Slide 16.3: SHR instruction

SHR Instruction

The SHR (shift right) instruction performs a logical right shift on the destination operand, replacing the highest bit with 0. The lowest bit is copied into the Carry flag, and the bit that was in the Carry flag is lost.

SHR — Shift Logical Right

Usage: shr dest, count

Flag	O	D	I	S	Z	A	P	C
Result	*			*	*	?	*	*

Shifts the destination right by “count” bits with zeroes shifted in on the left. The Carry flag contains the last bit shifted out.

	Clocks
Operands	286	386	486	Size Bytes
reg, 1	2	3	3	2
mem, 1	7	7	4	2-4
reg, CL	5+n	3	3	2
mem, CL	8+n	7	4	2-4
reg, immed8	5+n	3	2	3
mem, immed8	8+n	7	4	3-5

Logically shifting any unsigned operand right by n bits divides the operand by 2ⁿ.

Division of signed numbers by shifting is accomplished using the SAR instruction because it preserves the number's sign bit.

Shift Positive Right	Shift Negative Right	Shift Arithmetic Right
.data value SDWORD 32 .code shr value, 03 mov eax, value call WriteInt	.data value SDWORD -32 .code shr value, 03 mov eax, value call WriteInt	.data value SDWORD -32 .code sar value, 03 mov eax, value call WriteInt
Output	Output	Output
EAX =	EAX =	EAX =