Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 7-15
PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS
The SHR instruction shifts the source operand right by from 1 to 31 bit positions (see
Figure 7-7). As with the SHL/SAL instruction, the empty bit positions are cleared and
the CF flag is loaded with the last bit shifted out of the operand.
The SAR instruction shifts the source operand right by from 1 to 31 bit positions (see
Figure 7-8). This instruction differs from the SHR instruction in that it preserves the
sign of the source operand by clearing empty bit positions if the operand is positive or
setting the empty bits if the operand is negative. Again, the CF flag is loaded with the
last bit shifted out of the operand.
Figure 7-6. SHL/SAL Instruction Operation
Figure 7-7. SHR Instruction Operation
1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1
X
Initial State
CF
0
0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0
1
After 1-bit SHL/SAL Instruction
0
0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0
0
After 10-bit SHL/SAL Instruction
Operand
1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1
X
Initial State
CF
0
0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 1
1
After 1-bit SHR Instruction
0
0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0
0
After 10-bit SHR Instruction
Operand