Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

Vol. 2B 4-301
INSTRUCTION SET REFERENCE, N-Z
SHRD—Double Precision Shift Right
Description
The SHRD instruction is useful for multi-precision shifts of 64 bits or more.
The instruction shifts the first operand (destination operand) to the right the number
of bits specified by the third operand (count operand). The second operand (source
operand) provides bits to shift in from the left (starting with the most significant bit
of the destination operand).
The destination operand can be a register or a memory location; the source operand
is a register. The count operand is an unsigned integer that can be stored in an imme-
diate byte or the CL register. If the count operand is CL, the shift count is the logical
AND of CL and a count mask. In non-64-bit modes and default 64-bit mode, the
width of the count mask is 5 bits. Only bits 0 through 4 of the count register are used
(masking the count to a value between 0 and 31). If the count is greater than the
operand size, the result is undefined.
If the count is 1 or greater, the CF flag is filled with the last bit shifted out of the desti-
nation operand. For a 1-bit shift, the OF flag is set if a sign change occurred; other-
wise, it is cleared. If the count operand is 0, flags are not affected.
In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix
in the form of REX.R permits access to additional registers (R8-R15). Using a REX
prefix in the form of REX.W promotes operation to 64 bits (upgrading the count mask
Opcode Instruction
64-Bit
Mode
Compat/
Leg Mode Description
0F AC SHRD r/m16,
r16, imm8
Valid Valid Shift r/m16 to right imm8
places while shifting bits from
r16 in from the left.
0F AD SHRD r/m16,
r16, CL
Valid Valid Shift r/m16 to right CL places
while shifting bits from r16 in
from the left.
0F AC SHRD r/m32,
r32, mm8
Valid Valid Shift r/m32 to right imm8
places while shifting bits from
r32 in from the left.
REX.W + 0F AC SHRD r/m64,
r64, imm8
Valid N.E. Shift r/m64 to right imm8
places while shifting bits from
r64 in from the left.
0F AD SHRD r/m32,
r32, CL
Valid Valid Shift r/m32 to right CL places
while shifting bits from r32 in
from the left.
REX.W + 0F AD SHRD r/m64,
r64, CL
Valid N.E. Shift r/m64 to right CL places
while shifting bits from r64 in
from the left.