Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 58)

44 Assembly Language Programming for the 68000 Family

Unfortunately these two instructions do not accomplish the desired result.

The first MOVE instruction has destroyed the contents of register Dl.

This is the value that must be placed in register DO by the second MOVE.

This second MOVE will erroneously result in the value of register DO

not being changed. To perform the swap correctly, a temporary storage

location is needed. This can be a register or a memory variable. The

instructions

MOVE.L D0,D2

MOVE.L D1,D0

MOVE.L D2,D1

will perform the swap correctly. However, register D2 has thus been

used as a temporary storage location, and we may not wish to destroy its

contents either. The use of a memory location as a temporary frees the

registers but will cause the instructions to execute at a slower speed.

The 68000 EXG (exchange registers) is our salvation. We can swap

between any of the 16 registers but not between two memory locations

or between a memory location and a register. We can write the above

program as:

EXG.L D0,D1

The EXG instruction will operate on bytes, words, or longwords when

the proper instruction extension is specified. Of course, we can’t swap

two constants or a constant and anything else.

I didn’t mention it above, but a value cannot be moved into an address

used. Its general form is

MOVEAI.<size>] <ea>,An

<size> = W or L

Any operations involving an address register as a destination can only use

the word or longword forms. In the case of a word form, the word is

sign-extended to 32 bits before being used. The entire address register is

always used.

Addition and Subtraction

While moving values from one register to another is an important

part of assembly language programming, arithmetic operations such as

addition and subtraction will allow you to start writing programs that

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Motorola M68000 Betriebsanweisung Seite 58