Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 148)

154 Assembly Language Programming for the 68000 Family

The purpose of the arithmetic shift is to preserve the sign bit. You will

recall that the sign bit is the high-order bit when using two’s complement

representation. One use of the shift instructions is to multiply or divide

a number by a power of two. Without the arithmetic forms of the shift

instructions, incorrect values would result. The corresponding mnemonics

for the left shifts are ASL and LSL. Even though it would at first appear

that these are the same instructions, they actually differ slightly. While

it is true that the result in the destination of an ASL and an LSL will

be identical, the two instructions differ in how they affect the condition

code register. The arithmetic versions of the instructions will conditionally

set the overflow bit depending upon whether the most significant bit is

changed at any time during the shift operation. The remaining bits of the

CCR are conditionally set for both versions of the instructions.

There are three forms for the shift instructions:

<shift>[.<size>] DxfDy

<shift>[.<size>] #<data>,Dy

<shift> = ASL, ASR, LSL, LSR

<size> = B, W, or L

<data> = 1-8

The first form specifies a shift count in register Dx and the destination in

1 and 8. For counts larger than 8, two or more sequential shifts with

immediate operands can be used. Naturally, if the shift count is to be

variable, the first form is most useful. The third form shifts the contents

of a memory location by one bit only. Furthermore, the data width is

restricted to one word. If you need to manipulate a byte or longword

from memory, you will have to move it into a register first, shift it, and

then move it back to memory. Additionally, anything but a shift of a few

bits would also require moving the data to a register.

If the DO register contains 543216, the shift

LSL.W #1,D0

results in A86418 in DO. Likewise,

LSR.W #1,D0

results in 2A1916. And

MOVEQ.L #10,Dl

LSL.W Dl,DO

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Motorola M68000 Betriebsanweisung Seite 148