Computer Hardware User's Guide

LSH Logical Shift
13-136
Syntax LSH count, dst
Operation If count ≥ 0:
dst << count → dst
Else:
dst >> | count | → dst
Operands count general addressing modes (G):
0 0 any CPU register
0 1 direct
1 0 indirect (disp = 0–255, IR0, IR1)
1 1 immediate
dst any CPU register
Opcode
31 2423 16 8 7 015
000 01 1
0 10
dst count
G
Description The seven LSBs of the count operand are used to generate the 2s-comple-
ment shift count. If the
count operand is greater than 0, the dst operand is left
shifted by the value of the count operand. Low-order bits shifted in are zero
filled, and high-order bits are shifted out through the carry (C) bit.
Logical left shift:
C ← dst ← 0
If the
count operand is less than 0, the dst is right shifted by the absolute value
of the count operand. The high-order bits of the dst operand are zero filled as
they are shifted to the right. Low-order bits are shifted out through the C bit.
Logical right shift:
0 →
dst → C
If the count operand is 0, no shift is performed, and the C bit is set to 0. The
count operand is assumed to be a signed integer, and the dst operand is as-
sumed to be an unsigned integer.