manipulate the

the

of

fastest

110

to

USER READWRITE

The third typeofmemoryisthe User Program Memory Space. throughout

$1FFF. A detailed map

RAM,

by

the

for

its

The message "7167 BYTES FREE" is a result

locxation

performing a check on whether or not

If the number was less than

greater than

upto32KofRAM

furnished with the

are

loaded and BASIC holds allofthe program variables.

Later on, we

program overlays.

this

110,

and Screen Memory are located from a programming standpoint.Asyou can see

map,

the

various tasks, including

1024

7167,

will

MEMORY·

book.) In a standard 8K

of

first

1024 bytesofmemory are reserved for the operating systemtouse

and cycles through the memorytodetermine which locations are available, thereby,

you probably have added your own memory. BASIC can automatically check as long as the added memory is continuoustothe memory

PET.

This memoryisreally the working memory in the machine;itis where programs

discuss some techniques to expand

R.A.M.(RANDOM ACCESS MEMORY)

all

PET,itis

the

memory is included in figure 2.3, showing where the

the

bufferingofdata from the cassettes and other

the

ReadlWrite Memory is working correctly.

7167,

you may have a hardware problem.11the number is

located from location

of

an

analysisofBASIC which starts at

this

memory by using tape files and

(We

will call

this

area

$0000tohexidecimal

that

RAM

ROM,

110

devices.

comes

SCREEN MEMORY

The screen memory is physically composedofthe

make up the PET's standard memory. It is constantly being used by the

which takes

ROM,

thus displaying characters on the screen.

As

mentioned during the power-up discussion,

no direct control over it.

*PIA - Peripheral Interface Adaptor

**VIA - Versatile Interface Adaptor

For information about these and related chips, see 6502 Hardware Manual.

the

individual bytesofmemory and uses themtoaddress a special character generator

same kindofchips that are used

CRT

this

process is

totally

automatic, and

5

to

control electronics,

the

programmer has

1024

8192

o

RAM

RAM User BASIC program

Operating system and BASIC working storage

-------------

User Variables

Expansion RAM area-24K

32768 33792 34816

35840

36864

49152

59392

61440

65536

RAM TV

1---------------

ImagesofTV RAM

Images

ImagesofTV RAM

Expansion ROM area·12K

ROM

1/0

ROM operating system

FIgure 2.3. PET memory map

of

TV RAM

BASIC

On

every

cycleofthe

($8000) in

corner

giving us 8

dots

toptobottom.

controller

on

whether or

character, etc. There are

screen by POKEing the

second character set. After you have played to try lower

To understand

the

screen

of

the screen. Each character in

an

8 row

each

timeitis

This givesan8

automatically

notitis

two

character sets stored in

this

featuretoseeifyour

case letters

this,

TV screen

memory

high

addressed. These

scanning

memory

for

the

let us review

and processes

characteronthe

bit

changes

address 59468 with a 14(a12turnsitback) which

graphic set

(1/60ofa second),

wide 8 bit the

addressingofthe

the top

PET

performs

thatisavailable in

how

characters are represented in

the

hardware

the

screen

the

memoryisaddressed

screen. The character ROM

dots

are

serially

tall

character with no spaces between characters. The CRT

lineofa character,

the

ROM. You can change

with

this

data

starting

fedtothe

character generating ROM, depending

the

screen a

way. The second character set

the

first

6

starts

into

second

with

the

least

at

the

upper left-hand

the

character generator

thatisused generates

screen, working from

lineofof

the

character set on

turnsitto

little

set.

the

bit, you

PET

may

and in

address

lefttoright

a

the

want

substitutes

the

memory.

eight

times,

and

CHARACTER

The standard ASCII code is used to represent charactersinthe

In

the PET,

screen.

REPRESENTATION IN PET

the

8th bit (bit 7) is used to signify BASIC command words or graphics characters forthe PET

MEMORY

main memory. (RAM)

~

1

I

;;"1~

tf\:Si::Ild

1:.lC1\:S1

tJ\::Jlt:l

~~'1

~1~"

e1e, e11e e111

1e~"

il:l'"

1"1"

11:111

11CSld

11'" 111" 1111

Exampleinthe

Aisrepresented

Shifted

A (a

"I

~I

41

PET:

spade)

~

"

NUL

::>OH

!,;IX

c.IX

1:.01

I;.N~

tH..':K

~c.L

lJ:;

HI

L'

VI

H

t..:H

:;0

:;1

is

~ ~

~

1

VLc.

1)(,;1

1 1

~

:;~

V("

VL3

.1)(,;4

NIlK :;YN c.llJ LIlN

c.~1

bUJ;l:

I;.~'.,;

,:;

fj~

kti

V:;

Figure 2.... ASCII ch8r8ct

0100 0001 11000001

~

$

:<

..

(

)

•

+

/ 't'

~

1

"

1 f\

;;

"

4 V I

~

b

'(

'"

~

1

~

(;

c.

,

(;

H X h

1

J

K

<.

~

L

'1

N

•

...

set (7 bit

code)

1 1 1

~

1

~

Q

k b r

:;

1 1

~

i,W

a

c s

d t

u

V f v

W

y

L k

"

J m

t n

..

"

..

i

0 z

1

0

1

p

..

u

..

x

"

The

screen

There are only64characters from the standard ASCII set that are normally printable.

memoryisorganized

with

a different

representatIon

from

the

main

PET

.

lJ

"I

~I

1

41

I

;;<:1~

i::It:H'JI:I

e~e,

i::Ildl\::J

ee11 e,

••

"'1~1

tl11\::1

~111

1,,~~

1,,~,

1",e

1~11

11ee

11~'

11W

1111

"

•

1

"

p

"

11

~

(;

V

c.

,

(;

H X

1

J

K L +

L

N

0

Q

k

:;

I $ 4

U

V W

Y

"

J

,.

~

1

"

~

..

~

:<

..

(

)

•

/

(j

1

,

e 1

;;

"

~

b

'{

~

"

<.

~

>

"t'

memory.

figure 2.5.

ASCII

64

ch.,.cler

.'1(6bll code)

7

These are the same characters that are directly available on the

is

The representation in screen memory 6;

giving us a six bit code for the keyboard characters.

or

The graphic, additional

shifted characters, setIsrepresented by a 1Inbit sixofthe screen memory, giving an

64

displayable characters.

derived from the standard ASCII set by dropping bit

PET

keyboard.

This gives the characters are organized so that they are

Ii

I

~,1i:::J

~

......

I:H:U::H

."1"

1l~11

• 1

....

111..,

1111

..

11111 G W

111

....

111111

1..,

..

11111 K

lUIl::J

11111 111

..

1111

following table for

01

~I

41

~ ~

~

A Q 1

Ii

(; j)

t. U

t V

H

I V

J

L M N 1

0

PET

displayable characters.Itshouldbenoted that allofthe graphics

~

1

p k "

S

1

X

"

L

,

J =

+-

justashift

~

1 1

~

"

•

"

..

<

)

*

+

•

/

from the normal keyboard character.

~

1

..

~

4

~

0

'(

II

Y

(

)

"

1 1 1

~

•

I

I 0

I

,

J

L 1

\

/

I

~

1

•

I I

"

r I

)<.

4-

1

•

+

I

..

"

1

~

•

..

I

•

~

I

•

,

-

1 1

1

,

...

T

~

I

-

.J

•

,

"

,

'.

,

FIgure 2.8.

Example: This gives us the following conversions:

In

Character

A

+

1

~

Note the reduction from seven of

A and1.In

field consists

a white

If memory representation. Both PRINT and direct Input from the keyboard result in automatic translation between the screen memory and the main memory.

dot

the operating systemisused, it automatically translates the values from ASCII Into the screen

screen memory, the 8th bitisusedtostore reverse field. The reverse of

taking the dot pattern from the character generator and reversing It, replacing

with black and a black dot with a white.

main memory

01000001 1100

0001 00110001 00110001 10110001 01110001

bit

ASCIItosix bit gives the effectofchanging the order

PET

graphic charaet...

In

screen memory

00000001

00000oo1

Nt

(7 bit

code)

8

USE OF THE SCREEN MEMORY,

There are three waystoget data the appropriate memory addreSs normal updating

As

longasthe

and the

PET

memory only during

the useofthe screen

of

the

screenistoo

PET

directly

are contending

times

memory

controls

when

r"

into

the screen memory. The

the

desireci'translatedc"fiara:cter. This is programmed only when

slow.

the screen, thereisno

for

accesstothe memory. The routines In

the

screen memoryisnot being used for display. This slows

downtoabout40percentofthe

apparent

speed

firstofthese;stoPOKE into

effect

obtainable

from the fact

the

PET

that

change the screen

with

a POKE. The POKE,

the screen

however, gives a visual effectofflashing dots, because the screen Is displaying the character

that

Is being passed from the shouldbedisplayed at it

runs the more flashing there will

that

PETtothe

particular

be.

screen memory, rather

position. When a program pokestothe screen,

than

the character

that

the

faster

The second waytoget data

onto

the screen is

the

keyboard. During a

time

when keyboard input enabled, the character being struck on the keyboard is automatically displayed on the screen. The third approach

is

by useofthe PRINT commandinBASIC. When

PRINT

"ABC"

..

is typedtoBASIC,itresults in the next line being printed as:

ABC

Thisisa printofa literal field in which all characters between the quotes are printed. The next position at which a character

by a flashing signal called a cursor. The cursor is a visual indication

will

be displayediftyped on the keyboard is indicated

to

the userofthe next

print position in screen memory.

What Is

1/60thofa second, an

computer the screen memory be

By moving of controlofthe

physically

(the PET) and steps a

showninalternating normal and reverse

the

keyboard and some software

happeninginthe

interrupttothe

pointer

pointer,

keyboard.

we can

machine Is

PETisgenerated. This generates a real-time

blinker

Is reversed in the 8th

print

counter. When

output

called

that

every

time

the screenisrecycled, about

clock

this

counter reads

bit.

This causes the reference character

field,

giving as visual

any place on the screen. Thisisdone by using a

the screen editor,

effectofblinking.

which

37,

the character referenced

manipulates

screen memory under

on the

to

combination

is

by

9

Chapter

3.

BASIC KEYBOARD UNIT

Whenever keyboardtothe

Keyboard key is struck. Only after a carriage return is program, and then a

There are is

the

the keyboard The

is

similartothatofa

However, some

the

blinking

screen memory.

data

;s transferred by the

two

exceptionstothis,

useofGET,

dataisaccessed

PET

keyboard

important

1.

Becauseofthe

number

2.

The

3.

The various keys

4.

The characters

longer require

convenient

5.

All standard characters are available by useofthe

pad

number

cursor

whole

which

has

has pad

to have them available

line

willbediscussed

been optimized for use as a

typewriter

changes have been made:

high

been added to has

for

which

shifting.

appears on

is transferred at once.

neither oneofwhich

directly

so a

useofnumbers and

allofthe

screen movement and

are normally

These characters are

unshifted,sothatacomplete

shift

keys. These

the

interrupt

using

touch

the

mathematical

without

screen,

routinetothe

the

keyboard

in a

later

machine

typist

rightofthe

the

does

calculations

editing

shiftofthe

shifting.

graphics

the

computer

screen memory each

data

causes

section,

language programs.

computer

not

feel

with

main keyboard.

operators in a form

are

numbers on a standard keyboard no

quite

often

give

transfers

transferredtothe operating

the

cursortoblink. Oneofthem

and

the

other one is when

keyboard,

totally

locatedonthe

used in BASIC, anditis

the

outofplace.

the

computer, a calculator-like

64-character

PET

significant

data

though

that

the

is normal for BASIC.

numeric pad.

graphics

from

the

time

a new

organization

set is

line

drawing

,

ability.

PET

KEYBOARD

The keyboard the

upper or are always used a

thin,

transparent left in placetoprotect peelitoff

There are 64 printed characters on the

same keys. The restofthe are obvious: be diplayed in reverse

The reverse key ;s operational

function (the

shifted

throughtothe

the

shiftofthe

The keyboardisscanned

routine using a being repeated 8

over,

Until

Is struck. The

from

the that

consistsof73 keys,

shifted

by uSing

like

is operational untilitis terminated by a RETURN pressed or printed, or by pressing reverse-off

reverse key). This

the

lefttoright

keyboard scan

keyisreleased,noother

later

characters

unless

character

function

other.

CRT

times

oneofthe

plastic

the

sticky

field·

keys, so

using

controller.

scan. The keyboardisorganizedona 2 x 5

across

routine

scanned keyisthen

displayedonthe

film

covering

keys against

sideofa pieceofmasking tape so astoavoid

keyboard

return or

blackonwhite.

on

conceptofreversaloffunction,

that

a 6520 PIA, a four

the

keeps

including

two

the

cursor

a memory basis. From

the

Each

keyboard. To

keyboard

two

shift

keyboardtobe

shift

keys is pressed

the

key

top

which should be removed. This

scratching

keyboard with 64 upper case,orshifted

consistsoffunction

right and

complementary

time

the

final

scans

consideredtobe

dUring

linetoten

Interrupt

implement

valueofthe

are acknowledged

keys,

left.

Reverse on

timetotime

functions

occurs

either

shipping.Toremove

the

oneofwhich

operational.

simultaneously.

characters. Someofthe

allows

up and down, right and

are usually combined, with one being

line

decoder

from

row

matrix

noise

last

scan

key

all subsequent characters to

the

keyisstruck,

and

the

CRT,

with

in a buffer.

unlessalater closure.

may

be pressedtocause

Lower characters

Each key has

protection

the

film,

carefully

scratching

characters on

the

and N key

The

the

functions

the

left

is carried

interrupt keyboardisscanned

matrix

roll

scanned key

algorithm

was

keytops.

does

I

not

11

I

M .

-

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~E!!!6"><:l:

OIIa.l:«Cal

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0

~

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III

to

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a

•

..

•

..

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•

..

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•

..

•

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•

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~@J~[EJ[!J

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on

:

~

• m

1!

·

'"

t

J •

Iii

~

-

..

• •

·"

, -

l~

c

'"

0:

~

"

~

"

~

:

~

.

~

•

"

O@(!)@](!)

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o@C!l~[!J

apoo9(J

/

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)

pJeoQ,';a)j

/\1

II

: : : :

12

give classical N key roll over in

the order

The keyboard Is

BASIC,

of

the normal

that

for

shift

The Is pressed, the software sets a special

All key closures are translated using a ROM·based look·up table for the key. The

bit8of

Once the hardware translation queue. The keyboard queue is loaded every time a new key closure

characters can be transferred

ThiS

input

stack

to

determine whetherornotitis

queue. The keyboard queue

characters moved down, and a load index pointer decremented by one.

that

they are struck.

left

scanning the

that

checks for

functionsofscanning. The user can take advantageofthis

row.

keyisa special mUltiple key closure andistreated separately. If

the ASCII character whichisthen translated

queueisscannedbythe

may be scanned by a user program. The user program can look at the

but

does

allow

last

row, which contains

the

stop keytosample the

shift

is

done, the encoded valueistransferred Into a10character keyboard

to

the screen. "

GET

routine

greater than zero;ifIt is,

is

located at 527·536. The

for

legitimate

input

switch

whichisusedtochange the decodeofthe key.

into

directlytoallow

first

rejectionofnoise and trappingofthe

the

stop

key. This

1/0 device, Without having

by reading the

eitherofthe

the

screen representationinthe standard way.

is

sensed and is unloaded as soon as

input

without

that

means

character may be taken

that

thereisdata in the keyboard

keys

allows

goingtothe screen. The

the routine in

to

perform any

input

character

two

shift

keyisencoded

pointeratlocation

out;

all subsequent

keys

into

525

is

This

new key closure Both the GET and keyboard interrupt or

Whenever the screen first level enables the cursor

cursor

repeated, tryingtokeep th keyboard queue empty. The second level flashes the cursor and translates and stores characters from the keyboard

keyboard queue. Meanwhile, the first level operating system always watches the

carriage return. After the carriage return the operating system. The restofthe operating system does not see the characters until they have been typed and a carriage return operating system.

An printing

control

keyboard queue shift-run Is encountered, the routine

RETURN" run

a dangerous routine, unless

could

with

the Interrupt masked.

position.

interesting

out

to

is

automatically transferred in the proper order.

