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

4-278 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
the end of the shift operation, the CF flag contains the last bit shifted out of the desti-
nation operand.
The destination operand can be a register or a memory location. The count operand
can be an immediate value or the CL register. The count is masked to 5 bits (or 6 bits
if in 64-bit mode and REX.W is used). The count range is limited to 0 to 31 (or 63 if
64-bit mode and REX.W is used). A special opcode encoding is provided for a count
of 1.
The shift arithmetic left (SAL) and shift logical left (SHL) instructions perform the
same operation; they shift the bits in the destination operand to the left (toward
more significant bit locations). For each shift count, the most significant bit of the
destination operand is shifted into the CF flag, and the least significant bit is cleared
(see Figure 7-7 in the Intel
®
64 and IA-32 Architectures Software Developer’s
Manual, Volume 1).
The shift arithmetic right (SAR) and shift logical right (SHR) instructions shift the bits
of the destination operand to the right (toward less significant bit locations). For each
shift count, the least significant bit of the destination operand is shifted into the CF
flag, and the most significant bit is either set or cleared depending on the instruction
type. The SHR instruction clears the most significant bit (see Figure 7-8 in the Intel
®
64 and IA-32 Architectures Software Developer’s Manual, Volume 1); the SAR
instruction sets or clears the most significant bit to correspond to the sign (most
significant bit) of the original value in the destination operand. In effect, the SAR
instruction fills the empty bit position’s shifted value with the sign of the unshifted
value (see Figure 7-9 in the Intel
®
64 and IA-32 Architectures Software Developer’s
Manual, Volume 1).
The SAR and SHR instructions can be used to perform signed or unsigned division,
respectively, of the destination operand by powers of 2. For example, using the SAR
instruction to shift a signed integer 1 bit to the right divides the value by 2.
Using the SAR instruction to perform a division operation does not produce the same
result as the IDIV instruction. The quotient from the IDIV instruction is rounded
toward zero, whereas the “quotient” of the SAR instruction is rounded toward nega-
tive infinity. This difference is apparent only for negative numbers. For example,
when the IDIV instruction is used to divide -9 by 4, the result is -2 with a remainder
of -1. If the SAR instruction is used to shift -9 right by two bits, the result is -3 and
the “remainder” is +3; however, the SAR instruction stores only the most significant
bit of the remainder (in the CF flag).
The OF flag is affected only on 1-bit shifts. For left shifts, the OF flag is set to 0 if the
most-significant bit of the result is the same as the CF flag (that is, the top two bits
of the original operand were the same); otherwise, it is set to 1. For the SAR instruc-
tion, the OF flag is cleared for all 1-bit shifts. For the SHR instruction, the OF flag is
set to the most-significant bit of the original operand.
In 64-bit mode, the instruction’s default operation size is 32 bits and the mask width
for CL is 5 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