Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 59)

Getting 5tarted 45

actually perform meaningful tasks. The general form of the add and

subtract instructions are:

[<label>] ADD[.<size>] <source>,<destination> [<coinntent>]

[<label>] SUB[.<size>] <source>,<destination> [<cowment>]

As with the MOVE instruction, <size> may be B, W, or L.

The ADD instruction forms the sum of the source and destination

operands, which may be words, bytes, or longwords, and replaces the

destination operand with this sum. Both operands may be signed or

unsigned numbers. SUB works like ADD except that the source operand

is subtracted from the destination operand. Once again, the result replaces

the destination operand. The source operand may be any register or

memory location, or a constant. The destination operand may be a data

source and destination operands cannot both be memory locations. At

least one operand must be a data register, and the destination operand

cannot be an address register.

Here, and in later chapters, we will express instructions in a more

proper manner by indicating the allowable type for the source and

destination operands. The ADD and SUB instructions have the following

forms:

ADDI.<size>] <ea>,Dn

ADD[.<size>] Dn,<ea>

SUBI.<size>] <ea>,Dn

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

<ea> is a general way of expressing an effective address. An effective

address generally includes the data registers and the contents of mem

ory locations. Each instruction has more complicated limitations on the

effective addresses of instructions. You should consult Appendix C for

these details. Dn indicates that any of the eight data registers DO through

D7 can be used.

The following are all valid ADD and SUB instructions:

ADD.L D2,D3

SUB.W #5,D0

ADD.B D6,COUNT COUNT IS A BYTE LOCATION

As mentioned in the discussion of the. MOVE instruction, the size, in

bytes, of the source and destination operands must be the same. When

a constant is used, it must be capable of being represented by the num

ber of bytes of the destination operand. If a two-byte (word) constant is

specified, it cannot be used with a MOVE.B, ADD.B, or SUB.B instruc

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Motorola M68000 Betriebsanweisung Seite 59