The routine then moves

trick

a linetothe

BASIC, new line numbers may be entered

is

into

the

store a new value during a time

input

editor

for the more advanced programmeristo

the

keyboard queue. When controlisreturnedtothe

routineisoperational, a special two·level operating systemisin play. The

to

is

screen, forcing a carriage return

LOADJRUN

writtenina

routine take careofthat

flash and writes data from the keyboardtoscreen memory at the current

the

is

sent. This

sequence whichisimplemented by the keyboard scan program when a

allows

automatically

machine

cursor one character further down in memory. The process

printed,

for total

language

that

you are scanning and changing the queue.

automatically by only operating during the

this

routine automatically transfers the entire line

editingofthe line, priortohandingitto

into

the memory. Another exampleofthe useofthe

forces "LOAD, CARRIAGE RETURN,

with

the interrupt masked, because a

input

stream for a

use the

the keyboard queue and then returning

PETtowrite

input

routine,the load followed by the

its

own programs.

RUN

into

the

CARRIAGE

to

By

is

be

It should bad effects can happen

noted

that

this

keyboard queueisonly

to

your system. The

ten characters long and

only

known recovery from exceeding

13

ifit

is exceeded, dramatically

this

queueisto

power

j

..

..,......back

--lftyboanl and you do

SCREEN EDITOR

Typing on the keyboard, while the cursorisactive, transfers what is typed on the keyboard the screen. This function is like a simple computer terminal which requires youtoretype a whole line until you get understood with a examples are much more

To

follow these examples, two concepts are necessary. One is

system The second concept is

countered, are treated by BASIC as characters

In programming mode). BASIC is executing each instruction like print as soon as you type and does make

The done the line:

is

golingtointerpret

this

section you are operating the computer in what is known as a direct mode. (i.E. rather than

hit

carriage return.

first thing

this

with your users' guide. However,

on

and

start

over. When fooling with

not

have the Interrupt turned

it

right, but

PETtoIllustrate

the

PET

difficult

the?

that

when we follow

lets

you edit your mistakes before you enter a line.

it.

The user should follow discussions on his own

to describe thantosee.

the same as PRINT.

that

you will wanttohave printed onto the screen.

•

We

will

see in

the

machine useful as a super calculator.

that

we wanttodo is have

the

future that

the

we

the

queue, remember

off,

the

operating systemIsgoingtokill

that

when we type a ? the BASIC operating

a?

by a ",all characters after

this

is not the way most programs are operated. It

machine type a simple message.

hope by now that you understand a

thatifthe userIstypingon the

the",

you.

directly

The

editorisbest

PET,

as manyofthe

until

the

next"

it

into

the

You

should have already

little

better.

is

system

We

type

to

en-

R E

?"HI THERE"

T

U

R

N

You will see a key on the keyboard, the cursor moved automatically one place next character, and nothing else happened until after the carriage return. When the carriage return occurred, the

Let us ?HI THERE For thoseofyou who are touch typists, you may have already made keys. previously struck character. This key is called the delete key, located keyboard.

If

overstrikeofthat

out HI THERE

talk

In

we

strike the delete key once, you

that

BASIC responds by printingHITHERE. It shouldbenoted that each timewestruck

to

the right, allowingusto

HI THERE appeared almost immediately on

about the simplest function; that is, immediately correcting a mistake. Retype

B.

What we were tryingtotype wasHITHERE

ordertoallow

PET,

youtoimmediately correct

will

position.

a blank line, and READY.

We

can now finish typing

this

see that the B has disappeared. Typing the P results in

the

screen.

PET,

but we

mistake, there is a key which allowsusto

ET;

then

hit

carriage return, causing the

hit

the character B ratherthan

this

mistake with the PET's close

in

the upper right-hand sideofthe

type in the

the

line

erase a

PET

to print

P.

an

the

The delete key is want from where you are and then retype. This is the simplest form

fundamental editing tool which allows youtostrike

14

out

as many charactersasyou

of

editing. It is implemented by

decrementing the pointer is.

We

screenpolnter

can go back and erase the striking the delete key. Notice two one character at a time, and before you see as

it

doestooverstrike one. Also, notice

organized so

Because

of

it

blink. This phenomenon occurs becauseittakes15times as longtoblink

that

deleting

the

fact

that the character at the beginning deletes line above, however,

the

40th characterofthe

thisisa pretty slow wayofediting.

(2)ifyou strike fast,

the

previous character in memory moves the pointer back over

the characters scan from righttoleft

of

fr9m where you are by one and striking a blank over where the screen

READY

facts

as you are striking;

thatisright.in .frontofour

cursor by

just

(1)ifyou strike slowly, the cursor

the

cursor will actually move several characters

continuously

will

move

2characters

that

1.'he

PET

wraps around the screen. The screen memory is

that

character.

in

40~01umn

the line, and then striking the delete

chunks, for example deleting

key

at the beginningofthe

line,

previous line. Just keying back40strokes erases the READY from the

There are three cursor movement keys on your

it

moves

CURSOR RIGHT AND LEFT

up and down, and the third key movesithome (upper left-hand corner) and clears the screen.

PET.

One key moves the cursorrightorleft;

the second key

The cursor right key mbves the pointer one charactertothe right. If we strikeitnow five times, you see

thatitmoves us five columns over. It accomplishes

left

The cursor

that

type we strikeittwo

moves

the

time. Obviously, by doing this, we are able

achieved by use

CURSOR

keyison the same key as the cursor right andisevoked by shifting priortostriking. If we

four times, you will see that now we are back one charactertothe

more times,itmoves us around the cornerofthe previous line. Cursor left,ofcourse,

cursor pointer one character lessinmemory. Goingtothe

to

edit the screen. However, faster editing can often be

of

the

cursor up and down keys.

UP

AND DOWN

The cursor down moves the pointer40columns to the right from same Visual effect as moving positions with To cause

the

cursor right. The cursorisnow on the same position on the screen, but down one line.

the

cursortomove up, hold down the shift key while striking the cursor up/down key once;

it

down one line on

the

this

by changing the cursor pointer in memory.

rightofwhere we started. If

left,itmoves one character at a

its

current position. This givesitthe

screen. Foranexample,

try

spacing over forty

gets us backtoour original position.

Cursor up moves

the

screen memory pointer

"up"40characters from

its

current position, or rather,

characters less in screen memory than the current position.

will

just

this

40

SCREEN EDITING

We

can now use

PET

message. Once you are there, you can now delete the T by striking the delete key. all the characters now see the deleteisactually a matterofmOVing substituting

INSERTIDELETE

Before analYZing insert and delete, we should be reminded any single line may concerned with starting from the position the

left,

replacing the character preceding the cursor. The cursor is then movedtothe positionofthe

the

t9

a blank.

the

cursor movement characterstogetupin position on the second H in

the rightofthe character being deleted are movedtothe

all

the

characters in memory

that

the

screen memory is organized such

consistof40or80

characters on a line. Whenever

of

the cursor,tothe endofthe

characters.

(See

section on screen memory.) Insert and delete are

the

delete key is struck, allofthe characters,

line, are automatically shifted one character

replaced character.

is

The last character in the line

automatically blanked. Insertisthe reverseofthis

15

the

You

will

left

one character. You

left

one, rather than just

process, If we want

HI THERE

notice

that

will

that

to

fix the line that we just

HERE.

In ordertodo that, we have to make a spaceinwhich to type theT.To the shifted insert key with a single stroke. After striking which says HI THERE than one character, strike the insert key more than once; this moves all the characters on the linetothe right, and the cursor pointstothe first character characters on the line. For example, positioned over the H, then delete allofthe extra

got

through taking the T out of, we need to put a T back between the blank and the

accomplish this, we strike

T,

you will note

PET,

with the cursor blinking over the first characterofthe insert.Toinsert more

that

this

now creates a screen

•

of

the~nsert.

if

we

hit

the

insert key three times, type

1's;

we will then see that the back and forth movement

This then allowsusto insert several

1's

until the cursor

is

in the line is automatically handled and we end up with a perfectly recomposed message. It should be noted the screen. This

that

in no time has the computer respondedtothese commands, other than making a change on

is

because we have not yet pressed carriage return to tell the

PET

that

the line is

complete. That

is

why we have been talking about a screen editor. All editing is accomplished between the keyboard

the

and the screen memory, without interfering in any way with the user data, whether corrections.

to

compose perfect text and handitto

itbeBASICorthe

It is best symbolized by:

user program,toworry about the intermediate stepsofmaking

the computer without the programmer whoisusing the

rest of the operating system. This allows

What You

LINES ON A

Physically, a line on the screen consistsof40 computer business, many data inputs are organized for data can be put into ao columns than into characters per line, the user is given all the flexipilityofan allowing the screen beginning COMETOTHE AID OF THE next line. right above is only consideredtobe

The thing that allows the beginning

See

Is What You Get.

PET

SCREEN

columnsofinformation. However, traditionally in the

ao

column data cards and,ofcourse, much more

40.

Therefore, although the

80-eolumn line. Thisisaccomplished by

to

define more than40characters as a line. If we move our cursor overtothe

of

the line below HI THERE, and start typing NOWISTHE TIME

PARTY,

You

will soon see the screen considers

we will see that after typing theE,the space is automatically on

thistobe an aO-column line although the HI THERE

a 40-column line.

PETtoaccomplish thisisthat

of

the line. Each line has a marker that indicates whetheritis

Internally, thereisa tableofpointers at the

continuation line. This pointer Is kept In the negative bit position up

or

cursor down occurs, PETtotheir kept which

proper

Is

the

line

beginning pointerfor the first complete position Is then kept as a separate pointer telling the Whenever scrolling occurs,the line pointers are moved up second line

is

maintained

the

editor

examines

number. At any

until

the

line

the

statusofthese line pointersinordertoInitialize the

time

while the cursor is on the screen, thereisa separate value

line

from which the cursoroperates. The screen

PET

whetheritis

in

such a way that the conceptofthe first line

disappears on the screen. This line pointer tableislocated

memory locations 553·577.

PET

screen can display only

FOR

ALL GOOD MEN

the beginning of a

of

the indexpointer. Whenever a cursor

lineora

greaterorless than40characters.

40

TO

the

PET

in

Now

that

we understand insert at the beginning. the curSor

up

and left, until the cursor blinks on the NofNOW. If we insert twice, we can then type a

To

that

print

the

PET

can allow80columns, let us see what happens when we do the

this

line, we havetoput a1"at the beginningofthe characters.

(it shouldbenoted that this causes the characters on the linetoall movetothe

PET

return, the

prints NOWISTHE TIME on two consecutive lines, spaces a line and types

16

right).lfwenow

READY.

We

move

carriage

If we

1"

go up and make a change in the middleofthe line, we can the carriage returninthe

the

even though basic rule is

cursorisblinking on theP,a carriage return causes the entire linetobe

that

when a carriage returnisstruck, regardlessofwhereitoccurs in the line, the entire line is transferred, whether apparent

SCROLLING

Now when we try base we attempt

as

you useitwhen writing programs.

that

we have a mixtureof40-and

to

move

of

the screen. Hitting the next cursor down causes the entire screentomove up one line. Any time

to

print past the thousandth character on the screen, the screen editor automatically moves

line. If we spaceupto

the

word PARTY the first time

itbea 40-or 80-eolumn line. Sophistication in using the editor will become more

the

cursor

BO-eolumn

off

the bottom. To do this, we just cursor down until the cursor is at the

lines on the screen; let us investigate what happens

See

thatitmakes no difference wherewehit

thatitis

on the screen, now

the entire screen up one line.

the

Lines move upon

I screen. This Is accomplished

to

be scrolled

Is

screen by a one lineortwo

off

the top,

in

hardware by checking

the

81st characterthroughtothe thousanth character are movedtothe

line

jump

depending on the statusofthe top

the top

line

pointer plus one.Ifan

the screen memory, and the bottom80charactersofmemory are filled with blanks. If only a line Istobe moved

the

bottomofmemory.The cursorispositioned automaticallyinthe same position at the bottomofthe

at screen as

it

was when you triedtomove the cursor down;orin the caseofa carriage return and/or

printing, the cursor

off

the

top, the 41st characterismovedtothe

is

moved automaticallytothe

first, etc., and40characters are blanked

left

....

and sideofthe bottom line.

reprinted. The

line on

80~01umn

the

line

top

4O~0lumn

of

This processistotally automatic andiscaused by attemptingtoprint carriage returnorspace

off

the

bottomofthe screen. Thereisno other program control over the movement.Aswe will see when we write

that

a program

held down

HOME AND CLEAR

causes scrolling, the scrolling speed on the

while printing is occurring, the scrolling will be slower by a factorof20.

Striking the home key moves

the

cursortothe upper left-hand cornerofthe screen (the first location

PETistoo

fast to read. If the reverse key is

the screen memory). Holding the shift key down and pressing the clear key gives you a blank screen with the cursor blinking in the upper left-hand corner. This is accomplished screen positions and again homing the cursor. Clear

The

PET

basically moves data from the keyboardtothe screen and then when a carriage return is struck

moves the screen data

into

a program. This allows the user the

screen without having any effect on the program that prOVided

on the screen. This allows the entire screen

to

a//ow movement around the screen and to insertordelete,aswelf as type over any character

to

or

home can be given at any place on the screen-.

is going

to

act as an editing place for user-controfled input.

by

moving blanks into all thousand

flexibilityofmaking a correction on the

receIve the corrected version. Keys are

of

17

j

Chapter

The combination creative process. Someone who knows how

4.

of

instructionstosolve a particular problem cannot be taughtina text book. It is a

BEGINNING BASIC

to

use the computer uses his intuitionorcareful planning figure out instruction sequencestoallow solutionofhis problem. All that we can coverinthis all the problems. This book cannot teach you use the

PET

can be -except whenitis provided with pre-programmed softrware -is a tooltouse for solving

to

solve your particular problem. It can, however, teach you how

PETasan

instrument.

THE PRINT STATEMENT A computercan calculate numbers all day but We

will begin our discussionofBASIC with the PRINT statement for

itisof

no value unless the computations can be displayed.

that

reason.

to

book and

to

When typing text, PRINT can be abbreviated

as

? A statement such

PRINT

"HELLO"

as

this:

isanInstruction to the computer tellingitto displayonthe screen all characters between the quotes--in this

case a wordofgreeting.

isaninstructiontoprint the productof1024

Itisusefultonote

it

write a line, print 'A' and on a second line print 'B',itis possibletowrite the line:

which contained as

Unless the directed to the built-in screen. screen, under the the is accomplished by the screen computer character

will

print the squareof1024, a few spaces, and then the cubeof1024. Detailsofthe exact format is in

you can write down.

computer

physical

automatically

on the screen.

that

BASIC allows youtoprint more than one value at a given time. Rather than having

the next section. The point hereisthat

has been

controlofBASIC andan

representation on the screen is25fines by40characters, the

On

the other hand:

PRINT

1024·S

multiplied

PRINT

10241

2,

you can printasmany values across a seriesoflines

instructed

The

characters are

automatically

scrolls the screen up oneortwo

otherwise by meansofCMD command,

printedinthe

editor

which is keeping trackofthe screen position. Although

folding over the 41st character onto the

·8.

1024 t 3

all

print

full

lines whenitreaches the one-thousandth

print

position

printingofup to 80 characters

on the

line.

The

are

The

command character which print

variable.Ifthe data is presented in this form, the position automatically characters, return extra spaces, variable than seven characters,

BASIC obeys the foffowing roles for

and

after

no leading

If

the

number notation. For example, .0034 is the

number

PRINT has two

allows

followed

tabulates printing

prints

B,

a ; (semicolon) is used. PRINT A;B; results

or

trailing characters sent.Ifthe field to be

is fess than

falls between these values, the

for

by

'B'

followed by a carriage return.Inorder to cause BASIC to

'B'

major

printing

forms under the

the field specified

a carriage return. (2) Data presented in the form PRINT A,

'A'

startingatthe current screen

is printed.

will

.01orgreater thanorequal to 999999999.2, BASIC printsitusing

The

be printed

printing

printed

as 3.4

cursor is

after

controlofBASIC. (1)

after

the

print

fieldisprinted

position

in

the

'A'

being printed, then foffowedbyno

left

at the endofthe

spacing 20 characters, when using PRINT A,B.

characters.

