c64-programmers_reference_guide-01-basic_programming_rules.pdf

INTRODUCTION

about

This chapter

talks

how

BASIC stores and

manipulates

data.

The

topics

include:

1) A brief

mention

of the operating

system components

and

functions

as well

as the

character

set used

in the

Commodore

64.

The formation

of constants

and

variables.

What

types

of variables

there

are.

And

how constants

and variables

are stored

in memory.

The rules for

arithmetic

calculations,

relationship

tests,

string

han-

dling,

and

logical

operations.

Also included

are the rules for

form-

ing

expressions,

and

the

data

conversions

necessary

when

you're

using

BASIC with

mixed

data

types.

SCREEN DISPLAY CODES

(BASIC

CHARACTER SET)

THE OPERATING

SYSTEM

(OS)

is contained

The Operating

System

in the

Read

Only

Memory

(ROM)

chips

and

is a combination

of three

separate,

but interrelated,

program

modules.

1) The

BASIC Interpreter

The

KERNAL

The Screen

Editor

The

BASIC

Interpreter

is responsible

for

analyzing

BASIC state-

ment

syntax

and

for

performing

the

required

calculations

and/or

data

manipulation.

The BASIC Interpreter

has a vocabulary

of 65

"keywords"

which

have

special

meanings.

The

upper

and

lower

case alphabet

and the digits

0-9

are used to make

both

keywords

and

variable

names.

Certain

punctuation

characters

and

special

1-1 lists tl1e

symbols

also have

meanings

for

the Interpreter.

Table

special

characters

and

their

uses.

The KERNA handles

most

of the

interrupt

level

processing

in the

system

(for

details

interrupt

level

processing,

see Chapter

5).

The

KERNAL also

does

the

actual

input

and

output

of data.

The Screen

Editor controls

the output

to the video

screen (television

set) and the editing

of BASIC program

text.

In addition,

the Screen

Editor

intercepts

keyboard

input

so that

it can

decide

whether

the

BASIC PROGRAMMING RULES

Table 1-1. CBM BASICCharacter Set

NAMEand DESCRIPTION

CHARACTER

BLANK-separates

keywords

and

variable

names

SEMI-COLON-used

in variable

lists to format

output

;

EQUAL SIGN-value

assignment

and

relationship

testing

PLUSSIGN-arithmetic

t (

addition

or string concatenation

(concatenation:

linking together

in a chain)

MINUS SIGN-arithmetic

subtraction,

unar{. minus (-1)

ASTERISK-arithmetic multiplication

SLASH-arithmetic division

UP ARROW-arithmetic exponentiation

LEFTPARENTHESIS-expression

evaluatiorl

and

functions

RIGHT PARENTHESIS-expression

)

evaluatibn

and

functions

PERCENT-declares

variable

name

ani integer

NUMBER-comes

before

logical file num~er

in input/

output statements

DOLLARSIGN-declares

variable

name

s a string

COMMA-used

in variable

lists to forma

output;

also

separates

command

para

eters

PERIOD-decimal

point

in floating

point

onstants

QUOTATION MARK-encloses

string

cons ants

COLON-separates

multiple BASIC statem

nts in a line

QUESTION MARK-abbreviation

for the ke

ord PRINT

LESSTHAN-used

in relationship

tests

GREATERTHAN-used

in relationship

test

PI-the

numeric

constant

3.141592654

characters

put in should

be acted

upon immediatelyl

or passed

to the

BASIC Interpreter.

The Operating

System

gives

you two

modes

BASIC loperation:

1) DIRECTMode

PROGRAM

Mode

1) When you're

using the

DIRECTmode,

BASIC statem~nts

don't

have

line

numbers

front

the

statement.

They

are

executed

whenever

the

.:~IIIIII:~/. key is pressed.

2) The

PROGRAM mode

is the

one

you

use

for

running

programs.

BASIC PROGRAMMING RULES

using

all

your

When

the

PROGRAM

mode,

BASIC

statements

in front

of them.

