Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 60)

46 Assembly Language Programming for the 68000 Family

tion; in this case the destination operand is only a single byte. However,

if a one-byte constant is used with a word or longword destination, the

assembler is able to generate the proper machine instruction. The 68000

assembly language includes a mechanism in which numeric constants are

automatically sign-extended to 8, 16, or 32 bits as needed. The following

instruction is not legal and would be flagged as an error by the assembler:

ADD.B §1000,DO

The source and destination operands can be the same.

ADD.L DO,DO

results in the DO register being doubled.

Note that the ADD or SUB instructions do not allow the more general

form

ADDl.<size>] <ea>,<ea>

SUB[.<size>] <ea>,<ea>

This would eliminate the possibility of adding a constant to a memory

location, or subtracting a constant from an address register. There are a

couple of additional forms of the ADD and SUB instructions that help to

eliminate some of these restrictions.

ADDA[.<size>] <ea>,An

SUBA[.<size>1 <ea>,An

<size> = W or L

allows adding or subtracting to an address register.

ADDII.<size>] #<data>,<ea>

SUBI[.<size>] #<data>,<ea>

allow adding or subtracting an immediate value to a memory location

or data register. This instruction cannot be used to add or subtract

an immediate value to an address register. For example, the following

instructions are correct:

ADDI.L #1000,COUNT

ADDA.L #2,A5

Many assemblers allow the use of the mnemonics MOVE, ADD, SUB,

and so on without qualifying the instruction; the assembler decides what

instruction to use. For example, if you write

1 2 ... 55 56 57 58 59 60 61 62 63 64 65 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 60