E-03

most

significant9digits

When

the

field

printed

and - 1234567890.5 is

is a number, BASIC

The

standard

statement has ended in the form

startingatthe current screen

B,

then spacing over

not

send the carriage

'B'

field.Ifthe variable A is more

to be

printed

are printed, plus a decimal

is a string, there are

first

as

- 1.2345678E +

19

print

single

then BASIC

10

checks

its

scientific

point

size.

Og.

If

if

needed. Trailing zeroes

a number (unless

It

shouldbenoted screen, unlike the

PET,

of

the data screen being covered.

most

which causes the screen to advance the screen

after

it

is

printed

that

in order to

BASICs, the apparent space between fields is always a skip (cursorright)characterin

the decimal

as

a string).

tak.

lull

point

are

not

printed. BASIC always

advantageofthe

PET's

pointer

prints

ability

byone character;

to compose text material on the

a skip character

it

does

not

resultin any

after

Because the PET allows the inclusion

programmer has

as literals are clear screen, home cursor, cursor right, left, up anddown. By use

compose fields displayable on the PET screen.

We

previously discussed

located at memory location

full

controlofthe screen

of

any length and in any siZe starting in

how

the

8000

ASCII code.concatenated with two additional bits. Oneofthese

is

the upper·lower case bit.

When printing

characters

to

the

scr,en,

Into

the screen memory form.

the print subroutine In the operating system automatically translates ASCII

characters for the screen printer. The home character moves the printer pointer screen. representation for blank

In

The

clear character moves the printer pointertothe

in

allofthe 1000 characters on the screen.

BASIC, numbers are representedas5-byte binary quantities, exceptinthe special caseofinlegers,

which are represented in two bytes,

of

all

cursor

print

PET

screen memory consistsofa thousand charactersofatorage

positioning

position.

as literal characters within a string, the

The

cursor control characters availabletouse

of

these literals, one can

anyoneofthe 1,000 character

positions

hexa-decimal. Characters are representedinscreen memory in six

bits

is a reverse field and

The

various screen control characters are

to

beginningofthe

As

far as prinlingisconcerned, BASIC prinls integers the same as

screen, and Inserts the

the

second one

simply

movement

the beginningofthe

bit

does floaling point numbers.Infact, BASIC automatically converts integers to 110ating point and then the

inlo

floating point print routine converts the floating point numbers

printable characters.

it

VARIABLES

We

have already seen functions and then can print intermediate values

~

thai

the

PET

can be used as a large calculator which performs mathematical

Ihe

results. However,inmany cases, programming consistsofdeveloping

or

performing operations until something equals a certain value.Inorder

to

implement programming al any level, we needtoestablish the useoffunctions which can have a variety

of

values at programming as a variable. If you then a book on beginning algebra,

anyone

time. A function

are

or

that

can have any valueisdefined in both algebra and in

not familiar with the conceptofa variable through mathematics;

perhaps oneofthe very rUdimentary texts on BASIC might helpyou.

Allofour discussions after this witt concern themselves with the useofvariables. In

BASIC, variables are defined by two character alpha numerics. If the variable is a numeric variablethen it has no

trailing character. The character A is consideredtobe

different variable. Characters

A1isa third variable, but all three are definedasnumeric values. If

the variableA.Characters AA is a

the variable contains alphanumeric data,itIs defined as a string. A string variable now ends with a$.Thus, Aand

A$

are numeric and string values respectively and are different variables. AA$, likewise, is different

is

from AA, etc. BASIC distinguishes a variable by the fact that the first character alphabetic character. The second character maybeeither numericoralphabetic.

always

An

an

integer variable

ends with %, e.g.A%.

ARRAYS

Arraysisthe fourth typeofvariable which can be definedinBASIC. Arrays are differentiated by the

20

parentheses which used

inanexpression.

follow

them. Parentheses define the particular value withinanarray whichisto

be

A(D,1)refers to A%.

All may be specified in the same program. Specific for each a variable.

Equal is used in two ways: If encountered in mathematical function: the value to the left the right. Otherwise, when following a variable such as in the expression A value in A with the resultantofthe expressiontothe right.

Originally BASIC required the word LET before any variable assignment, but in

and may be

To

understand how variables operate in BASIC, try the following examples on your

press

RETURN

CLR

the

first character in the second rowofa two-column array and is different fromA,A$

definitions

of

the typesofvariables follows, but first let us address some examplesofhow one uses

an

IF-THEN typeofstatement, equal means

of

the expression is compared and must equal

omitted.

A =2 is equivalenttoLET

after each command you enter.

A =

2.

The command CLR sets all variables in

and memory allocation techniques

=2+2,

?A

PET

prints

Now type

A=2+2

D.

?A

This time

Now type

PET

prints

4.

?B

PET

prints

Now replace

B=2"A

O.

the

valueofB with

twice

the value inA,by

typing

?B

PET

prints

Now change

A=2+3

8. the

valueofAbytyping

?A

PET

prints5.If you now type

?B

PET

prints8,the

says B

= 2*

same value as before. Untilwegive a new expression for B or re-execute the one which

A,

the valueofB will remain

8.

the

=means replace

PET

the

LET

PETtozero.

PET.

Remember

and

standard

value

of

the

is optional

to

FLOATING POINT VARIABLES BASIC always assumesoperation, oroperates

variable is assigned space in memory for a standard floating point number.

Four bytes contain a binary representation 9

digits Each variable is also assigned a 1-byte exponent limited less thgan

STRING VARIABLES A string variable can contain a function, whether it

character. There is a specific set

precisionofa decimal number. Accuracyofmost

-34

yield numbers too smalltodistinguish from zero.

of

variables that allow extraction and packingofdata into strings which

totally,

of

that precision. It gives us the capabilityofspecifying about

in floating point arithmetic.The:refore, each normal

calculations

to

having a maximum value

be

a number, graphics character, or standard ASCII

21

is limitedtothis

representation.

of

+33. Exponents

willbediscussed later on. The stringislimitedtothe80charactersofthe set 'of functions th'at allow the constructionofstrings upto255

INTEGERS

characters

input

buffer. Thereisa specific

(see

later text).

As we have indicated, an integerissimply a whole number. Floating point variables are stored in BASIC with five bytes; one for the exponent and four for the mantissa, which gives

accuracyof9 digits.

In

an

many cases, variables can be expressedinmuch simpler numbers.Inordertoallow the user most

in

memory efficiency, particularly

PET

has implemented the conceptofstoring certain numbers as two-byte integer values. Any integer

value between minus 32,767

of

highest bit

USE

OF

the number containing the Sign.

PROGRAM

AND

DIRECT STATEMENTS

Throughout the text, until now, we have been using the program technique which allowedusto

to

PETto respond directly

it

directly, as we typeitfrom the keyboard and hit carriage return. Thisisso-called direct mode.Inthis

mode, we can use the

the print statement.Inthis

PET

as a super calculator. For Instance, if we want the

divide the result by a third, we can ask

get

should

the answerof2 followed by a keyboard, except when it is super calculator and for teaching with the

the caseofarrays which can take significant amountsofmemory, the

to

plus 32,767 may be storedinthe formofa two-byte number with the

case, BASICisobeying the command we are giving

PETtoadd two numbers and

it

the question

READY.

in

the processofexecuting a BASIC program.Inaddition to using it as a

PET,

the direct mode is quite useful for debugging

The

?(2+8)/5.

PET

If you have typed that on the

will obey any statement given it from the

PET,

of

get the

you

computer programs. Variables can be assigned intermediate values and then small sectionsofthe

be

program can

executed with working correctly. Break points can commands, again in direct mode, without having

or

debugging

in the caseofusing the

true computing element, one has

GOTO

statementstoassess why any particular pieceofcode is not

be

put in programs and current statusofvariables checked with print

to

modify your main program. However, except for

PETasa super calculator, in ordertoget the computertoact as a

to

write or load a BASIC program. The difference between execution

direct mode and a program is that several statements canbegrouped together in logical order and then

of

BASIC will execute all

the statements before asking the user for control.

tn

Suppose

easily accomplish

very simple. Any statement you want by a

A good habittodevelop when typing in linesofa programisto 1,2,3,

program. All you need

To

print HI THERE, vertically, each lineofour program will type one letterofthe message. we are going

to

start with line10and make each line a multipleof10.

we

want BASICtoprint ourHITHERE message vertically as opposedtohorizontally.

this

in a program but not very easily in adirect statement. Rules for program entry are

to

be treatedbyBASIC as a program statement must be preceded

line number. A line number may be any number from 0to63,999.

use increnmentsof10or100.

etc., use

10,

20,

30.

This will give you space latertoadd

to

remember is that BASIC interprets each line number in order.

lines and make corrections in your

Instead

10?"H" 20?"1" 3O?"T" 40?"H" 50?"E" 60?"R" 70?"E"

Whether you are typing tell the

PETtotake a look at what you have typed and act accordingly. The lines ten through seventy

a programorgiving direct commands like

RUN,

you have gottohit

RETURN

in

22

We

can

of

to

constitute a program which The program is now resident in memory.

tells

the

PETtoprint out HI THERE.

To

execute the program, type

printed in the vertical format:

H

I

T

H

E R E

T.

You will note that we do not have a space between the Iand

of

the multiple

program by typing LIST. This gives

10?

PRINT

20?

PRINT

30?

PRINT"T"

40?

PRINT

50?

PRINT

50?

PRINT

70?

PRINT

tenisthat we can now insert a correction between lines20and

us

the program printed as follows:

"H" "I"

"H" "E" "R" "E"

Oneofthe reasons we use the numbers in

Now type:

25?""

Press return and relist the program, and we will see that line25is run the program now, we get:

H

I

RUN.

This gives us theHITHERE

30.

First, display the

inserted between lines20and

30.

If we

T

H

E

R E

This example demonstrates the useofline numbers and the abilitytoinsert lines numberstomake a correction in a program.

There return. Then following the I on line down key, inserted the cursor-down (it looks like a reverse field executed.Donot forgettohold down

When we now run the program, you see

is

another waytoget the same effect. First delete the space by typing25followed by a carriage

list

the program and see that line25has been deleted. Now position the cursor on the space

20,

and insert a cursor down. First by hitting the insert key, and then the cursor

if

you

don't

hit the insert key first, the cursor will move down immediately. But because you

Q), the cursor will not move until instruction

shift

before striking insert.

this

also gives you the effectifa space on the next line. This

20

would not always be true, except we had been cheating and using the automatic scrolling capability the

PET

which clears

have received such a nice result. Try programming a home 5?"HOME", then try a clear

The

screen the screen. takes programs and L·I·S·T starts

editor The

at

out

the field. Had we programmed ahome priortoprinting a program, we would not

will

allow

you to take a program and make changes on anyofthe lines you display on

list

commandhas several features to help you

prints

the first line number in memory and

the contentsofthe basic program which is stored in memory.

get

the right lines to the screen to edit. List

lists

to the screen device

5?"ClEAR".

The

command

all

the instructions to the

is

of

23

e,nd.

The

longer-programsteaturesoflist

lists

just

line

20,

LIST 10-50 which numbers lines from line 50 to correct your PET

fromth~·fJeginning

any

pieceofprogram

just

to see

which

lists

lines

of

the program through line50included,

the

endofthe program. Some

whichiscurrently

what

they do with

our

allow

you to

10

through50included,

combinationofthe above can be use(nO

little

stored

program.

in memory.

list

onlyasingle

Try

line-nurtlber

LIST-50 which

and

LIST

50-

which

eachofthe above commands,on,

lIST

means'

list

Iists-aH'oUhe"

find"and'

2fJwiifcft'

61Pt#e

BASIC is an interpretive language relatedtothe direct commands we are executing. BASIC executes a

command by taking the last line typed

words and expressions which

it

which Is the token for PRINT),Itinterprets

Command words are stored In memory

As

a program

lineisentered

into line number and searches through memory, it

is the same line number, then the entire lineinmemoryisdeleted and a new line is inserted In memory.

toItand analyzing

recognizes. Every timeitencounters a key word such as PRINT

this

word

with

bit

8ontotell BASIC

RAM

memory through the useofthe carriage return, BASiC

untilitfinds the same number,orthe number Just greater. If

In the pre·interpreted stateall the key words are replaced

to

This allows the interpreter

by

is the data typed in PRINT, even though

BASIC is called

an

keyword that needs

the programmer such as literals, pointerstothe variables, and the keywords.

it

takes five characterstotype,

interpreter because the actual execution of the instructions is donebyanalyzing the

tobeexecuted in the program line,then executing that keyword under the control

store commandsinthe most memory·efflcient form. The

the

line working from

into

a command which means somethingtoBASIC.

lefttoright looking

thatitIs a command word,orkey word.

with

the single character token forthe keyword.

only

data stored

only

takes one character in memory.

fOr'

(or?

takes

key

the

of a seriesofsubroutines. This is a trade-off which results in very memory-efficient storage programs but longer execution

times

tokens in memory and stores them on

of

coding

use BASIC

data on tape or in memoryisnot transferabletoother machines. Itisgenerally not possible to

instructions

than wouldbetrueofa machine language program. Because

I/O

devices whenever a programisloaded and saved, the actual

typed in from other machines.

PET

BASIC uses

,

When editor carriage and

To listitfirstonthe same

There are two ways to execute a BASIC program.

first instructionofthe

more numberofthe

print

direct mode, operates

course,

LITERALS

In quotes.

you create a BASIC program you are

and

the

BASIC line editor.

retumtransfersitto

delete

delete a line, you type the line

clears

old

lines.

line

number

all

the program variables

instructions,

first

THERE

insteadofHI THERE. RUNisexecutedindirect

must

specify

screen

program

and

with

new

and

ENDisencountered,orthe stoP keyispressed. RUN

instruction

the

same as RUN except

the line

The

main

memory.

number

aiteritthen type a carriage return to re-enter it.Toreplace a line,

text

and

in order,

to be executed. For example,ifyou type RUN30,

numberofthe

screen

type carriage return.

and

startingatthe

operating editor

The

immediately

initializes

allows

BASIC line

The

lowest

that

first

statementtobe

the program pointers.

noneofthe variables are re·initialized.

our HI THERE examples we have used PRINT commands

In

the

PET

these are called literal strings. Dataisalso kept in the

under two levelsofeditor:

you to change characters within a

editor

followed

firstoftheseisto type RUN.

number. Execution

allows

by

mode. A GOTO statement,

with

you to

a carriage return.

Thenitexecutes each

may

executed, e.g. GOTO

characterstobe printed enclosed in

the screen

add

new

continues

have as an

our

PET

in binary floating·point

character

line

until

lines

and

modify

To

modify

The

command

until

sample program

also

The

30.

a line,

enter

there are

argument

executed

GOTO,

the

RUN

no

the

will

in

of

24

numbers. Muchofthe data you wanttowork with in programsisnot numeric but alphanumeric••the way

talk

we These characters are specified contained within a set

To the

more sophisticated techniques. We

Graphics characters are stored as original 64 character set and they are stored the lower characters on the keys. A literal can be used print HI THERE.

back and forth as human beings.

to

the

PET

with literal strings. More specifically a literalisany value

of

quotes.

allowthe maximum compositionofscreen data,the

abilitytostore and execute cursor control characters which are fedtoIt by meansofliteralsorother

have already discussed in a section on

an

PET

keyboard input how the

extensiontothis

in

memory with a special indicatortodifferentiatethem from

PET

has a special setofgraphics characters and

PET

stores

set. Graphics are produced by shifting from the

to

draw a line just as easily as it can be used

its

data in ASCII.

to

Any combinationofcharacters within the cursor movement and the reverse field.

you are typing in a literal whenever a quotation mark

t1"lP

is typed until the the screen in a format so that the software which transfers the input linetoBASIC will transfer them as control characters

You can see the cursor movement characters flagged with reverse field within a literal. Type a single quote and see heart. Cursor down Is a bracket and cursor left dots. Insertisa shifted

You cannot entera character in reverse field into a literal but you can turn on reverse field with the control character before your character between quotes are those which are interpretted as control characters.

Delete

been typed on a line, you lose the or

You should note at least one time while you are editing that you have fallen into the aforementioned trap of return, then cursor up

is

a carriage returnistyped.

tryingtomove the cursor after a quote has been typed.

the

this

only

that a closing quotation mark is entered, all characters are transferred directly

if

thatisappropriate.

happen. Reverse field looks likean"R".

