SAL|SAR|SHL|SHR

Shift

SAL: Shift Arithmetic Left
SAR: Shift Arithmetic Right

SHL: Shift Logical Left
SHR: Shift Logical Right

Right Arithmetic Shifts preserve the sign by filling with the same value as the sign bit.
Right Logical Shift always fill with 0s.

Left Shifts always fill with 0s, thus SAL and SHL are synonymous.

SAL|SHL

SAL and SHL are synonymous.

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 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 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).

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 instruction, 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 64-bits and sets the mask width for CL to 6 bits. See the summary chart at the beginning of this section for encoding data and limits.

Operation

DEST: first operand
COUNT: second operand
countMASK: 0b00111111 (= 63) for 64-bit operations, 0b00011111 (= 31) otherwise

SAL|SHL

tempCOUNT = COUNT & countMASK;
while tempCOUNT != 0 {
    CF = MSB(DEST);
    DEST = DEST * 2;
    tempCOUNT = tempCOUNT - 1;
}

// determine overflow
if COUNT & countMASK == 1 {
    OF = MSB(DEST) ^ CF;
} else if COUNT & countMASK == 0 {
    // all flags unchanged
} else {
    // COUNT not 1 or 0
    // OF undefined
}

SAR

tempCOUNT = COUNT & countMASK;
while tempCOUNT != 0 {
    CF = LSB(DEST);
    DEST = DEST / 2; // signed divide, rounding toward negative infinity
    tempCOUNT = tempCOUNT - 1;
}

// determine overflow
if COUNT & countMASK == 1 {
    OF = 0;
} else if COUNT & countMASK == 0 {
    // all flags unchanged
} else {
    // COUNT not 1 or 0
    // OF undefined
}

SHR

tempCOUNT = COUNT & countMASK;
while tempCOUNT != 0 {
    CF = LSB(DEST);
    DEST = DEST / 2; // unsigned divide
    tempCOUNT = tempCOUNT - 1;
}

// determine overflow
if COUNT & countMASK == 1 {
    OF = MSB(tempDEST);
} else if COUNT & countMASK == 0 {
    // all flags unchanged
} else {
    // COUNT not 1 or 0
    // OF undefined
}

Flags Affected

The CF flag contains the value of the last bit shifted out of the destination operand; it is undefined for SHL and SHR instructions where the count is greater than or equal to the size (in bits) of the destination operand. The OF flag is affected only for 1-bit shifts (see Description below); otherwise, it is undefined. The SF, ZF, and PF flags are set according to the result. If the count is 0, the flags are not affected. For a non-zero count, the AF flag is undefined.