Atari 800 Manuel de l'opérateur télécharger pdf (Page 78)

A

=

USR(ADR(E$))

But

there is still another

possibl€

hitch. Th€ 6502 machine

language code is not

fully relocatable;

any absolute memory references

to the

program

are certain to

fail. For

example, suppose

yoru

progran

has a

jumpto-subroutine

oSR)

insiruction

that refe$ to a subroutine within

the object code. This instruction

would rell ii rojump

Lo a specinc address. Unforruna-tely. your program

has no

way ofknowing

at

what

specific address that subrouiine is stored

and thus

will

almost ce ainly

jump

to

the

wrong

addre$. The

probl€m

arises from

the

fact

that BASIC might move your

object

program

almost an}'.wh€r€ in memory.

There

are s€veral solutions to

this

problem.

The simplest solution is to write

fully

relocatable code; that isJ code with

no

JMP'S,

no

JSR'S

and no data tables

enclosed within it. Put all data tables

and subroutines onto

page

6. Ifyou stil

need

more space,

put

very large data

tables into the

BASIC

string and

point

to

them indirectly.

Replace long

JMP'S

with

a bucket brigade of branch

instruciions. These

techniques shodd allow

you

to

wdte large

machine

languag€

programs.

Exarnple 1.

10

20

30

40

60

70

80

90

;

ROUTINE

EXCLOR

;

PERFORMS

EXCLUSIVE OR OPERATION

ON

;

TWO BYTES

PASSED THROUGH THE

STACK

;

PASSES RESUI,TS

DIRECTLY THROUGH USR FUNCTION

t

0000

00cc

00cD

00D4

00D5

0600

68

0601

68

0602

85CD

0604

68

0605

85CC

0607

68

0608 45CD

060A

85D5

o60c

68

060D 45CC

060F 85D4

0611 60

0612

Example 2,

O12O RESLTH

0130 xxcloR PLA

o140

0150

0160

o170

0180

0190

0200

o210

o220

o230

0240

0250

TXMPORARY

HOLDING LOCATION

TXMPORARY

HOI,DING LOCATION

ADDRESS FOR

PASSING RESULTS

ADDRESS FOR PASSING

HIGH RDSULT

SAVE HIGH BYTE

SAVE LOW BYTE

PERFORM HIGH

EXCLUSIVE OR

STORE RESULT

PERFORM lOW XXCLUSIVE

OR

STORE RESULT

WHAT

COULD BE SIMPLER?

TEMP]-

$0600

$cc

$CD

$D4

$D5

O1OO TEMPH

O11O RESLTL

10

20

30

40

50

60

PLA

STA TEMPH

PLA

STA TEMPL

PLA

EOR TEMPH

STA RESI-TH

PLA

EOR

TEMPL

STA RESI,TI,

RTS

.END

I

nourr^" ^otr

;

GENXRATES NOTDS

I,UTH CONTROLLABI,E

ATTACK AND DXCAY

;

TIMIS

;

CALL FROM BASIC

WITH COMMAND:

;A=USR(1536,

F, A, P, D)

1 2 ... 73 74 75 76 77 78 79 80 81 82 83 ... 91 92

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