"a" and cursor up

is

the

shiftofthat character and looks like avertical linethrough the 5th column

"T"

which looks like a second vertical line.

is

printed. The only characters that are allowedtoappear in reverse field

editing character that will still work within a literal. Once an odd numberofquotes has

abilitytomove the cursor about the screen until either a closing quote

to

edit your mistake.

PET

keyboard may be typedinas a literal and

PET

has a special modeinthe screen editor which assumes that

is

typed. From the

Home isan"S"

is

the shifted "a" or hole character. Cursor right

EithM

type a phoney closing quoteoracarriage

time

that the first quotation mark

and clear is a shifted

this

includes all

is

"S"

a right

to

or

of

Another method

function.

to

try mode

you struck the insert function.Itis suggested that you make up your own examplestoplay with this.

Examples may also be suggested

The depth recommend

with the use

It

do a cursor movement, the control characters will be flagged with reverse field

is

easytoget

abilitytoreadily manipulate the graphics and the cursor movement characters can allow whatever

of

graphicalcapabilityyou have thetime and patiencetoprogram.The computershould be fun.

of

inserting cursor control characters into already existing

has the same effect asanopening quote. For example,ifyou type insert three

outofbecause you need only enter as many new characters as the numberoftimes

to

you as you make a few editing mistakes.

that

you develop your own programming

of

Imbedded graphics and cursor movement characters. Remember

skills

with the text and contlonually experiment

textisto

that

use the insert

just

you cannot

times

and then

as before. This

hurt

25

We

the

machine;..

stuff.

REVERSE FIELD

We

have shown in the examplesofquote mode and insert how once a mode has been established for a

line, the carriage return. Reverse field works in the same way. It remains is

typed or a carriage return is entered.

As

describedina previous section on screen memory, reverse field characters are stored with a special

bit on spots effect and doubles the number

useful that as well.

Here is

field on and type This gives us a

Reverse field remains on from the first

subsequently typed on the screen previously mentioned. This applies equally to keyboard inputaswellascharacters printed from a literal string.

the"

worst

PET

will

to

indicate the black spots in the characters coming from

willbeblack.

it

is not only implemented on the

an

exampleofhow reverse field works: Clear the screen and type HI (space). Next

that

can happen Is

continue with that function untilitis either cancelled by a new control characterora

As

you will see when you typeanexample,

of

TH

ERE

line in which we have highlighted THERE.

.Finallytype reverse field

that

you clear the screen accidently after typingina bunch

in

effect until a reverse field

ROM

will be all white and all the white

this

gives a very desirable highlighting

potential characters which the

PET

display but in someofthe

off,

(shifted reverse field on), type (space), PET.

time

the control characteristyped and alt characters

will

be printed in reverse field until the mode is terminated as

PET

can display. This feature is so

PET

hard copy printers

off

character

hit

of

reverse

we

To get the inserted. characters THERE until the titeral screen memory, a screen position

is received

character or a carriage return is typed.

TERMS AND OPERATORS

The

communication with BASICiseitherwith numbersorwith alphanumeric literals. Numbers are always presented in decimal form even though the microprocessor in the to

keep the representing precededbya $··e.g. $0010 is equalto16.

As

BASIC recieves Jines, the interpreterdivides the charactersitsees into several classes. Such

commands, functions and operators. PRINT is a commandtoBASIC with a specific function

perform. A function can be something

the keyboard, you get a constantof3.14159265, which can be usedinan

PETtotype

TYPE

?"HI (reverse field on) THERE (reverse field

occupy

is not in reverse field yet. The effectofthe quoteisto postpone the actionofa control character

is

by

the program which printsiton the screen. Reverse field remains on until a reverse field

two

straight,

itindecimal

out

in reverse field we use a literal with the control character for reverse-field-on

off)

PET". Note that the reverse field on and

a space on the screen when programming and

interpreted. Since the reverse field is turned on by setting a bitofeach character in

is

not required for reverse field onoroff

PET

will

assume that whenever we are talking about a number,

form. Later when we

like square rootora variable, or a special function. Whenever you type

talk

about hexadecimal numbers, they will always

that

they appear in reverse field, but the

when the streamofcharacters

PET

operatesinbinary mode.Inorder

we

that

expression.

are

as PET

off

be

can

non

An

operator is a character

in

evaluating an expression. The following setofoperators are defined for BASIC:

Plus sign

(+)

causes

thatisinterpreted by BASICasan

two

valuestobe added together using floating point representation with the results

arithmetic function which istobe

26

performed

being calculatedina floating point accumulator. The accuracy is limitedto9 significant digits. Minus

of

subtracts the value to the right

•

is the BASIC multiply. The value to the right

is

BASIC's divide. All the numberstothe rightofthe slash

the slash.

the minus from the value to the leftofthe minus sign.

of

the multiply is multipliedbythe value to the left.

are

divided

into

the expression to the left

of

t means exponentiation. All the values

Open and close parentheses cause values inside them parentheses are evaluated as a single value. Parentheses may be nested and are evaluated outward,

of

starting from the innermost set Sally" Subtration. Expressions within parentheses parentheses. The following setofexamples shouldbetried on your

operators and

Addition ?2+2

Subtraction ?4-2

Multiplication

will help you remember the precedQnceofoperators Multiplication first, then Division, Addition,

their

precedence.

parentheses.Inorderofprecedence, the memory aid

to

the

left

of t

are

raised to powerofthe value on the right.

tobesingle expressions. All expressions inside

are

evaluated first starting from the innermost set

PETtoshow the operationofthe

"My

Dear Aunt

of

? 6*2

Division

? 12f2

UseofParenthesis

?

4+8/2

?(4+

8)12

OrderofOperations

?(2+4·(8-4)12P

FUNCTIONS

There are three functions which

PET.

The firstofthese

keyboard character ofTI

be

always TI$ and the value Tl are

indicated, every time a screen refresh occurs, value expression three byte binary number whose value is stated evaluate the amountoftime beginning the end. This function is accurate to

Tl$ presents and accepts data

used,

two

and

evaluated as

is

measured as a 24·hour real-time clock. It is available to the programmer in

TI,

which gives the value the current number that BASIC is keeping. This number is keptasa

of

the sequence and then the difference calculatedbysubtracting that variable from theTIat

it

always presents data in string form with two characters for hours, two characters for minutes,

characters for seconds.

isTT:

to

the valueof3.14159265 etc. It canbeused anywhereinany expression and will

this

two

are

available in BASIC which are, at the timeofwriting, unique to the

Whenever this character is usedinan

number. Example:

ways to communicate with the real

(1/6Oth

that

a particular operation has taken,TIcanbestored in a variable at the

1160

of

a second.

in

the formofhours, minutes,and seconds. When the expression TI$ is

The

valueoftime in

expression, BASIC translates it from the

?ll.

time

clock

within BASIC.

of a second), a value within the

in

numbersof60ths of a second, or so called jiffies.

the

computer is kept in a 24·hour clock. Ifitis ten

PET

its

As

is updated. This

binary formbythe

previously

To

27

minutes clock, set

the

past 1 p.m. in

type

the

expression TI$ =

the

afternoon, TI$

with

the

clockto2:45 and30secondsinthe

would

be printed as 131000. To set the valueofthe

number being typed in

afternoon,

type

TI$ ="144530".

quotes

in 24-hour

time.

For example, to

real

time

As a personal experience, you some

additional

the

computer

Care

must

be takeninuseofthe zeroatmidnight. waited

until

reading, go

zeros

Oneofthe

the

variableTIis

the

real

should

back

and

time

clock.

value TI. Remember

authors

greater

screen. This expression never reached

compensate value when

Functions range anywhere fromn,which

compute

forthe

for

it

might.

thisisto

watch

are preprogrammed

the

sineofa number. When BASIC

function,

calculates

the

for when the

capabilitiesofBASIC

i:;

a predefined

resulting

simple. If A equals sineofn radians,

In

this

statement,

by expanding

space

after

forA.In

the

sine is

we are

the

valueofn,

the

expression:

computed,

actually

evaluating

it is

using

multiplied

set the value TI$ =

print

it.Aswith

wrotealoop

than a

the

computed

in a program for

constant

time

mightgothrough

function,

encounters

value, and uses

the

expression

A=SIN(n)

two

functions,n,and sine; BASIC

the

function

A = 2'SIN(n)

by 2 and stored in

to

the

right

all the

that

other

the

expression TI

variables,

graphics

and the valueofTI

value asTIgoes

which

can be treated as a

to sine,

the

which

code for

function,itevaluates

value in the

wouldbewritten:

sine and

finally

A.

time

now and

automatically

display

when

through

midnight,

is a

capabilityofBASIC

command.

would

storing

after

you have done

the

power-on sequence

where

the

midnight.

the

displayisputonthe

The

and readjust

single

value.

the

The use is really

evaluate

the

result in

this

the

goes

back

program

only

way

the

stored

Functions

to

expression

variable

to

is

to

quite

The

trigonometric expressions degrees

to

radians,

functions,

sine, cosine,

for SIN, COS, TAN all have as

multiply

the

numberofdegrees by n/180. For example:

tangent

their

only

and arc

tangent

argument an angle given in radians.Toconvert from

?SIN(90'nI180)

calculates

Sinof90 degree. To

obtain

the

cosineof45 degrees:

PRINT COS (45'nI1801

To

compute

the

tangentof40

degrees. For example:

?TAN (40'nI180)

Eachofthese functions are computed by tables. Because n is

values should be The

accuracyofBASIC radians. Expressions only

to tentothe BASIC. The Answers are

less

ninth.

function

always

than 1000 degrees

functions

which

Arc

arc

tangent

is five parts in tentothe

use

the

tangentisthe

computes

given between

or

6n.

values in radians are a

only

inverse

the

value in radiansofthe

plusorminus

trigonometric

17.

The

tenth

functionofthe

accuracy

result is in radians.

?ATN(.5)

To convert

the

numbertodegrees use

the

following

example:

?180fn~

ATN(.5)

are all available in PET BASIC. The

limitedto9

as long as

is 5 parts in

significant

valueofn

function

the

specified

digits,

argumentisbelow

whichisaccurate

expression given on

1010.Innormal use

as a

the

in general,

20

function

argument.

the

in

28

The

following

functionofarc

the

Both

above expression by division checked

by zero

for

zero and

valueofone.

general

expressions

tangent.

expressions

180frr.

which

(It

shouldbenoted will

before

can be usedtocompute

ARC SIN

(Xl=ATN(XlSQR(-X'X+1)

ARC COS (Xl = - ATN(XlSQR( -

give

the

resultsinradianstobe convertedtodegrees by

resultina

using

the

thatinboth

basic

error. Before

expression arc sine, X

the

valueofarc sine and arc

X·X

+

1)+

expressions

using

the

shouldbechecked

1.5708

thereisa

expression,

cosine

mUltiplying

the

as a

total

possibilityofperforming

the arc

cosine

should

foritbeing equaltothe

a

be

MATHEMATiCAL

The

largest ?overflow smaller number

legal

error. The

will

result in an

ASS

Absolute

positive

valueisspecifiedinthe

number.

INT

This

expression

example:

For

Other additional the argument

than

number

the

inaccuracy.

four

foraninteger,

FUNCTIONS

number

Thereisno

basically

inherent

might,infact,

that

smallest

underflow.

rounds

inaccuracyofdropping

However,

the

BASIC can

magnitude

form ABS(X). The

inherent

the

handle

that

accuracy

PAINT ASS(

current

is

± 1.70141183 E+

can be

function

loss.

valueofthe

INT(.23)

INT(-2.5)=

distinguished

returns

For

example:

-145).

parametertothe

=0

-3

INT(1.79)=1

significant

small

inaccuraciesinthe

be stored in BASIC as 3.99999999. When

result

is3,not

4.

digits,

argument

38. from0is

the

this

expression

could

Any

larger

valueofthe

this

number

2.93873588 E -

lowest

introduces

problems.

numberisused in

gives

an

39.

expressionasa

integer.

no

For

example,

the

Any

SGN

This

expression

- 1

if

the

signisnegative. For

returns

a 1ifthe

signofthe

example:

SQA

This

function

resultisan ?ILLEGAL QUANTITY ERROR.

entire

The is ·2.71828183.

range.

following

calculates

two

functions

the

square

send

rootofany

themselves

EXPONENT

The

parameter

defines

the

powertowhich

numberisgreater

?SGN( -

- 1

45)

?SGN(+10)

1

number

Accuracyofthe

greater

?SQA(16)

with

natural

the

base Eisraised. The

29

than zero, a zeroifthe

than

zero. If a

expression

algorithms.

is 5

parts

The

limitofthe

minus

in 10tothe

algorithms

parameter

valueiszero. and a

numberisused,

tenth

for

are base E

is 88.02969189.

which

the the

A number greaterthan that will result inanoverflow. A formofthe expression is

only allows the flow function forE,other

functions

?EXP(1)

are availablebyratioingtothe Log:

EXP(X).

Although the

PET

Basic

To calculate the

logrithmic

function is given with the parameter LOG(X) which is logged to base LOGtobase

10,

the expression is written:

LOG(X)/LOG(10)

E.

RANDOM

The random functions are useful for many statistical programs and games. Basic random

an

provided. The random numbergenerator uses

be

The argument can

random number sequence generated

either non-zero, or negative. Positive numbers always return the next valueofthe

by

a numerical algorithm in BASIC. It always starts

value, or seed power-on. However, the seed for the random can'

Repetitive access to the random function

in a program is not random because the relationshipofthe time is predictable from the time that the program is initialized. random number is the first one. A solution

RND(-TI)

give a close

to seed a number sequence, and use

to

theorhetically pure random number for

algorithm which develops a value between zero and one.

be

initializedbyusing the minus value.

So

in a fixed program sequence, the

to

this

is to use the

RND(+1)

statistical

time

to generate random seed, use the

for the numbers in the sequence. This should

analysis and definitely gives a random

functions

with

the same

only

are

truly

sequence for game play.

The RND

number generation sequence. This technique can predictable repeatable sequence can be obtained

of

a minus number is not truly random at all. The parameter is passed as a seed to the random

be

used in debugging programs in a sense that a

by

RND minus tor program development. Suppose in a game program you want to simulate rolling a six-headed die. Initially, you can see the random number generator with the instruction

D~RND(-TI)

Subsequently, you can compute the valueofthe die with

= INTI6'RND(1) +

D

1)

PEEK, POKE:

PEEK contains the memory address in decimal in the number between a and

than hexadecimal

is a function which allows the user to look at any location in the

PET

which to wanttolook at the resultisa decimal

255.

BASIC is currently constructedsothat the contentsofany address greater

COOO

is automatically returned as zero. This is a legal constraint, posed by the

PET

memory. The parameter

company who wrote the BASIC software to protect their copyright.

Example:

POKE

POKE is not a function but is written like a command. It allows the usertodeposit a number into read/write memory. The parameters are specified in a list after the command. The memory address the actual value character A at the first location

To

look at memory location

of

wheretoput the information. It may range from a to 65536. The second parameter is

to

be deposited. It must

25,

the expression is written:

?PEEK(25)

be

between aand

of

the screen memory we would write

POKE 32768,1

255.

110

first

parameter is the

For example,ifwe wanted to put the

Some locations in memory cannotbechanged (ROM) and others should not be changed (BASIC and

system variable

RAMorI/O).

If you POKE the latter,bepreparedtoreset your machine.

30

or

USR

The

USR

is a

function address located at memory location one and programming

FRE

This function

used in an expression,

FRE forces a BASIC

blocksothat

for

tells

they

which

a detailed description and useofthis

you

how

is designedtopass a parametertoa lanuage program using the

many bytes are teft in memory. Althoughitis a true function sinceitcan be

it is normally used in direct mode in the form:

action

can be

called garbage

efficiently

allocated.

two

in the

?FRE(O)

collection.

PET.

function.

This

consolidates

See

the section on machine language

all unused bytes into one large

jump

Several

functions

existtoaid in formatting

data

whenitis

printed on the screen or hardcopy printer.

TAB

This format function places the cursor at the column specified in the argument. The argument goes through the INT routine. The legal range ignored. Note: TAB uses skip characters, not spaces.

is

(}I;;;X",,255.

If the cursor is past the location specified, the tab is

pas

This function returns the positionofthe cursor. The position is reset to zero at each carriage return.

Note: HOME and CLEAR do not affect

POS

even though the cursor is settothe first column.

SPC

This format Legal range

function

is

(}I;;;

X""

prints

255.

Note: SPC(O)

out

the numberofskips

put

256

skips.

