Intel® 64 and IA-32 Architectures Software Developer’s Manual
Volume 2 (2A, 2B & 2C):
Instruction Set Reference
SAR/SHR Description
Shifts the bits in the first operand (destination operand) to the left or right by the number of bits specified in the
second operand (count operand). Bits shifted beyond the destination operand boundary are first shifted into the CF
flag, then discarded. At the end of the shift operation, the CF flag contains the last bit shifted out of the destination
operand.
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; 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.
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 negative 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).
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