Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 81)

Conditional and Arithmetic Instructions 67

referencing an address register. However, only word and longword forms

of the instruction can be used with an address register. The following

instruction would increment the value of memory location COUNT:

ADDQ.W #1,COUNT COUNT IS A WORD

At first it may seem that these instructions are identical to the ADDI

and SUBI instructions. In order to understand why the ADDQ and SUBQ

instructions are better, we have to delve into the actual machine instruc

tions generated when these mnemonics are used. The ADDI and SUBI

instructions will generate a 16-bit opcode word and one or two additional

16-bit values corresponding to the immediate operand. With the ADDQ

and SUBQ instructions, only the 16-bit opcode word is generated. This

results not only in a savings in memory locations but also in a significant

increase in speed because the programmer doesn’t have to perform the

memory fetch to obtain the immediate data. In the case of the ADDQ

and SUBQ instructions, this data is contained in the 16-bit opcode word.

Along a similar line, a special MOVE instruction, MOVEQ, is pro

vided. This instruction allows an immediate source operand that must be

represented in 8 bits or less, giving a signed range between —128 and

+127, or an unsigned range between 0 and 255. The destination is always

a data register, and the full 32 bits are used. This instruction is only avail

able in the longword form. It should be used whenever possible, since

the number of bytes required will be less than the corresponding MOVE

instruction. For example,

MOVEQ #100,DO

is preferable to

MOVE.L #100,DO

Many assemblers will automatically use the MOVEQ version whenever

possible, even if it hasn’t been specified.

The 68000 provides many instructions that can be functionally dupli

cated by other instructions. When a programmer uses the more appro

priate instructions, programs are more readable and take up less space

in memory. This latter consideration is important for large programs, or

when you only a small amount of memory is available. We will make ex

tensive use of the ADDQ and SUBQ instructions for a variety of purposes

throughout this book.

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Motorola M68000 Betriebsanweisung Seite 81