NOTES

specified

in the argument (which goes through INT).

31

Chapter

Useofdecision logic in writing programs.

A

may GOTO written with

IF-THEN has three forms:

5.

major

advance in BASIC programming is the

be done in

is written to

two

ways

specify

--

unconditionally

a target

a space between GO

line

number where

and

TO.

IF (condition) THEN (statement)

IF (condition) GOTO (line number)

IF (condition) THEN (line number)

ELEMENTARY PROGRAMMING

ability

with a GOTO

PET

BAS/C

GO TO 50

GOTO 100

to loop

execution

will

back

and

conditionally

will

a/ways branch. GOTO

recognize both forms.

re-execute lines

with an IF-THEN.

of

a program. It

maya/so

be

Conditions are written provides six relational operators:

Until now we have been developing programs which do single familiar with ascending order.

Ifwewantedtotake and print numbers betwenn 1 and 20, their square and square root values on the screen, we could write the linear program as before:

The big disadvantageofthis is that we would havetokeep typing in lines until the 20th line.

UNCONDITiONAL LOOPING

However, with our conceptsofvariables and the additionofa computes values and prints them out The program reads as follows:

Line 10 prints a heading for the columnofnumbers. Itisexecuted

the

10 20 30

as

two arithmetic expressions separated by a relational operator.

<,

>,

=,

<

>,

<

=,>=.

functions

concept that says that first line10is executed, then line 20, and other line numbers in

PRINT PRINT

PRINT

1,1,1 2,2"2, 3,3"3,

10 PRINT

SQR(2) SQR(3)

200 PRINT 20,20"20, SQR(20)

lOOP,

without

"VALUE","SQUARE",

havingtotype such a long program.

SQUARE ROOT"

in serial order.

we can write a program that

only

once.

PET

You

BASIC

should be

201=1+1

Line 20 computes the next numbertouse. The first time

it

never been referenced so

Line30is like lines 10-200ofthe previous program except that the constants have been replaced by a variable.

Line 40 contains a GOTO command which

BASIC stores text lines so

to

quickly

that

line.

limited

to branchingtoa lesser line number but It can branchtoa greater number too. You

interpreter can execution

GOTO is not

hasaninitial

that

examine

a pointertothe next line precedes each line. Using

only

30 PRINT

the

line

value

of

O.

1,1"1,

40

GOTO 20

directs

execution backtostart again at line 20.

numbers, determine

this

lineisexecuted, I has

SQR

(I)

If-

a line does exist, and transfer

this

technique, the

32

will

see a

As before, we

key

is

useofthe

factorof20.

To

stop

programtoresume where it

While address·small

and

with

programming. editortocorrect

CONDITIONAL LOOPING

The IF-TH

THENis

future

pressed. Rapid

the

this

printing

mistake

then

execute,

the

computed

EN

executed.Atestisspecifiedbyputting

= equal

<>

not

exampleofthe

type

RUNtostart

reverse key

loop, press

program

statement

equal

makes

just20numbersonthe

into

the

insteadofprintingaheadingatthe

value.

Luckilyitis

line40to

scrollingofthe

slows

the

program. You

allows

>greater than

<

less

than

>= greater <= less

If

A<B

then

thanorequal

print

"A

LESS THAN B"

conGeptofusing

our

program. The program

screen

the

scrolling

STOP key. When you

left

offorRUNtobegin at

useofthe

Jumpingtothe

most gotoline

youtospecifyacasetotest

to

GOTO, it

should

visibleinthis

20.

memory

rate.

screen. However, before we address

see a

40 GOTO

wrong

case. By

You have

GOTOtoskipaportionofcode.

will

continuetoprint valuesofI

makes

Holding

wanttorestart a program

does

not

common

topofour

placeinthe

now

oneofsix

the

down

the reverse key

the

beginning.

really

helpusto

error and

10

program. We

programisthe

stopping

fixed

the

andifthe

relational

screen

its

the

program we

first

in a

caseistrue,

operators

almost

either

solve

cure. If we retype:

long

until

impossibletoread,

slows

will

most

between

the

scrollingbya

type

CaNTtocause

the

problem

that,

letusintroduce

intersperse

common

can

lifeofprogram bugs.

the

error

use

the

statement

two

expressions.

the

we

the

STOP

tried

heading made

screen

after

but

the

to

a

in

the

If

the

expressionistrue,

expressionisfalse,

what

they

mean,

In

ourexample,

. The IF-THEN

to

limit

then

finally

We can also

restore line

The program

Most

BASIC

vestige to

force

IF-THEN IF-THEN line

lets

the

program and cause

drop

write

40:

will

interpreters

of

when BASIC operated

program

instructions

number

the

instructions

the

program

remember

we can add

us make a varietyofdecisionsatthe

through

theIFstatementtoskip

execute

executiontoend at a

the

through20values and when Iisequalto20,goto

required youtoinclude

have three

where GOTDisimplied.

jumpstothe

that

the

statement:

actionstohappen.Inthis

instruction.

non-interactively

forms:

on

the

arrow

pointstothe value you

40

IF I <= 20 TH

around

351F

specific

The firstisIF

same line

40

GOTO

50 END

an END

point.

The

with

the IF

numbered

EN

time

the

unconditional

from cards. END canbeused

expression

third

line. If

GOTO

case, we

20

statementtofinish

formisIF expression THEN

20

we are

50

statement

you

are in

would

executing

execute

GOTO

liketosee

GOTO. Add

line

the

number. The

are executed. If

doubt

program from 1to20

your

about<and>

less

than

the

program. This

two

new

END

statement.

program. Thisisa

optionallyinPET

followed

allows

lines

second

by a

the

and

other.

us

and

BASIC

is

33

statementtobe

tabletodrawaline

executed

before

between

proceedingtothe

the

10th and 11th valueonthe

32

IF I =10THEN PRINT " "

Expressionsinthis

screen.

form

might

change

our

Ifwetrytoexecute

screen

IF-THEN are when

theZwould

50. However, in

the The IF-THEN

toIimit jump legitimate makesaprogram

becauseofthe

only

placing

notbeexecuted,

PRINT X

plus

and

letsusmakeavarietyofdecisionsatthe

the

program

the

computing

additional

conceptoftesting

DATA ENTRY

Before a

The

more waystoget

computer

program

complex

could

payroll

information

this,

you

statementatline

two;

either

programming

Z =A

willbeexecuted

and

cause

element.

really

work.

program

require

program

into

will

see

thatalineisnow

32. It

should

the

IFX=5THEN

because

the

line

either

IFX=5THEN

if

X =

5.

actionstohappenatvarious

values

The int-9lligent

can

perform

only

simple

might

variables.

that

allows

combinationoflogical

useful

data

such

need rates,

work,ithastobe

hours,

drawn

on

be

the

between

noted

line.

that THEN For

example,

the

logical

statementisexecuted.

50:Z=A

drops

PRINT

the

as YESorNO

throughorexecutes

X:Z=A

time

we are

computertobe usedasboth

and

executing

points.Itis

abletoaccessadata

responsestoa

tax

information.InPET BASIC

the

decisions

tenth

and

eleventh

conditionsoftheIFand

in:

statement

the

program.

conceptofthe

with

gameorsimulation.

valueonthe

This

unconditional

control

repetitive

element

baseofsome

there

Take

care

allows

operations

are

us

and

sort.

A

two

READ AND DATA STATEMENTS

Onlyashort cards program. the

When BASIC began GET parameters

included

program.

allowed

The

syntaxofREAD is the verb followed

time

The

that

ago

with

the

command

running

direct

communication

change

when

there

program.

READ

was

in an

but

notasoften--e.g.

READA,B,C,D

werenotimeshare

Thus,

DATA

statements

designedtopullout

interactive

with

environment

the

BASIC

tables,

by

a

listofvariables into which theDATA

systems,

were

this

through

program.

etc.

BASIC

typed

DATA

READ

could

and

into

variables

timeshare,

has

been

not

accept

scattered

which

verbs relegatedtoinputting

input

throughout

couldbeused

such

as INPUT

is to be read.

other

READ processes DATA statements as they are encountered in the program. DATA statements at and 30 and end

might

be processed

of

lines are considered terminators

by

a READ statement at line

10

DATA2,-53,

IE10

20.

DATA is processed sequentially

and

20 READ A,B 30 DATA 3.14, 1,06E23

Blanks quotes String

and

graphic

are

consideredtobe

data

canbetyped

characters

without

are

automatically

delimiters

quotesifit

for

literal

does

thrown

characters.

not

away

contain

unless

literals.

they

are

surroundedbyquotes.

50 DATA ABC, DEF

than

the

by

and

line

10

commas

The

34

Commas within quotes

will

notbetreatedbyBASICasfield terminators.

60

DATA

",",","

It is also possibletotype mixed alphanumeric and data fields. Numeric fields maybetreated as alpha.

10

DATA 123, ABC,

20

READA,A$,

345

B

It ;s adviseable for the programmertoknow how many data statementshehas put into the machine or

of

use some kind program will index

adelimeter at the endofthe data. Ifitis not done, the data is continuously read, and the

its

way through allofthe data statements. Finally, DATA willbeexhausted and when

the next

READ

is encounteredan?OUT OF DATA

ERROR

will occur. Sometimes you may also see this errorifyou carriage return through READY on the screen

PET

because the

thinks

you already told ittoREAD

Y.

SYNTAX error results when

READ really

and DATA are implemented in the following manner: The

not

partofthe first line data statement pointer is directed command

initiates

a search for a DATA statement token.

There is one other command available

an

attempttoread alpha field into a numeric variable is made.

first

byteoftext contains a zero. This

but

is a dummy line

to

this

byte. Sinceitis

to

the programmer which allows himtoreuse the stored data.

consisting

onlyofa terminator. When

pointingtoa terminator, the

RUNistyped, a

first

READ

RESTORE restart the DATA search back to the beginning of memory. The following program would correctly operate continuously re-reading DATA;

10

DATA

20

1=1

30

READA:PRINT A

10,

20,

30,

40,

50,

60,70

401=1+1 50IF1<8

60

RESTORE

70

GO

TO

THEN

20

30

INPUT

When interactive response to DATA requirements became possible, the conceptofINPUT from the

to

keyboard was introduced. Since the classical input device

statements was

limited

by this device.

BASIC was a

TTY,

the formatofinput

is

Operation

The

sequence.

of

INPUT is considerably enhanced when coupled with the powerful

formofthe

statementIsthe verb INPUT

followed

by

INPUTA,B,

When BASIC editor, characters

After

carriage

interpreted

Leading blanks are supressed, soifyou are

blinking

are

by

encounters

the

allowed

returnisreceived,

BASIC

cursor

up

using

this

for input. Because you are

until

its

Instruction,Itprintsaquestion

under

the

carriage

dataishanded

input

buffer

return is

and

inputting

issued

back

to

rulesofinterpretation.

a string which requires blanks or literals,itis

necessary to enclose the input characters within quotes. The editor

picksuponly the characters between the question mark and the current positionofthe cursor.

35

PET

a variable list. INPUT

satisfies

C

mark

to the screen then

controlofthe screen editor,

as

a

terminator.

BASIC one

character

at

a time. Data is then

screen editor.

the variables in

activates

cursor

the screen movement

This

allows

INPUT data may be delimited by commas as with the DATA statement. When more fields are provided than are actually required, BAStC responds with

and takes only those characters On

the other hand, when not enough data is inputted, BASIC witt respond with

inputofdata

from a pre-eonstructed form on the screen.

?EXTRA IGNORED

it

requires to satisfy the INPUT list.

1?

and begin blinking the cursor againtoget additional

input.

Ifanalphabetic field is encountered during the interpretationofa numeric field, BASIC responds

?REDO

In

PET,ifinput is fottowedbyonly a carriage return withnoother typing, it is considered by BASICtobe a

of

termination teletype BASIC when The stop key is not operative while the

INPUT has a special feature which allows you and in what form they are to be. A literal which follows the input command is printed prior carriage return is typed. For example:

it

would print:

and wait for you to calculate the third legofa right triangle:

10

INPUT "FIRST LEG"; A

20

INPUT "SECOND LEG";B 30IFA=OORB=OTHEN10 40? 50

GOTO

the program, sameasa

this

to

input your birthday in standard numeric formtovalueA.Here isanexampleofIN

"THIRD IS";

10

SOR

was the most convenient wayofterminating a program.

(A'

A +

B'BI

stop

PET

10

INPUT "BIRTHDAY"; A

FROM START

key. This particular featureisa carryover from the days

waits for input.

to

indicatetothe user what input characters are desired

to

the time the

BIRTHDAY?

with

a:

of

PUT

If you run

We

can change our programtose

and changeitto:

ThiS change, when you execute it, witt accept values typed as

acceptable, however, good programming practice protects the user from getting confusedasto how many fields go on a particular line. although to mix alpha and numeric values.

GET A major problem with INPUT is thatitdoes not

grinding halt while the user takes his time to enter some characters and strike RETURN. been equipped if

a key has been pressed.

The command is

this

program and put in values 3 and 4 respectively, you witt get a

howtocombine values on a single line.

10

INPUT "FIRST LEG, SECOND LEG";A,B

3,4.You

it

is definitely not good programming practice,itis

10

INPUT "NAME, BIRTHDAY";

allow

real-time programming. All processing comestoa

with

a special function which will yield one character at a time from the keyboardortell

GET.

GET

is identical in syntaxtoINPUT. It is possibletospecify a listofvariables

A$,

5.

We

delete line

will see that either form

A

20,

PET

list line

is

possible

BASIC has

36

10,

but

generally

single key closure. When a

only

thisisnot a good idea because

numeric keys

will

be accepted as input. All

numeric

the

purposeofGETisto

value is specified

GET A

others

?SYNTAXERROR

will

cause

scan

the

message:

the

keyboard and return with a

Useofthe

Otherwiseitwill

The

most

will

have a null value (length =

that

was pressed. See

GET

callsaroutine

contains

Since

queueisempty

The

following

In

this

numeric value is

have a value 1·9 for keys

desireable way to use GET is with a

a valueofnullorzero.Ifthere are characters,

the

lengthofqueueis10 characters,

when

routine

case,

""is

a literal

the

which

waiting

will

confusing

0);

next section examines

for

wait for a keytobe pressed and exit

which

contains

becauseifno key

1-9.

string

otherwise

the

a response. This is

the

string

for

a detailed

keyboard

calling

10

GET A$

20 IF

A$=""THEN

no characters and is a null string.

NOTES

has

variable. If a key will

contain

explanationofhow

interrupt

GET 10

particularly

buffer.Ifthe

the

firstistaken

times

been struck, the value returned is zero.

has

the

character

strings

bufferisempty, outofthe

In a

loopisa

useful In interactive

only

with

the

10

not been pressed,

corresponding to

work.

queue and returned.

good

waytoinsure

games.

valueofa key closure:

the

variable

that

string

key

the

37

Chapter

We with alphanumeric data. BASIC has a set may single variable.

PET

All

6.

have been describing numeric functions primarily, but almost any useful program also has to deal

be

expressedasa continuous connectionofcharacters whichisviewed by BASICasthe

BASIC, has a $ notation whichisusedtoexpress variables which are stringsofalphanumeric data.

of

the rules which applytonormal variables applytothe string variable.

ADVANCED PROGRAMMING TECHNIQUES

of

functionstodeal with these data. Also, all alphanumeric data

valueofa

Following the naming conventions, we can create a variable A$ not equal Type

A$="NOWISTHE TIME" and PRINT

of

string ali that can in

a string is an that is used is

We A$ formatted

including those that form control characters such as cursor down, and carriage return. This will

illustrated soon.

BASIC allows string expressions output device which accepts more than characters becauseofthe sizeofthe input buffer. This problem canbehandled by breaking strings into substrings before they are input individual characters can then be recombined into the original stringbyconcatenation.

a lengthupto

be

entered on a line. However, the limitation on the numberofcharacters that canbestored

255.

1/0

device and the constructionofdata is accomplished by the concatenationofstrings. The operator

+

can modify the expression A$ which we have been developing by typing

and you can see that the literal we typed in had a space at the beginning. Unlike numbers Which are

by

BASIC,the valueofthe

about70characters, dependingonthe numberofcharactersofthe line number--

You

can build strings larger than canbeentered. The accumulationofcharacters from

up

or

by using GET to input each character individually. The substrings

A$to

show the valueofthe string. This technique can define a

literalistaken literally. A string can contain

to 255 characters long. These canbeoutputtothe

79

characters. Input, however, is usually restrictedto79

to

A% and not equaltoA.

