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,

purposes only and

subjecttochange without notice.

first edition

"All

rights reserved"

this

manual is for information

Commodore Business Machines

901

California Avenue

Palo Alto, California 94303

TA61£OPCONTENTS

Chapter

Chapter4.

Chapter

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

•

Chapter 7.

ChapterS.

Chapter

PETcommunicationwiththeoutsideworld PET

interfacesand Commands and operations for peripheral devices IEEE·488bus

Machinelanguageprogramming .

Allocationofmemory Commands from BASIC Machinelanguage

Errorsanddiagnostlcs. Debug techniques BAStCerrormessages

errormessages

lines

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

monitor

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

......

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

functional blocks .

main memory " .

terms of built-in BASIC functions .

PET

RAM

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 .

contact identification .

IEEE

interface .

PET

port

7.12

Edge

connector J4 .

iii

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 .

7.23

7.24

7.25

7.26

8.1

8.2

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

. . . . .

.. ..

. . .

.. ..

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

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

. . . . . . .

.. .. ..

LIST OF APPENDICES

Detailed PET memory

BASIC

Expressions and operators

Space and speed hints

Main logic board assembly

Suggested reading

statements

commands

map

Chapter

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

a few simple

a short period after unpacking the shipping container. The potentials

arise

that

this book has

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

sending out from timetotimetousers.

very nature,islimited.

that

you and other users

Questions

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.

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

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

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

the preceding paragraph. Afteryou have used the

lightly

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.

your

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

examples we

invite comments on

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

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

other"'EY~~enCEt;

'1Ji:"teri~lwritten

have included a

PET

worksisthe

aRput BASIC, with different

listofsuch suggested reading

aspecffic

PET

need whichisnot covered

itself.Insome cases, we will

tYPlilS

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

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

available for purchase from your

PET

dealeroreither can

purchased from Commodore directly.

NOTES

Chapter

UNPACKtNGYOUA

PET

AN'lfTURNINGITON

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

concealed damage. If anything

amiss, reportly this immediatelytoboth the placeofpurchase and the

shipping agent.

PET

from

its

Remove your surface, then plug

into

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

any standard, grounded electrical

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

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

The screen memory transfer

the screen is done

,3:~~,~,

INTERFACE

view of

PET

PET. dot

this

time, you will see on

the computer memory which is controlling the screen.

microprocessor, and so, even when the computer is

current contents

the screen memory.

1.6A

FUSE -.,..:=/

ISlO

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

turns the

PET

on and

in the power-on

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,

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

computer and corresponds roughlytoone letter or

theory·

"S192

bytes".

But a few hundred are used by the

net available bytes.

you failtoget the power·updisplay the

To get the display, four different types

first

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

ROM

14k

RAM

address and

TV RAM

data-bus

keyboard

110

\ I

IEEE·4BB

user port

Figure 2.2.

PET

memory bus

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

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

ROM

contains a series

Only memories are not only

most

protection and

bits

that manipulate the

the

fastest

110

USER READWRITE

The third typeofmemoryisthe User Program Memory Space. throughout

$1FFF. A detailed map

RAM,

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

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

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.

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

control electronics,

the

programmer has

1024

8192

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

TV RAM

BASIC

every

cycleofthe

($8000) in

corner

giving us 8

dots

toptobottom.

controller

whether or

character, etc. There are

screen by POKEing the

second character set. After you have played to try lower

To understand

the

screen

the screen. Each character in

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

starts

into

second

with

the

least

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

the

want

substitutes

the

memory.

eight

times,

and

CHARACTER

The standard ASCII code is used to represent charactersinthe

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~

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

PET:

spade)

NUL

::>OH

!,;IX

c.IX

1:.01

I;.N~

tH..':K

~c.L

lJ:;

t..:H

:;0

:;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 f\

;;

4 V I

(;

H X h

•

...

set (7 bit

code)

1 1 1

k b r

1 1

i,W

c s

d t

V f v

L k

J m

t n

0 z

The

screen

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

memoryisorganized

with

a different

representatIon

from

the

main

PET

;;<: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

•

(;

H X

K L +

I $ 4

V W

(

)

•

e 1

;;

"t'

memory.

figure 2.5.

ASCII

ch.,.cler

.'1(6bll code)

These are the same characters that are directly available on the

The representation in screen memory 6;

giving us a six bit code for the keyboard characters.

The graphic, additional

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

displayable characters.

derived from the standard ASCII set by dropping bit

PET

keyboard.

This gives the characters are organized so that they are

~,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

~ ~

A Q 1

(; j)

t. U

t V

I V

L M N 1

PET

displayable characters.Itshouldbenoted that allofthe graphics

p k "

J =

justashift

1 1

•

)

•

from the normal keyboard character.

(

)

1 1 1

•

I 0

L 1

•

I I

r I

)<.

•

1 1

...

•

FIgure 2.8.

