Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 95)

Addressing Modes 81

not a label used on a memory location. Although some assemblers may

allow you to assemble the following:

MOVE.L ARRAY(AO),DO

ARRAY: DS.L 100

you will most likely run into trouble. The label ARRAY can ultimately be

located anywhere in memory and therefore should be represented by a

32-bit value. The displacement must be a signed 16-bit value.

Address register indirect with displacement is extremely useful for

data structures that contain records. A record can contain almost anything.

All records must be the same size if a simple array access technique is

to be used. Let’s say that we want a simple data structure containing

the names and ages of various people. We will restrict a name to be 10

characters, plus a byte for the terminating null character. An age can be

represented by a single byte. This gives us a total record size of 12 bytes.

We can declare an array of names as follows:

NAMLST: DC.B

'TOM

',0

DC.B

'ERIN

'rO

DC.B 3

DC.B

'KRISTIN

',0

DC.B 5

DC.B 0

Here a null name has been used to mark the end of the list. We can

access the name and age of a particular record by defining constants to

represent the displacements into the record for each component.

NAME: EQU 0

AGE: EQU 11

RECSIZ: EQU 12

Here is a program that will add up the ages of the names in this array:

CLR.L DO DO HAS SUM

LEA NAMLSTfAO AO -> ARRAY

LOOP: TST.B NAME(AO) FINISHED?

BEQ FINI YES

CLR.L Dl NO, CLEAR HIGH ORDER BYTES

MOVE.B AGE(A0),D1 GET AGE

ADD.L DlfDO ADD IN AGE

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Motorola M68000 Betriebsanweisung Seite 95