A$=A$

+"

FOR

ALL". Print

aJl

combinationsofbits

screenorto any

be

or

COMPARISON OF STRINGS

The ASCII table is definedinFigure

represented when two strings are compared. Characters within a set

to

the leftmost character

Using the ASCII table, we can compare a string containing

The

position. A string containing a blank

string are compared in sequence with the first unequal character defining Thus the same relational functions may

"A"

result is that the second string is greater than the first.

is less than the string

the endofthe field specified.

is

2.6.

It contains the order in which characters within

less than a

"ABC"orany string containing

"1",

whichisless thanan"A",

be

used for both strings and numbers.

an

"A"toone containing a

"A"

< > for unequal

=for equal

< for less than

> for greater than

be

Immediately the string comparison feature can

check

a telephone directory.

file or atelephone directory. Comparisons can alsobeusedtosearch ordered

appliedtohelp you construct ordered

38

the

PET

of

strings are compared starting at

"B"

in the same

whichisless than a

as the

the

first

character. All characters

relationship between the strings.

lists

such as a

lists

such as a file or

"8".

are

The

Try the

NUMBERS AND ASCII CODES Two complementary pairs

into character strings.

STR$ STR$ is a

numeric expression:

Positive numbers are preceded by a blank in the STR$ equivalent. Negative numbers have a sign in the corresponding position.

following

10

INPUT A$

20

INPUT 30IFA$=B$ 40

IF A$ 50

PRINT

functionofone

10X=3.1

20

?STR$

RUN

3.1

READY

(X)

program to develop a feeling

B$

<$B

"A$

THEN?

>B$":

"A$=B$"

"A$

GOTO 10

of

operations on

argument. It returns a string that is the character representationofthe

<B$"

GOTO

for

10

strings

sequences and

and numbers

matching

allow

functions:

us to put unconventional

things

VAL

is

the

VAL the first non-blank character

complementofSTR$.1t converts a string to a number which may be used for computations. If

of

?V

AL("Z")

the string is not numeric, then the value of the

function

is zero.

o

READY

On

the

other

hand, VAL will convert as many

?VAL ("3.14

3.14 an

VAL is REDO

CHR$ We

INPUT, but some devices require control characters example, a return characters by giving the ASCII code number. CHR$ is a ASCII representation. The value

excellent

from START.

have shown that strings may be assigned printable ASCII characters through either literalsordirect

with

10

A$ = CHR$(65) + CHR$(66)

20

PRINT A$ RUN AB READY.

AB")

functiontouse with INPUT sinceitcan preventaninexperienced user from causing a

PET

printer uses

no tine feed when it performs overprinting. CHR$ allows you to specify such control

shifted

of

the argument

digitsasit

carriage return as a specialterminatortoindicate a carriage

must

can uptoan

which

cannot be produced by normal means. For

functiontoconvert a number

be O=X< =255.

invalid character.

into

internal

In

the above examples,65is the ASCII code for

before concatenating them and printing them

"A"

and66is a

out.

39

"B".

We

converted the codestocharacters

ASC

turns

a character into

parameter is a string.

?ASC("A")

65

an

ASCII code number which maybeused in numerical calculations. The

If the string

of

the

?ASC("123")

consistsofmultiple

string.

characters, then

this

function

will

return

the

code

tor

the first

character

49

The ASCII

SEGMENT OF STRINGS

In

many casesitis

problem whereinresponse toanINPUT, a person's name is typed in. It might consistoftheir middle intiHal, and last name. It is important that for sorting, however, that not all Johns that the list

In

ordertobe abletoseparate partsofstrings functions. analyze pieces once as

convenience.

LEFT$, RIGHT$, and MID$

The

specified. is negative,orzero,orgreater than 255, then an ILLEGAL QUANTITY ERRORisprinted. RIGHT$(STRING

VARIABLE, I)gives

to zero,

or

greater than 255, an ILLEGAL QUANTITY ERROR is printed.

code

for

"1"is49.

desirabletoaccess

be

orderedbylast name.

Mostofyour programming

ofaconstructed

they

are

essentially

function

specified

If I

the

rightmost

just

partofa

and use

with

strings

string.

the

same

as LEFT$(string variable, t) gives

"I"

We

will

function.

charactersofthe

Three

string

them

will

consistofusing oneofthese

present

the

combinations

string expression. When

in developing an ordered list. Consider

first name,

be

together, but

in expressions, PET BASIC provides three

three

functions

useofthe

the

functions

are provided

leftmost

"I"

characters

"I"

and

mainly

is less

define

all three at

for programming

of

the

string

than,orequal

the

to

There are expression gives

length ILLEGAL QUANTITY ERROR is printed. For the characters from

The second expression is MID$(STRING VARIABLE, the

lengthofthe

returned. If zero,

of

All

of

characters,

the

string.

To

find the last

characterofthe

function.

LENGTH OF A STRING

The LEN

two

expressions

"J"

of

the

string,

"I"

or

greater than 255, and ILLEGAL QUANTITY ERROR is printed.

these variables

function

then

"I"tothe

string.

is greater than the lengthofthe

right

numberofcharacters,ora given

name

string

givesanexact

for MID$. The

characters from

this

will

give a null

endofthe

All the characters

combined

from

until

will

our

previous example, we can analyze characters

the

first

countofthe

first

most

the

string

define

blankisencountered.Toimplement

starting

string.Ifeither

"J"

greater

string are returned.

starting

string,

a new

function

numberofcharacters

general one is MID$(STRING VARIABLE,

with

the

"r"th

character. If

"I"

or

"J"

than

the

numberofcharacters

I)

whichisthe

in the

"I"

then a null

which

numberofcharacters

same as

position

string

allows

contained

negative,orgreater than

specifying

until

the

is returned andif"I"

us to

starting

this

"I"

is greaterthan the

leftinthe

a

"J"

endofthe

take

either

starting

program we need one more

in a

the

at a given

from

the

string.

40

I,

J). This

255,

string,

greater than

string

are

is negative,

left

number

position

rightmost

Non-printing

an all

of

characters and blanks are all counted as partoflength. Strtngs are stored in BASIC with a 3·byte vector. Two bytes are a pointer

the string We

can now write a general purpose programtoextract the last name from a full name.

is

stored and the

10 INPUT"NAME:FIRST,

third

byte Is the length, the

MI,

LAsrJA$

LEN

function extracts

to

the location In memory where

this

byte.

20I=LEN(A$)

30IFmld$(A$,I,l)=" "THEN

60 401=1-1 50IF1>0 60

PRINT

Two variantsofMID$ are used here. Line30uses the case where a length is specified as the first

parameter. specify a length

STRING

Strings are storedinthe

As

each new stringisadded, a chain grows downward from the-topofmemory. Storage Is optimized by never creating a copy the string points executed, both concatenation

A

LARGER

Using the string functions described thus far we can write a routine which will shuffle a deckofcards for us

and deal them

bridge. Note use

GOTO

"LAST

We

are using a lengthof1tosearch for the blank delimiting the last name. Line60does not

in

STORAGE

to

AS

or

EXAMPLE

30

NAME

the

where

and

INPUTisexecuted.

OF

out

one at a time. The following routine has applications In many games like poker or

of

the

= "; MID$(A$,

MID$. Everything beyond the positionofthe

space between

the

literal occurs in

BS

will

share the same copyofthe string. New strtngisrequired onlyIfa

STRING

PET

graphics card symbols:

FUNCTIONS

1+1)

the

endofyour BASIC program and the highest RAM locations.

of

a string assignedtoa literal. In

text

In memory.

likewise,ifan

blankistaken.

this

expression

case the vector for

AS

=

BS

is

109

REM

SET

UP

DECK

110

Ci="A.2.3

120

Ci=C$+"A.2.3~.S%.7.a.9.T.J.QIfI(."

130

C$=C$+"A+2+3+4+S+6+7+a+9+T+J+Q+K+"

140

O=C

190

REM

200

R=2*INT(LEN(Ci)*RND(I)/2+1)-1

201

Ni=MID$(Ci,R,I),Y$=MIDi(Ci,R+l,l)

430

REM

432

433 43S

C$=T$+MID$(C$,R+2)

439 440

4S0

4SS

460

470

$+"

PULL A

SHRINK

IFR}ITHENT$=LEFTi(Ci,R-l)

T$=""

REM

PRINT

PR

INTNi; IFLEN(Ci)}=ITHEN200 REM

END

INPUT"ANOTHER

GOT»10S

....

S*.7.a.9.T.J.Q..,;."

A+2+3+4+S+6+7+a+9+

CARD

THE

DECK

A

CARD

H, OF

DECK

DEAL

READY.

The string

rank. As a card is dealt, N$ contains the rank and time so

Statement of

cards.

assignment is

C$isinitializedtocontain a deckofcards. Two characters represent each card; the suit and

that

unique cards are always dealt.

105

clears

C$ls

too

the

screen. This is done just for show so that the program can illustrate the dealing

initialized in

large to

stateme~ts

fit

on one line.

110

through

WITH

Y$

ALL

_";2$

contains

140.

C$ls

S2

CARDS

1+J

+Q+K

+"

,GOT043S

the

suit. The

concatenated because the literal

deck

string,

CS,

shrinks each

41

Statement 200 uses In

201

the

index is used to pull N$(rank)Y$(suit) from C$ by

RNDto

generate an,index

into

C$.

The random index is in

the

MID$

function.

the

range 1to LEN(C$)

-1.

432 through 435 removes

argumentofLEFT$

440

prints

each card for our program asitis pulled. 450

cannot

the

be zero,

usertoreshuffle.

USER DEFINABLE FUNCTIONS

this

point

To

used

to approximate simple functionsintermsofother

A

functionisdefined

100 110 120

The

nameofthe letterora Thus the

we have covered all

many

more

certain

expression

might

INPUT B

INPUT C

DEF

FN

A(V)~V/B+C

functionis"FN"

letter

followed

following

functions

become

in a DEF

byaletterordigit.

are

valid

functions.

function

FNX FNJ7

FNKO

FNR2

card from

the

in that realm,

the

R>1

functions

string so

test in 432 prevents an ILLEGAL QUANTITY ERROR.

intrinsictoBASIC. Those

especially

in lineitbecomes very

unreadable. Fortunately ,the

statement:

followedbyany

names:

legal

thatitwill

tests

trigonometric.

tedious

not

be dealt again. Since

for

the

endofthe

While one

and from a

facility

variable name. Recall

deck

familiar

documentation

exists

in PET BASIC to

that

the

second

and 460

with

could

mathematics

write

allows

code to

standpoint

a variableiseither

the

are

a

define

a

The

most

severe

one

line

(BD-characters).

The

variable in defined are are

After

to be consideredtobe used

in the evaluation.

the

funtion

(imitationofuser-defined

parentheses

any

expression

definijion

example:

130

Z =FNA(3)

140?Z

When

the

DEFFN characterofthe with

the

name are where the be

definedintermsofother

dummy

spaceisconsumed

6.1

Figure

shows

statementisexecuted,asimple

name

two

variable is stored. The

by recursion.

some user-defined

String

global

has

pointers:

functionsisthat

functions

follOWing the variable

butitmay

(have the

cannot

have

same

be defined.

nameiscalledadummy

only

one

argument.

value as in the restofthe program),

has been executed, a user defined

variable

bit7.the

most

significant

an addressofthe

bit,

settoindicateitis a

text

where

codetoexecuteafunctionisre-entrant so

DEF FN. An

outofmemory

functions

which

error

are readytobe used in

they

mustbecontainedintheir

variable. A

Other

variables

function

can be used as in the

entryismadeinthe

function

the

function

will

occurintime

is stored and an address

PET

entirely

function

used

in the

expression

and

their

current

variable

table.

name.

Associated

thatafunction

as the available

BASIC programs.

on

may

be

values

following

The

first

of

may

stack

42

FUNCTIONS EXPRESSEDINTERMS

SECANT, SEC(X)

OEF FNA(X) = lICOS(X)

FOR X < >n/2

COSECANT, CSC(X)

OEF FNB(X) = l/SIN(X)

FOR

X<

>0

CONTANGENT, COT(X)

DEF FNC(X) = COS(X)/SIN(X)

FOR

X<

>0

INVERSE SINE, ARCSIN(X)

FND(X)

=A1N(XlSQR(-

FOR ABS(X) < 1

INVERSE COSINE, ARCCOS(X)

DEF FNE(X)=

-A1N(X/SQR(-X'X+l))+rr/2

FOR ABS(X) < 1

OF

BUILT·IN BASIC FUNCTIONS

X'X+1))

INVERSE SECANT, ARCSEC(X)

DEF FNF

(X)

= A1N(SQR(X'X

-1))

FOR ABS(X) > 1

INVERSE COSECANT, ARCCSC(X)

DEF FNG(X) = A1N(I/SQR(X'X

FOR ABS(X) > 1

INVERSE COTANGENT, ARCCOT(X)

DEF FNH(X) = -

A1N(X)

+rr/2

FOR ANY X

HYPERBOLIC SINE, SINH(X)

DEF FNI(X) =

(EXP(X)-

EXfl(-

FOR ANY X

HTPERBOLIC COSINE, COSH(X)

DEF FNJ(X) =

(EXP(X)+EXP(-

FOR ANY X

HYPERBOLIC TANGENT, TANH(X)

DEFFNH(X)

= -

EXP(-X)/(EXP(X)+EXP(-

FOR ANY X

+ (SGN(X)

-1))

X))/2

+ (SGN(X)

-1)'rr/2

X))'2 + 1

HYPERBOLIC SECANT, SECH(X)

DEFFNL(X)

= 2/(EXP(X) +

EXP(-

FOR ANY X

43

X))

HYPERBOLIC COSECANT, COSH(X)

DEF FNM(X)~2JEXP(X)-EXP(

FOR

X<

> 0

HYPERBOLIC COTANGENT, COTH(X)

DEF

FNN(X):

FORX<

INVERSE HYPERBOLIC SINE, ARCSINH(X)

DEF FNO(X)~LOG(X + SQR(X'X +

FOR ANY X

INVERSE HYPERBOLIC COSINE, ARCCOSH(X)

DEF

FNP(X):

FOR X > : 1

INVERSE HYPERBOLIC TANGENT, ARCTANH(X)

DEF

FNQ(X):

FOR ABS(X) < 1

INVERSE HTPERBOLIC SECANT, ARCSECH(X)

DEF

FNA(X):

FOR 0 < X < : 1

EXP(-X)I(EXP

> 0

LOG(X + SQR(X'X

LOG((1

LOG«(SQR(-X'X+1)+1)IX)

+ X)I(l -

-X))

(X)+EXP

1))

-1))

X))12

(-X))*2+

1

I

INVERSE HYPERBOLIC COSECANT, ARCCOSH(X)

DEF

FNS(X):

FOR X < > 0

INVERSE HYPERBOLIC COTANGENT, ARCCOTH(X)

DEF FNT(X)=

FOR ABS(X) > 1

GOSUB·RETURN

We have seen how to use the intrinsic function. The major limitation of DEF FNisthat it can consist of only a single algebraic expression anditmust fit

Often several lines of code will be repeated through a program. These program lines can be collected

one place and executed by a GOSUB command:

The lines of code are called a subroutine. GQSUB meansgoto the subroutine. It differs from GOTO that GOSUB remembers at which line numberitwas executing before the GOSUB and can return automatically to the following line after executing the subroutine code.

on

one line.

DEf

LOG«(SGN(X)'SQR(X'X +1)+ l)IX)

LOG«(X+1)I(X-1))12

FN

to create a single variable function which can be used like any

GOSUB 5000

in

A subroutineisstored as a series of linesinBASIC starting at the line number specified by the GOSUB. The last lineofthe subroutine must be a RETURN statement. This tells BASIC you want to resume executing the mainline code after the GOSUB.

