Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 80)

66 Assembly Language Programming for the 68000 Family

Many assemblers will allow the use of the CMP mnemonic for all

variations of the CMP instruction. The assembler figures out what version

to use by looking at the type of the operands in use. However, it is

somewhat sloppy programming to do this. You should always be aware

of what the actual 68000 instructions are. If you issue the following

instruction,

CMP.L VAL1,VAL2

you will lose out, No version of the CMP instruction will allow the use

of two memory operands.

ADDQ and SUBQ Instructions

Many programs require adding or subtracting the constant 1 from a

to perform this operation.

ADD.L #1,D0

increments the value in the DO register.

SUBI.L #1,VAR55

decrements memory location VAR55. Many microprocessors provide spe

cial instructions to increment or decrement a value. The 68000 instruction

set provides a better way to increment or decrement a number. The

ADDQ (add quick) and SUBQ (subtract quick) instructions are provided.

They can be used to add or subtract a value ranging from 1 to 8. Since

they allow small values other than 1, they are more flexible than their

counterparts on other CPUs. All arithmetic flags are updated just as they

are with the ADDI and SUBI. Here are their general forms:

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

SUBQ(.<size>] #<data>,<ea>

<size> = B, W, L

<data> is a value between 1 and 8 inclusive

The effective address can be almost any valid addressing mode including

an address register. There is no special form of these instructions when

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Motorola M68000 Betriebsanweisung Seite 80