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84 Assembly Language Programming for the 68000 Family

fact, we can let the assembler do the work for us. By specifying a label for

the displacement, the assembler will automatically compute the proper

offset value. If we wish to access the data at location COUNT, we can

use PC with displacement addressing:

MOVE.L COUNT(PC),D0

COUNT: DC.L 1000

There are a couple of advantages in using this addressing mode. First,

the length of the instruction will be shorter than if the long absolute mode

is used, and a full word of memory will be saved. Second, when we

use PC relative addressing modes, the program code becomes position-

independent. This means that we can move the program around in

memory without modifying it and it will still work. Of course, this is

assuming that no absolute addressing is used anywhere in memory, which,

with great care, can be done. It is a help that all the branch instructions use

this mode at all times. Only with JMP and JSR do you have a choice. This

technique is useful for dynamically loaded programs that will be loaded

into various locations in memory. It is somewhat beyond the level of this

book to discuss all the applications of these techniques; however, one

application is that of dynamically loaded device drivers in an operating

system. If you program the Apple Macintosh, you will always have to

write position independent code—this is a drawback of the Macintosh

programming environment.

A couple of restrictions apply. PC relative modes can never be used

for the destination operand of an instruction. Additionally, many assem

blers and systems support the concept of a program section. Normally,

PC relative modes cannot be used with a label that is in another program

section. The specific restrictions on the use of these modes is somewhat

system-dependent. Consult the manuals for your assembler and system.

Additionally, some assemblers will generate this addressing mode auto

matically for references to labels that are not forward references. A for

ward reference is a reference to a label that has not yet been defined. You

should take a good look at your assembly listings to determine the ad

dressing modes that your assembler uses. Just look at the extension words

to the instruction. If there is only one, then PC relative mode is used.

Addressing Mode Summary

Here is a table of all the possible addressing modes and their assembler

syntax:

1 2 ... 93 94 95 96 97 98 99 100 101 102 103 ... 255 256

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Motorola M68000 Betriebsanweisung Seite 98