44

Example;

10

REM

20

GOSUB

30

STOP

50

REM

60

RETURN

THISISTHE MAINLINE CODE

50

THISISA SUBROUTINE

If we could take a snapshotofexecution, we would see the lines executed in

10·20·50·60·30

Five bytes are pushed onto

line

the currently

number and

eX8cI,Jting

number and

text

text line number and the GOTO routine handles

address from

the

stack

when a GOSUB is executed: a GOSUB token, and

addressofthe

the

stacktoresume mainline execution. All

GOSUB. The line number following the GOSUBisstuffed

the

branch.

this

RETURN

FOR

order

two

bytes each for

restores the line

entries in frontofthe

GOSUB entry are also eliminated. The physical little

stack

Example

limitation

space left.

of

SUbroutines

on the numberofGOSUB'sineffect at one

time

Is 23. After

this

many there is very

Consider the factorial function:

1= 1 x 2 x 3 x

n

You

cannot define

simple routine

10 100 110 120 130

140

The

routine on lines 100·140 couldbeused many times during a program using different values for N. For

...

xn

this

function with the DEF

to

find n! for any given n (up

INPUT N

l=l:NF~l

NF~NF'I

1=1+1 IFI

<=N

GOTO

PRINT NF

110

FN

command.Onthe other hand, you can use

to

34). (NF denotes n factorial)

the

example, suppose you want a binomial coefficient:

into

the

following

The

program would

10

15

20

30

40

50

60

70

100

110

120

130 140

m!

r!

(m

r)!

be

PRINT PRINT"R= N=M:GOSUB100:X=NF N=R:GOSUB100:Y=NF N = M - R:GOSUB100:Z = NF

BC=X/(Y'Z)

PRINT BC END l=l:NF=l NF=NF'I

1=1+1

IFI RETURN

<=N

"M

=";:

";:

GOTO

INPUT M

INPUT R

110

45

TYPE

RUN

for the values

RESULT IS 462

M=11

R=6.

SUbroutines

to

used results. For example, in the subroutine on lines 100-140, the variable N is input and the variable NF output

When we make N equaltoM,A,and Of course, function such as printing a special form on the screen:

NESTED SUBROUTINES

The subroutine on page the binomial

The subroutine, denoted SUB

act

like a

"black

input

the

data and other fixed variables (or sometimes the same variable) are usedtooutput

as shown:

N

-+

some subroutines do not need inputted variables as they might just perform a specified

coefficient.

box"

or complex

I SUB100

NF=N!

SUB

6-14

itself

could be used as a subroutine in a program that repeatedly calculates

Merely change line70to

10,

beginning on line10and ending on line70has the following structure:

1-+

M-R

respectively, we get

1-+

function

NF

print form

70

RETURN

within the program. Certain fixed variables are

NF

equaltoMI,

A! and (M-R)!.

the

is

BC

~

M-+G

SUB10

-+

BC

(~)

R-+

N + t

NF

B

Subroutines that are used by other subroutines are called nested subroutines.Inthis nested in SUB20. Many programs have subroutines nested in subroutines in nested subroutines

limitisthe amountofmemory available.

only Subroutines can also

accepts specific characters from the keyboard, on the screen, subroutines.

CAUTIONS

A common error in using subroutines istoallow and result in a RETURN WITHOUT GOSUS

mightbecalled on many

be

nested in more than one subroutine.

prints

timesinthe

ERROR.

main program

a mainline executiontofall into a following subroutine

Put a STOP or END statement in your codetoprevent

An

input subroutine, for example,

a winking cursor, and prints the given characters

itself

and also in various other

case, SUB100 is

...

The

that

46

this

10

GOSUB RETURN

20

15

10

END

GOSUB

20

RETURN

Sometimes, you just once, then memory space. On the other hand, subroutines are incredibly powerful programming

to

structure your program into blocks.

might

it

have a tendencytomake everything

into

asubroutine. If agiven subroutine is used

should be incorporated into a program whereitis usedtosave execution

tools

and allow you

FOR-NEXT LOOPS

FOR-NEXT

with a single statement.

The

endofthe"loop is

simplifies

the

writingofBASIC programsbyallowing

specifiedbythe

FOR 1= A

statement

TO

B STEP C

one to

specify

complex

loop structures

Exiting a FOR-NEXT loop via a branch wllf leave the FOR to

assign

We

have seen how repeated operations canbeperformed using a counting variable such as I in the

the

maximum

may

result in NEXT WITHOUT FOR errors.

permitted

limit

to the variable then

as longaseach loop uses a unique variable. Useofidentical

entryonthe stack.

exit

the loop through a NEXT.

The

best way to handle thIs is

routine.

10

1=1

20

1=1+1

30IFI<=10THENGOT020

time

and

loop

In

this

case, any routine appearing in lines 21-29 will be repeated10times.Inaddjtion, the variable I will

have values which range from

1to10

in increments of

1.

This looping process can be genralized in the case:

101=A 20

I=I+C

30

IF

1<

=B

THEN GOTO

The valuesofI

Since

this

10

20

I

is

the counting variable, A is the initial value, B is the ending value, and C is the increment.

A,B,C may not

10

FORI=A(2)

On

the other hand, the counting variable can

subscripted

10

REM

20

NF= 1

30

will

range from AtoB in incrementsofsize

process is cumbersome to use, BASIC also prOVides you with the FOR-NEXT statement:

FOR

I= A

TO

B STEP C

+ 1TOJ'2

1(1,4).

When the increments areofsize

COMPUTATION OF FACTORIAL

FOR

1=1

TO

N

20

but

STEP-1

they can

be

C.

be

any valid arithmetic expression

any floating variable

1(C=1)yOU

need not include the

but

cannotbeinteger (1%) or

STEP

in the program.

47

40 50 NEXT

Note

how

computing

NF=NF·I

much shorter and more clearly

program

written

without

FOR·NEXT.

this

routine

is written comparedtothe

same factorial

Whenever a FOR is executed. madetoseeifthere afe any entries alreadyonthe and all program wh)ch each

NEXT to

GOSUB execution for

other

time.

matches

that

point.

a FOR entry

FOR entries

jumps

the

If no

that

outofthe

most match

also

puts

cannotbematched,

8 16·byte entry Is pushed

that

were made

afteritare

middleofa FOR

recent

stack

entryorthe

variable

stack

eliminated

loop

again will

onto

for

the

specified

is found, a NEXT WITHOUT FOR error

a 5-byte entryonthe

When all

stack. When RETURN Is executed,

the

FOR entriesonthe

the

same

stack.

from

not

Before

loop

the

thisisdone,acheck

variable.Ifso,

stack. Thisisdonesothat

use up 16-bytesofstack

as a parameter and resets

occurs.

the

stack

have been searched, a

that

stack

is

FOR entry

a

space

the

stack

is searched

pointer is

left

on

entered RETURN

a GOSUB entry. This assures

but

never existed,

eliminates

the

GOSUB

RETURN

stack

~hat

If

will

you GOSUBtoa

still

be abletofind

sectionofcode In

the

most

entry and all FOR entries made

which

recent GOSUB entry.

after

the

GOSUB entry.

a FOR

loop

Is

NESTED FOR·NEXT LOOPS FOR-NEXT loops, like subroutines, can be nested, That is, a FOR-NEXT lOop may be contained in another

and so on. When doing so, it

is

important

nottouse

the

same

counting

variable as

this

will result in ?NEXT WITHOUT FOR ERROR 10 FOR 15

PRINT

1=1

"I"

TO

10

20 FOR J = 1 TO 10 25 PRINT 30

FOR 35 PRINT 40

K=1

"K"

TO

10

50 NEXT 60 NEXT

Lines 40-60 correspondstowhich FOR. the

counting

the

NEXT.

of

the

above example are

variable used in

40

NEXT K

50

NEXT J

confusingatfirst

Optionally

the

one may

corresponding FOR but in no way isitrequired by BASIC to execute

60 NEXT I

PET

BASIC will also

allow

40

youtowrite one NEXT

J, I

A NEXT WITHOUT FOR error will result, however,

It is

interesting,

however,tosee

how

compact

the

expressions can be when they are nested.

glance because one cannot

specify

a variable

that

terminates

if

you are careless in

notation

appears and

48

tell

following

NEXT. The variable refers

all three FORs at one

specifying

how

the

powerful

which

orderofK,J,1.

the

FOR·NEXT

to

Some You For example,

will

Similarly, when you exit a FOR·NEXT loop using a branch, you should assign the counting variable the end value and then exit

instead

hints

may change

compute

the

of

the

valueofthe counting variable during the looping sequence.

10

FOR

1=1

TO

8 20X=X+1 3OIFI=7THEN 40

PRINT X

value

X=1+2+3+4+5+6+7=28

the

loop via a NEXT statement. For instance, you should use

10

FOR

1=1 20IFFNA(I)=OTHEN 30

NEXT:RETURN

10

FOR

1=1 20IFFNA(I) 30

TO

=0

1=8

10

THEN

1=10

RETURN

SUBSCRIPTED VARIABLES

Array variables need

less than

(number

elements. SUbscripts start at 0 and go uptothe maximum value

Limits on the numberofdimensions and sizeofa dimenslon are determined by sizeofmemory available and space available on a array elementsto256. Each array element requires at least 5·bytesofstorage.

If a single dimension array requires more than

first

the

Example: List

10 elements.

of

elements in each dimension) +1

A(0,0)-------A(0,8) A(9,0) A(9,8)

reference. Otherwise, a REDIM'ED ARRAY error will occur.

of

1 $100 2 3

4

5 6 7 $ 5 0

8

9 $

not

be declared with a DIM statementifthey

The

total

numberofelementsinan

line following a DIM.

account balances

$135 $57.86

<$

9 8

7>

$22

<$

6 3 >

<$

2 1 >

21

hav.e

only one dimension and contain

array can be computed by

by

the other subscripts. Thus

PET

BASIC restrict the total number

10

elements, the DIM statement mustbeexecuted before

A(9,8)

multiplying

contains

of

(9$1)*(8$1)

the

Suppose we needtowrite a simple program which allowed youtoINPUTanaccount number and a

to

transaction and keep a running total on each account. We could refer

49

each account balance as A1,A2,

A3, A4, A5,

10

20IFI=1THENA1=A1+C

30IF1=2THEN

etc.

etc. This is acceptable but would require a

INPUT "ACCOUNT, CHARGE";I,C

A2=A2+C

lotofparallel logictoaccomplish

the

summation

This list can be stored in a single variable which is actually a values and program can be reduced

10

20

30

The listwehave represented has 9 rows and 1column. Thusitis a 1dimensional array. A multiple column table can also

an

individual value is accessedbyan

to:

INPUT "account, charge"; I,C A(I)=A(I) GOTO

+ C

10

be

represented. Thisisa two dimensional array.

Account # Balance

1 $100 1 2 3 4 <$987> 5

6 <$63>

7 8 9

$135

$57.86" 1 $22

$50

<$21> 1

$21

index. The index we can use is the account number. Our

#oftransactions

listofvariables. This list isanarray of

1 1

1 1 1

1

To

Our table has 9 rows and 2 columns. and column index

In

ordertouse such a tableina BASIC program, you must provide a statement, to describe the number

rows and columns contained in

Such a description is a DIMension statement. For our table

Let us rewrite our program to update the column containing the numberoftransactions

10

INPUT "ACCOUNT, CHARGE";I,C

20

A(I,1)=A(I,1)+C

30

All

,2)

40

GOTO

Now suppose that account had a balance and each balance had a number sheetsofpaper on topofeach other and refering to each sheet by number.

= All

10

of

whereitis contained. For example, the quantitydenoted by a * is in row3,column

the

,2)

+1

we

had a table for eachof5 companies and each company had 9 accounts and each

access a certain entry position, you must specify the row index

array variable.

of

9 rows and 2 columns we could write

A(9,2)

DIM

of

transactions.

We

can describe

thisaspiling

1.

of

50

1

~ ~

2

~

3

~

We

have created by

matrices usedinmathematics. In

mathematics, a vectorisan

The above vector has n components and is called a vector

For example,

~(3,9,2)

is a vectorofdimension

Order is imporatant here since

w~(3,2,9)

W=Fv.

Vectors can be stored in memory using subscripted variables. These variables are used in the same way as the variables

variable or return a zero

this

example a multi-dimensional subscripted variable. These arrays correspond

3.

we

have seen so far

or

"""

ordered collectionofnumbers:

of

dimension

if

-X,

1%,

A$,

etc. That is, they call whatever value is stored in that

null (" ")ifthe value has not been previously specified.

n.

to

Like vectors, subscripted variables have the power notation. They are especially useful when combined with FOR-NEXT loops as the next example shows.

Example: Dot Product

The dot product

For example, in the four dimensional case,

and

w=(w"

Then

v·w

=

Suppose

Then a programtocompute the dot product v • w might look like

we

v=(5,6,7,11,4,6,11

w =(9,5,2,1,0,3,2)

FOR I = 1TO 7:READ V(I):NEXT

FOR I FOR I FOR I

of

two vectors v & w is a vector, denoted byv.w,

,

w

w~)

2

J

(V1

X

W"

V

2

had

= 1TO 7:READW(I):NEXT = 1TO 7:VW(I) = V(I)·W(I):NEXT

X w

,

V3

2

X w

,

V4

3

=1TO 7:?VW(I):N EXT

X

if

w~)

execute a large numberofoperations using a single

whose ith component (v.w), isv,x

51

I I I

W,.

DATA 5,6,7,11,4,6,1 DATA 9,5,2,1,0,3,2

SUBSCRIPTED STRING VARIABLES

It

was mentioned previously

decimal: A(I) integer:

A

%(1)

that

subscripted variables can be

siring: A$(I)

Subscripted string variables are extremely useful as showninthe next program which prints a bar graph of the U.S.

GNP

from 1966 through 1974.

GROSS NATIONAL PRODUCTS

(IN S BILLIONS)

The program listing

READY

10 SPACE $ 20 30 40

50

100FORI=OTOB

110 120 PRINT "R"LEFT$(SPACE$,Y)A$(S·(X 130Q"

66

67 68 69

10 $

11

12

$

753

$

796

$

869

$

936

982 $ 1063 $

1111 13 $ 1307 14 $ 1413

is:

="

FOR

1=1

TO7:READ A$(I):NEXT

FOR

i = 0

TO8:R

"'AD

V(I):NEXT

PRINT"'O SPE:(8)"GROSS NATIONAL PRODUCTQ"

PRINTSPC(12)"(IN $BILLlONSjQ"

X~V(1)145:Y=INT(X)

Y))

PRINT"

C R"STR$(66 +I)" $"STR$(V(I))"Q"

140 NEXT

200

DATA"l!!!!I"

210

DATA753,196,869,936,982,1063,1111,1307,1413

"I!!!!I"

"!!I"

'1"

"I"

")", , , , , ,

READY

52

The subscripted values A$(O),

A$(1),

...

A$(7) give accuracytothe graph by printing these graphics:

V(O),

V(l),

...

,V(B)

are the GNP's for eachofthe 9 years. The subscripted strings

string

A$(O)

A$(l)

A$(2) A$(3) A$(4) A$(5)

A$(6) A$(7)

THE HEADING

is printed in lines

prints out each bar and line

STR$(V(I)).

50and60and then a FOR·NEXT loop on lines

prints

null(by

default)

I

ASC

165

180

•

181

•

161

182

-

GROSS NATIONAL PRODUCT

(IN $BILLIONS)

'100-140

130

prints a cursor up and then the associated year, STR$(66 +I)and GNP,

prints out the eight bars. Line

170

167

(R)

120

Each bar is madeupof formula

Y= INT(V(lV45,

Y reverse field spaces and the string A$(B*(X·Y)). The Y is determinedbythe

=INT (GNP/45)

Here,45is purely a scale adjustment. The proportionsofthe bars remain the same when values other

45

than

Fine tuning on the bar length

Here B*(X·Y)

decimal part.

DIMENSION STATEMENTS

When using more than10subscripts for any variable, a dimension statement must be given. It takes the form, DIM redimensioned been used, a ?REDIM'D ARRAY in memory for the given number programming sense, therefore,

Because the variables are divide in storage between arrays and simple variables insertionofan additional simplevariable Is a storage area

the simple variable can be Inserted at

are used.

A$(8'(X-Y))

will

A$(K),

mustbeblock

is

accomplished using the subscripted string variable

range over the decimal values 0 through 7.99

where K is the largest subscriptofA$

without

a CLR statement or when a dimension statement appears after the variable has

ERROR

of

to

bit

more complicated once an array has been defined. First, the entire array

moved upwardbyseven bytes and the pointers adjusted upward +7.Finally,

occurs. When a dimension statement is made, space is reserved

variables, including the variable whose subscript isO.It is good

begin subscripts at 0 and not

the

endofsimple variable storage.

used in the program. When variables are

...

