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

7-16 Vol. 1

PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS

The SAR and SHR instructions can also be used to perform division by powers of

2 (see “SAL/SAR/SHL/SHR—Shift Instructions” in Chapter 4, “Instruction Set

Reference, N-Z,” of the Intel® 64 and IA-32 Architectures Software Developer’s

Manual, Volume 2B).

7.3.6.2 Double-Shift Instructions

The SHLD (shift left double) and SHRD (shift right double) instructions shift a speci-

fied number of bits from one operand to another (see Figure 7-9). They are provided

to facilitate operations on unaligned bit strings. They can also be used to implement

a variety of bit string move operations.

Figure 7-8. SAR Instruction Operation

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

Initial State (Positive Operand)

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

After 1-bit SAR Instruction

1 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

Initial State (Negative Operand)

Operand

1 1 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

After 1-bit SAR Instruction