Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 94)

80 Assembly Language Programming for the 68000 Family

LEA BLOCK,AO

ADDQ.L §2,AO ADJUST BY ONE WORD

MOVE.W -(AO),DO GETS DATA ELEMENT INTO DO

DS.W 99 FIRST 99 WORDS OF ARRAY

BLOCK: DS.W 1 THE LAST WORD OF ARRAY

In the next couple of chapters you will learn some additional uses

for this addressing mode. Before going on to the next addressing mode

I should mention one minor detail that you must keep in mind. When

you use register A7 with either predecrement or postincrement modes,

it will always be adjusted by a multiple of two bytes. This means that

byte operations cause the value 2 to be added or subtracted. This is due

to the special significance of register A7 as a stack pointer, the topic of

the next chapter. Another thing you may be tempted to do is to use the

CMPM instruction with this addressing mode. You cannot do this. CMPM

can only be used with the address register indirect with postdecrement

mode.

Address Register Indirect With Displacem ent

This addressing mode is also a variant of address register indirect.

However, in this case there is no predecrement or postincrement. Rather,

this mode provides the ability to include a constant displacement to be

added to the value in the address register before it is used to form the

effective address. This value is not used to modify the contents of the

address register, but only in forming the effective address. The general

form is d16(An), where d16 indicates a displacement value which is 16 bits

long. It is a signed value; therefore, it can represent both a positive and

a negative offset given by the contents of An. Let’s say that AO contains

the value $10000. The instruction

MOVE.L 4 (A0),DO

will move the word at location $10004 to register DO. Using a symbol for

the displacement, the above example could be written as

DISP: EQU 4

MOVE.L DISP(A0),DO

If a symbol is used for the displacement, it must be a constant value and

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Motorola M68000 Betriebsanweisung Seite 94