9 but A$ automatically truncates the

1.

53

If

large arrays assigned. much execution simple variable Is defined. The best strategy assign a valuetoall known simple variables brfore assigning arrays. This

will

optimize execution speed.

are

defined and initialized first before simple variables are

time

can be lost moving the arrays each

to

followinthis

caseisto

time

a

Functions

CLR

NEW

of

new

and CLR on data pointer:

String pointer equatedtotopofmemory

to

Data pointer Endofarray tabletostartofvariables Endofsimple variablestostartofvariables

String pointer equatedtotopofmemory Data pointer Endofarray tabletostartoftext Endofsimple variablestostartoftext

of

Start

variablestostartoftext

startoftext

to

startoftext

·1

+3

54

PRINCIPAL

POINTERS

INTO

PET RAM

g:

256 • PEEK(

• PEEK(

at initialization

1024 1025 1028

123

122 144 124 126

~

'"

o

'"

-

m

-

x

-

000

145

c.

!!?

'"

~

!!?

m

3

m

"

-

125 127

~

'"

o

'"

-

<

'"

~.

~

m

'"

m

c.

"

o

-

<

~.

~

m

'"

129 128

m

c.

"

o

-

'"

~

m

131 130 132

~

'"

8-

~

~.

<C

"

133

o

-

'0

o

-

3

m

3

o

.:<

'"

8192

typical program

BASIC

statements

1025

variables

1092

Figure 6.2. Principal pointers Into PET

arrays

1113

RAM

1175

strings

8184

Chapter

As

side

As outlined PET

contact identification convention for

7.

indicated in Figure

of

the

PET

that

in

Figure

7.1,

there are four connectors provided, accessible through

enable the usertointerface the computer with external devices.

7.2,

edge card connectors

PET

INTERFACES

are

AND

LINES

slotsinthe

utilized which are, in fact, direct extensionsofthe

main logic assembly board itself. There are two contactstoeach positionofthe connector. The

J1

MEMORY

.14

CONNECTOR

andJ2is also illustratedinFigure

EF-J-T=-~

EXPANSION

f

2ND CASSETTE INTERFACE

INTERFACE J3

PARAllEL

USER PORT

J2

IHE-4ll5

SERIAL

ELECTRICAL

J,

~,

NUMBER

1.6A

(SLO

SPEC

FUSE

E1LOI

7.2.

3·WIRE

POWER

AC

CORD

rear and

Figure

7.1.

Simplified viewofPET

fuse, line cord and interfacing connectors.

FROM PET

rrrr_"

MAIN

LOGIC ASSEMBLY

'_nov":

~

Insulation C t t

':

1 2 3 4 5 6 7 8 9

~:

: : : : : : : : : :

ABC

Figure

Rear

7.2.

to

0 E F H J K L M N ·

or

Edge·on View through slotsinPET (or Pin)

Simplified viewsofedge connectorsJ1and J2

illustrate contact Identification convention.

showing switch,

BOARD

_IJ

1011

""upper

12./

:':'

'lk

........

onac

lac

P;o)

Lower

Contact

IEEE-488 INTERFACES (Connector J1)

The standard IEEE·488 connector is not used on the

PET.

Instead, a standard12position,24contact edge

connector with .156 inch spacing between contact centers is provided. This permits

with all

of

the other connectionstothe

PET.

ittobe

compatible

Keying

Table 7.3

slots

are located between pins

shows

the PET

contact

2-3

and

identification

9-10.

characters,

57

the

connection

forastandard

IEEE

connector,

the IEEE mnemonics and the signal definitions.

Electrical'drlve capablllty and

PET Pin

Characters

Upper Pins

1 2 3

•

5 6

7

8 9

10

11 12

lower

Pins

A

9

e

0

E

F

Lower Pins

H

J

K

L

M

N

line

impedance matching is in accordance with IEEE·488 specifications.

Standard

IEEE

Connector Signal

Numbers

1 2 3

•

5 6

7

8 9

10

11 12

13

,.

15 16

17

18

19 20

21

22

23

2.

IEEE

Mnemonic

0101 DI02

010.3

0104 Data

EOI OAV

NRFD NDAC

lFe

SRa ATN GNO

0105 0106 0107 Data 0108 Data

REN GNO

GNO GNO GNO GNO GNO GNO

Signal

Definition/Label

Data

input/output

Data

input/output

Data

input/output

input/output End or Data valid

Not

Data Interface clear

Service request Attention Chassis

Data Data

Remote enable DAV

NAFD NDAC

IFCground

SAO ground ATN Data ground (0101-8)

identify

ready

for

not

accel:?ted

ground and IEEE

cable shield drain wire

input/output input/output input/output input/output

ground

ground ground

ground

line #1 line line

line

data

line line line

line #8

#2 #3 #4

#5 #6 #7

Table

7.3.

PET

contact Identification characters.

IEEE-488 Identification characters,

associated labels and descriptions.

RECEPTACLES FOR THE IEEE INTERFACE

A

listoffrequently used12position,24contact receptacles that are suitable for connection to the

edge card connectorJ1and J2isshown here:

Table

Manufacturer Part

Cinch 251-12-90-160 Sylvania

Amp Amp Amp

7.4.

Receptacles recommended for

connectors or parallel user port.

Number

6AG01-12-1A 530657-3 530658-3 530654-3

1-01

PET

IEEE·488

58

PET

IEEE·488 CONNECTORS

The IEEE·488 standard receptacles are not directly connectabletothe

these are shown in Table

7.5,

and belongtothe Cinch Series57or Champ Series (Amphenot). Also shown

are their matching plugs.

PET

edge connector; some

of

Connector

Manufacturer

Cinch

Amp Amp

Commodore has available a 1

contact

your

local

dealerorCommodore

meter

PARALLEL USER PORT (Connector J2)

The lines for

edge connector with a

this

interface are brought out from

.156

inch spacing between contact centers.

connectors.

Keying

Table 3·1 shows the PET

Note that the connections the

connector is used;

that

slots

are

located between pins

identification

1-12,

the

top

PET

service departmentorqualified dealers. When using the incorporated

this

jumpers someofthe

the top connectors 1-12 be used

Identifier Description

standard

dual

and

the

PET

Solder-plug

displacement

connectors

connector-PET edge connectOf, cable. Please

delivery.

main logic-boardtoa12position,24contact

5710240

5720240 Solder-receptacle

552301-1 Insulation

552305·1 Insulation

Table 7.5. IEEE

fong 1£££-488

tor

price

1-2

and

10-11.

characters, the corresponding labels and

lineofcontacts

top

contactstothe bottom contacts. It is

only

with extreme caution.

(see

Figure

See

7.6),

plug

rec.,ptacle

Table

are

7.4

for suitable mating

their

descriptions.

primarily intended for use by

ROM

diagnostic, a special

strongly

advised

Identification

Character

1

2

3 IEEE·SRQ

4

5 Diagnostic

PET

Signal

Label

Ground

T.

V.

Video

IEEE·EOI

Sense

Table 7.6. Parallel user port information.

pin identification

signal labels

Digital ground.

Video

output

used in diagnostic routine for verifying

the

video circuittothe

connectiontothe

Direct the

IEEE·488 port.Itis

ing

operationofthe

nostic routine. Direct connection

the IEEE-488 port. It ing

operationofthe

nostic routine. When this pin

up

the

PET software jumps to the diagnostic routine, rather than routine.

characters,

and

their descriptions.

Signal

Description

used for external display.

is

held low during

the

corresponding

display board.

SRQ

signal on

usedinverify·

SRQinthe diag-

to

the

EOI signal on

is

usedinverify-

Eorinthe diag-

the

BASIC

power

Table

continuedonnext

page.

59

Table 7.6. Parallel user port information (continued).

Identification

Character

6

Signal

label

Tape #1 Used with READ verify cassette tape #1 read function,

7 Tape #2 Used with the diagnostic

READ

8 Tape Write

9

10

11, 12

A S

TV.

Vertical diagnostic. May be used for external

1.

V.

Horizontal

GND

CA1

verify cassette

Used with the diagnostic routine

verify

operationoftheWRITE func·

both

tion of

T.

V.

vertical sync signal verified

TV

display.

T.V.horizontal signal verified diagnostic may be used for

Digital ground. Digital ground. Standard

6522VIA.

edge sensitive

C PA0

0

E

F H

J

K

L

M

N

PA1

PA2

PA3

PA4 PA5 PA6 PA7

CS2

GND

Input/output

f--

and can be programmedindepend· ently

of each

or

output.

SpeciallfO Digital ground.

Signal

Description

the

diagnostic routine

routine

tape#2read function.

cassette ports.

input

linestoperipherals,

other

for

pinofVIA.

TV

of

input

to

in

display.

VERSATILE INTERFACE ADAPTER

The lines on

the

bottom sideofthe user port connector originate from a Versatile Interface Adapter

(VIA MOS Technology part #6522).

The signals CA1, address

E840.

The parallel port

PAO·7,

(Decimal address

and CB2, are directly connectedtoa standard 6522 VIA located at hexadecimal

59456).

consistsofeight programmable bi-directional I/O lines for the eight lines,CA1, which can also connection, CB2. This has most

of

the abilitiesofCA1, but can also act as the input or

shift register.

A detailed specification for

port are utilized by the

in

signals

any way.

the

VIAisbelow. All signals on the VIA

PET

for internal controls. Please note

PAO·7,aninput handshake line

be

used for other edge-sensative inputs and a very powerful

outputofthe VIA

that

are

not connectedtothe

that

the user should avoid interfacing these

60

user

Table

7.7

shows the

decimal

and hexadecimal addresses in the

PET

associated with the

VIA.

Decimal

59456 59457

59458

59459

59460

59461 E845

59462

59463

59464

59465

59466 59467 59468 59469 59470 59471

Hexa-

Decimal

E840 E841 0001

E842

E843 0011

E844

E846

E847 0111

E848

E849

E84A E84B E84C E84D 1101 E84E E84F 1111

$E840+ Addressed Location

0000

0010

0100

0101

0110

1000

1001

1010

1011 Auxiliary Control register.

1100

1110

Output

I/O

I/O Port A Data Direction

Read Timer 1

Access Timer 1 Latch low order

Access Timer 1 Latch high order

Read low

Access high order byte

Serial I/O

Peripheral Control register. Interrupt Interrupt

Output

register for I/O register for I/O

with handshaking.

Port

B Data Direction

register.

Counter byte Write low order byte.

order byte. Write

Latch high order byte and initiate

byte.

and reset Write Timer 2 interrupt.

2; reset Counter

write,

without

to

Counter

count.

order

Counter

to

low order byte

butdonot

Shift

register.

Flag register (lFRl.

Enable register.

register for I/O Port

handshaking.

port port

low order

Timer 1 Latch

high

to

Timer 1

byteofTimer

interrupt.

reset

of

interrupt

B.

A

2

of

Timer

on

A,

Table 7.7.

VIA

6522

Decimal and Hexadecimal

PROGRAMMING THE USER PORT

Data lines using a software

PAO-7

are individually programmed to

function

POKE 59459 command to place a number into

shows a practical exampleofinput/output selection. The programming need

drive the pins programmed as

only

be carried out at the beginning. From then on POKE 59471 can be used

outputs,

and PEEK(59471)

61

addressesinPET.

for

input or

will

read all the inputs.

output

the

data direction register. Table 7.8

as required. This is done by

to

Command

Statement

POKE POKE POKE

Programming

59459,255 59459,0 59459,240

table

7.8.

01

Binary

Representation

11111111

00000000

11110000 PA0-3

Parallel user port example.

lines

PAO·7

for

Inputfoutput operation.

lines

PA0·7 PA0-7

PA4-7

Mode

Output

Input Input

Output

SECOND CASSETTE INTERFACE (Connector J3)

This interface is brought out from the

with

.156

inch spacing between contact centers

PET

main logic boardtoa 6 position,12contact edge connector

(See

Figure

7.9).

A keying slotislocated between pins 2-3.

This port is intended for use with the Commodore second cassette system only. Any other connections are made at the risk

of

the user. Please note that + 5 volts is

not

intended for use asanexternal power

supply.

Table 7.10

typical

receptacles

shows

the PET

that

Note A·1, In

Identification

Characters

identification

are

suitable

Insulation 6

for

FROM PET MAIN LOGIC ASSEMBLY BOARD

/"

~12345

Rear or Edge-on View

Figure

B-2,

etc

some special units, pins 1 through 6 were

Label Description

A·,

B·2

e·3

D-4 Read Read line from cassette. E-5

F-6

GND Digital ground.

+5

Motor Computer controlled positive 6 volts for

Write Write linetocassette. Sense Monitors closureofmechanical switch

characters,

the

second

./

·~I

:~:==:===:

ABC

7.9.

Simplified

with

contact

.. imply a pin Atopin1, pin Btopin2,connection.

Positive 5 voltstooperate cassette circuitry

only.

cassette motor.

cassette when any

labels

cassette

===:

D E

th~ugh

view01edge

Identlllcation.

and

connector.

F

~:5

"'F"

slotinPET

connector

not

connected.

buttonispressed.

descriptions.

Top

View

Upper

Contact

./

(or Pin)

Lower

Contact

{or

Pin}

J3

Table 7.11

on

shows

some

Table

7.10.

second

PET

pin Identification characters, labels and associated descriptions.

cassette

interface port.

62

Manufacturer

Sylvania Viking Viking Viking

Amp

Sullins Cinch

Table 7.11. Aselection of suitable receptacles lor connecting

with

the

PET

second

cassette

Identifier

6AJrJ7

-6-1

Al·rJl 2KH611AB5 2KH6/9AB5

2KH6/21AB5 53rJ692-1 ESM6·SREH 25rJ·06-9rJ-170

edge

connector

J3.

MEMORY EXPANSION CONNECTOR lConnector

The memory expansion the

6502

microprocessor. Figure

connector used. The spacing between

Note

that

the40top

"A"

edge

connections.

connector

edge

"B"

connections

provides access to the buffered and decoded input/output lines from

7.12

shows a

simplified

contact

(or

m_{(~mm

InsulatiO(

Table 7.13 shows the

Table

SIDE B

"'"

: : : : : : -

1 2 3 4 5 6 33 34 35 36 37

SIDE A RearorEdge on View through slotinPET

Figure 7.12. Simplified view of

Identification.

PET

pin numbers, line labels and fine descriptions.

Side A

Pin Numbers

Al

A2

A3

A4

A5

A6

A7

AS

~

Table 7.13. Memory expansion connector.

continuedonnext

BA3

BA4

BA5 BA6

BA7

All

side B

Line

Labels

BA0

BAl

BA2

Line

page.

Address bit

Address bit1.used for memory expansion.

Address bit2,used for memory expansion.

Address bit 3, used for memory expansion.

Address bit4,used for

Address bit 5, used for memory expansion.

Address bit 6, used for

Address

labels and line descriptions.

J4)

view of the 40-position-BO

centers is

pi~S)

0.1

inch.

are ground returns for the corresponding40lower

~

contact

Upper

(o':°;f~;

edge

connector

contacts

Buffered.

bit

Buffered.

383940

J4 with contact

grounded.

Line Description

rJ,

used for memory expansioll.

memory

7, used for memory expansion.

PET

pin numbers.

expansion.

Lower Contact

(or

Pin)

edge

•

63

Table 7.13. Memory expansion connector.

Line labels and line descriptions (continued).

PET

pin numbers.

Side A

Pin Numbers

A9

A10

All

A12

I

A13 A14 A

15

AlB

A17 SEL 2

A1S SEL 3

A19

A20

A21 SEL 6

A22

A23

A24

A25 SEL B

A2B A27

A2S A29 A30

A31 A32 A33 BD0 A34 A35 BD2 A3B BD3 A3l A3S BD5 A39 A40

I

,

I

line

Labels

BAS

BA9

BA

113

BAll

NC NC NC

SEL 1

SEL4

SEL 5

SEL

SEL 9

SEL A

Address bit 8, used for memory expansion.

Buffered.

Address bit 9, used for memory expansion.

Buffered.

Address bit 10, used for memory expansion.

Buffered.

Address bit 11, used for memory expansion.

Buffered.

No connection.

No

connection.

I

,

No connection.

I

,

4K byte page address select for

locations 100IJ·1FFF.

I