You can

have

than

must have

line numbers

BASIC statement

in a line of your program,

but the number

one

statements

limited

the

fact

that

you

can

only

put

char-

acters

logical

screen

line.

This means

that

if you

are

going

over

the

80 character

limit you

have

put

the

entire

BASIC

statement

that

doesn't

fit

new

line

with

new

line

number.

hit DI!IiIII before

NOTE: Always

type

NEW and

starting

new

program.

The Commodore

64 has two complete

character

sets that you can

use

either from the keyboard or in your programs.

In SET 1, the upper case alphabet and the numbers 0-9 are available

without pressing the IDIIiI key. If you hold down the IDIIiI key

while typing, the graphics characters on the RIGHT side of the front of

the keys are used. If you hold down the [i key while typing, the

graphics characters on the LEFT side of the front of the key are used.

Holding down the IDIIiI key while typing any character that doesn't

have graphic symbols on the front of the key gives you the symbol on the

top most part of the key.

In SET2, the lower case

alphabet

and

the numbers

0-9

are available

without

pressing

the

IDIIiI

key. The upper

case

alphabet

is available

when

you hold down

the

IDIIiI

key while typing.

Again,

the graphic

symbols

on the LEFTside of the front of the keys are

displayed

by press-

ing the

key, while the

symbols

on the

top

most

part

of any

key

without

graphics

characters

are

selected

when

you

hold

down

the

IDIIiI

key while

typing.

To switch

from

one

character

set

to the

other

press

the

and

the

IDIIiI

keys together.

PROGRAMMING NUMBERS AND VARIABLES

INTEGER, FLOATING.POINT AND

STRING CONSTANTS

Constants

are

the data

values

that

you put in your BASIC statements.

BASIC uses

these

values

represent

data

during

statement

execution.

CBM

BASIC can

recognize

and

manipulate

three

types

constants:

INTEGER NUMBERS

FLOATING-POINT

NUMBERS

STRINGS

BASIC PROGRAMMING RULES

constants

Integer

are

whole

numbers

(numbers

without

decimal

-32768

points).

Integer

constants

must

between

and

+32767.

In-

teger

constants

do not have

decimal

points

or commas

between

digits.

If the

plus (+)

sign

is left

out,

the constant

is assumed

to be a positive

number.

Zeros

coming

before

a constant

are

ignored

and

shouldn't

used since they waste

memory

and

slow down

your

program.

However,

they

won't

cause

error.

Integers

are

stored

in memory

as two-byte

binary

numbers.

Some

examples

of integer

constants

are:

-12

8765

-32768

+44

-32767

NOTE:

Do NOT put commas

inside

any

number.

For example,

always

type

32,000

32000.

If you

put

a comma

in the

middle

of a

number

you will get

the

BASIC error

message

?SYNTAX ERROR.

Floating-point

constants

are

positive

negative

numbers

and

can

contain

fractions.

Fractional

ports

a number

may

shown

using

decimal

point.

Once

again

remember

that

commas

are

NOT used

be-

tween

numbers.

If the plus sign (+)

is left off

the front

of a number,

the

64 assumes

Commodore

that

the number

is positive.

If you leave

off

the

decimal

point

the

computer

will

assume

that

it follows

the

last

digit

the

number.

And

as with

integers,

zeros

that

come

before

a constant

are

ignored.

Floating-point

constants

can

used

in two

ways:

1) SIMPLE NUMBER

SCIENTIFIC NOTATION

Floating-point

constants

will

show you up to nine digits

on your

screen.

These

digits

can

represent

values

between

-999999999.

and

+999999999.

you

enter

than

nine

digits

the

number

will

rounded

based

on the

tenth

digit.

If the

tenth

digit

is greater

than

equal

to 5 the number

will

be rounded

upward.

Less than

5 the number

will

be rounded

downward.

This could

be important

to the final

totals

some

numbers

you

may

want

to work

with.

Floating-point

numbers

are

stored

(using

five

bytes

memory)

and

are

manipulated

in calculations

with

ten

places

of accuracy.

However,

BASIC PROGRAMMING RULES

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