Example: This gives us the following conversions:

Character

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...

screen memory

00000001

00000oo1

(7 bit

code)

USE OF THE SCREEN MEMORY,

There are three waystoget data the appropriate memory addreSs normal updating

longasthe

and the

PET

memory only during

the useofthe screen

the

screenistoo

PET

directly

are contending

times

memory

controls

when

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

by useofthe PRINT commandinBASIC. When

"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

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

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

combination

Chapter

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

similartothatofa

However, some

the

blinking

screen memory.

data

;s transferred by the

two

exceptionstothis,

useofGET,

dataisaccessed

PET

keyboard

important

Becauseofthe

number

The

The various keys

The characters

longer require

convenient

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

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.

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

protection

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

not

M .

O-l:a.~:g

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

OIIa.l:«Cal

E'-°c-III°

:::I

zi)!?O~!?

./;;

III

•

~@J~[EJ[!J

O@C!J[!J[!J

~@J@[EJ(!)

O@(!)~[!J

•

~@)~[E)@

](!)

O@C!J[!J@

~~[!)(B[!J

O@[!)~G

~@][!JlE

O@C!J[!J(!)

~@[!JlEC!J

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• m

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• •

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, -

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O@(!)@](!)

•

~@~(B(!)

o@C!l~[!J

apoo9(J

D '" •

)

pJeoQ,';a)j

/\1

: : : :

give classical N key roll over in

the order

The keyboard Is

BASIC,

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

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

done, the encoded valueistransferred Into a10character keyboard

the screen. "

GET

routine

greater than zero;ifIt 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.

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

perform any

input

character

two

shift

keyisencoded

pointeratlocation

out;

all subsequent

keys

into

525

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

automatically transferred in the proper order.

The routine then moves

trick

a linetothe

BASIC, new line numbers may be entered

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

screen, forcing a carriage return

LOADJRUN

writtenina

routine take careofthat

flash and writes data from the keyboardtoscreen memory at the current

the

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

It should bad effects can happen

noted

that

this

keyboard queueisonly

your system. The

ten characters long and

only

known recovery from exceeding

ifit

is exceeded, dramatically

this

queueisto

power

..,......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

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:

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,

and

start

over. When fooling with

not

have the Interrupt turned

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.

•

will

see in

the

machine useful as a super calculator.

that

we wanttodo is have

the

future that

the

queue, remember

off,

the

operating systemIsgoingtokill

that

when we type a ? the BASIC operating

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"

into

the

You

should have already

little

better.

system

type

en-

R E

?"HI THERE"

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.

overstrikeofthat

out HI THERE

talk

strike the delete key once, you

that

BASIC responds by printingHITHERE. It shouldbenoted that each timewestruck

the right, allowingusto

HI THERE appeared almost immediately on

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

What we were tryingtotype wasHITHERE

ordertoallow

PET,

youtoimmediately correct

will

position.

a blank line, and READY.

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

the upper right-hand sideofthe

type in the

the

line

erase a

PET

to print

the

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

fundamental editing tool which allows youtostrike

out

as many charactersasyou

editing. It is implemented by

decrementing the pointer is.

screenpolnter

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

doestooverstrike one. Also, notice

organized so

Because

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

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.

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

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

edit the screen. However, faster editing can often be

the

cursor up and down keys.

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;

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

SCREEN EDITING

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

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

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.

The last character in the line

automatically blanked. Insertisthe reverseofthis

the

You

will

left

one character. You

left

one, rather than just

process, If we want

HI THERE

notice

that

will

that

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

you will note

PET,

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

that

this

now creates a screen

•

the~nsert.

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

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

because we have not yet pressed carriage return to tell the

PET

that

the line is

complete. That

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.

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

column data cards and,ofcourse, much more

40.

Therefore, although the

80-eolumn line. Thisisaccomplished by

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

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

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

cursor down occurs, PETtotheir kept which

proper

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

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

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

the indexpointer. Whenever a cursor

lineora

greaterorless than40characters.

the

PET

Now

that

we understand insert at the beginning. the curSor

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

that

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

right).lfwenow

READY.

move

carriage

If we

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

you useitwhen writing programs.

that

we have a mixtureof40-and

move

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

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

be scrolled

screen by a one lineortwo

off

the top,

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

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

printing, the cursor

off

the

top, the 41st characterismovedtothe

moved automaticallytothe

first, etc., and40characters are blanked

left

....

and sideofthe bottom line.

reprinted. The

line on

80~01umn

the

line

top

4O~0lumn

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

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

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

is going

act as an editing place for user-controfled input.

moving blanks into all thousand

flexibilityofmaking a correction on the

receIve the corrected version. Keys are

Chapter

The combination creative process. Someone who knows how

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

BEGINNING BASIC

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

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.

book and

When typing text, PRINT can be abbreviated

? A statement such

"HELLO"

this:

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

case a wordofgreeting.

isaninstructiontoprint the productof1024

Itisusefultonote

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

the other hand:

1024·S

multiplied

10241

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

available

full

lines whenitreaches the one-thousandth

position

printingofup to 80 characters

outputs

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

the

number notation. For example, .0034 is the

number

PRINT has two

allows

followed

tabulates printing

prints

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

trailing characters sent.Ifthe field to be

is fess than

falls between these values, the

for

'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

fieldisprinted

position

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

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

- 1.2345678E +

single

then BASIC

checks

its

scientific

point

size.

