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42 Assembly Language Programming for the 68000 Family

without having to remember different mnemonics for all these instruc

tions.

The general form of the MOVE instruction is

[<label>] MOVE[.<size>] <source>,<destination> [<coroment>]

The <size> following the MOVE indicates the type of operation to be

performed. It must be a B, W, or L, standing for byte, word or longword.

If the size is omitted, the default value is taken as word. The MOVE

instruction takes the value of the source operand and places a copy of

it into the destination operand. The source operand is not changed. The

destination operand may be a data register or a memory location, but

not a constant. The source operand may be a data register, memory

location, or constant. A number of other instructions have source and

destination operands. Always remember that the direction of the data

flow is from the left operand to the right operand. You may have used

an assembly language for another computer (for example, from the 8080

or 8086 families) in which the flow is reversed. Be careful when you start

out programming the 68000 so you don’t make a mistake.

Let’s assume that the DO and D1 registers contain the following values:

123 4SG

We now execute the following instruction:

MOVE.L D0,D1

The DO and D1 registers would contain the following values after execu

tion:

123

Notice that the previous value in the D1 register has been lost and that

the new value is identical to that contained in the DO register. Also note

that the contents of the DO register remains unchanged. We must also be

careful that the size of the source operand in bytes matches the size of

the destination operand in bytes. This is normally not a problem with the

registers, since they will always accommodate a full longword. However,

specifying a memory location that is actually a byte, when we really

want a word or longword, will result in faulty program behavior. Your

assembler will not be able to catch this mistake, and it is a common cause

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Motorola M68000 Betriebsanweisung Seite 56