Og.

needed. Trailing zeroes

a number (unless

shouldbenoted screen, unlike the

PET,

the data screen being covered.

most

which causes the screen to advance the screen

after

printed

that

in order to

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

the decimal

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

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.

previously discussed

located at memory location

full

controlofthe screen

any length and in any siZe starting in

how

the

8000

ASCII code.concatenated with two additional bits. Oneofthese

the upper·lower case bit.

When printing

characters

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

The

clear character moves the printer pointertothe

allofthe 1000 characters on the screen.

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

which are represented in two bytes,

all

cursor

PET

screen memory consistsofa thousand charactersofatorage

positioning

position.

as literal characters within a string, the

The

cursor control characters availabletouse

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

beginningofthe

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.

VARIABLES

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

performing operations until something equals a certain value.Inorder

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

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

anyone

time. A function

are

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

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

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

always

integer variable

ends with %, e.g.A%.

ARRAYS

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

parentheses which used

inanexpression.

them. Parentheses define the particular value withinanarray whichisto

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

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

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

IF-THEN typeofstatement, equal means

the expression is compared and must equal

omitted.

A =2 is equivalenttoLET

after each command you enter.

A =

The command CLR sets all variables in

and memory allocation techniques

=2+2,

PET

prints

Now type

A=2+2

This time

Now type

PET

prints

PET

prints

Now replace

B=2"A

the

valueofB with

twice

the value inA,by

typing

PET

prints

Now change

A=2+3

8. the

valueofAbytyping

PET

prints5.If you now type

PET

prints8,the

says B

= 2*

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

the valueofB will remain

the

=means replace

PET

the

LET

PETtozero.

PET.

Remember

and

standard

value

the

is optional

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.

variables that allow extraction and packingofdata into strings which

totally,

that precision. It gives us the capabilityofspecifying about

in floating point arithmetic.The:refore, each normal

calculations

having a maximum value

a number, graphics character, or standard ASCII

is limitedtothis

representation.

+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.

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

memory efficiency, particularly

PET

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

value between minus 32,767

highest bit

USE

the number containing the Sign.

PROGRAM

AND

DIRECT STATEMENTS

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

PETto respond directly

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

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

case, BASICisobeying the command we are giving

PETtoadd two numbers and

the question

READY.

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,

get the

you

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

program can

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

debugging

in the caseofusing the

true computing element, one has

GOTO

statementstoassess why any particular pieceofcode is not

put in programs and current statusofvariables checked with print

modify your main program. However, except for

PETasa super calculator, in ordertoget the computertoact as a

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

BASIC will execute all

the statements before asking the user for control.

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

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

start with line10and make each line a multipleof10.

want BASICtoprint ourHITHERE message vertically as opposedtohorizontally.

this

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

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

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

can

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

tells

the

PETtoprint out HI THERE.

execute the program, type

printed in the vertical format:

E R E

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

the multiple

program by typing LIST. This gives

10?

20?

30?

PRINT"T"

40?

50?

70?

tenisthat we can now insert a correction between lines20and

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:

RUN.

This gives us theHITHERE

30.

First, display the

inserted between lines20and

30.

If we

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

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

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

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

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

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

the program through line50included,

the

endofthe program. Some

whichiscurrently

what

they do with

our

allow

you to

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

which Is the token for PRINT),Itinterprets

Command words are stored In memory

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

This allows the interpreter

is the data typed in PRINT, even though

BASIC is called

keyword that needs

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

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

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

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

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

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

The

command

until

sample program

also

The

30.

a line,

enter

there are

argument

executed

GOTO,

the

RUN

the

will

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.

the

PET

with literal strings. More specifically a literalisany value

quotes.

allowthe maximum compositionofscreen data,the

abilitytostore and execute cursor control characters which are fedtoIt by meansofliteralsorother

have already discussed in a section on

PET

keyboard input how the

extensiontothis

memory with a special indicatortodifferentiatethem from

PET

has a special setofgraphics characters and

PET

stores

set. Graphics are produced by shifting from the

draw a line just as easily as it can be used

its

data in ASCII.

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

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

thatisappropriate.

happen. Reverse field looks likean"R".

"a" and cursor up

the

shiftofthat character and looks like avertical linethrough the 5th column

"T"

which looks like a second vertical line.

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

edit your mistake.

PET

keyboard may be typedinas a literal and

PET

has a special modeinthe screen editor which assumes that

typed. From the

Home isan"S"

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

"S"

a right

Another method

function.

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

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

easytoget

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

graphicalcapabilityyou have thetime and patiencetoprogram.The computershould be fun.

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

you as you make a few editing mistakes.

that

you develop your own programming

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

the

machine;..

stuff.

REVERSE FIELD

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.

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

indicate the black spots in the characters coming from

willbeblack.

is not only implemented on the

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

you will see when you typeanexample,

ERE

line in which we have highlighted THERE.

.Finallytype reverse field

that

you clear the screen accidently after typingina bunch

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

reverse

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.

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

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

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

that

expression.

are

as PET

off

can

non

operator is a character

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

performed

+ 118 hidden pages

Commodore 2001-8, PET 2001-8 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual