SAL/SAR/SHL/SHR

Shift

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
REX + C0 /7 ib SAR r/m8**, imm8 C Valid N.E. Signed divide* r/m8 by 2, imm8 times.
C0 /7 ib SAR r/m8, imm8 C Valid Valid Signed divide* r/m8 by 2, imm8 time.
REX + D2 /7 SAR r/m8**, CL B Valid N.E. Signed divide* r/m8 by 2, CL times.
D2 /7 SAR r/m8, CL B Valid Valid Signed divide* r/m8 by 2, CL times.
REX + D0 /7 SAR r/m8**, 1 A Valid N.E. Signed divide* r/m8 by 2, once.
D0 /7 SAR r/m8, 1 A Valid Valid Signed divide* r/m8 by 2, once.
REX.W + C1 /4 ib SAL r/m64, imm8 C Valid N.E. Multiply r/m64 by 2, imm8 times.
C1 /4 ib SAL r/m32, imm8 C Valid Valid Multiply r/m32 by 2, imm8 times.
REX.W + D3 /4 SAL r/m64, CL B Valid N.E. Multiply r/m64 by 2, CL times.
D3 /4 SAL r/m32, CL B Valid Valid Multiply r/m32 by 2, CL times.
REX.W + D1 /4 SAL r/m64, 1 A Valid N.E. Multiply r/m64 by 2, once.
D1 /4 SAL r/m32, 1 A Valid Valid Multiply r/m32 by 2, once.
C1 /4 ib SAL r/m16, imm8 C Valid Valid Multiply r/m16 by 2, imm8 times.
D3 /4 SAL r/m16, CL B Valid Valid Multiply r/m16 by 2, CL times.
D1 /4 SAL r/m16, 1 A Valid Valid Multiply r/m16 by 2, once.
REX + C0 /4 ib SAL r/m8**, imm8 C Valid N.E. Multiply r/m8 by 2, imm8 times.
C0 /4 ib SAL r/m8, imm8 C Valid Valid Multiply r/m8 by 2, imm8 times.
REX + D2 /4 SAL r/m8**, CL B Valid N.E. Multiply r/m8 by 2, CL times.
D2 /4 SAL r/m8, CL B Valid Valid Multiply r/m8 by 2, CL times.
REX + D0 /4 SAL r/m8**, 1 A Valid N.E. Multiply r/m8 by 2, once.
D0 /4 SAL r/m8, 1 A Valid Valid Multiply r/m8 by 2, once.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA 1 ModRM:r/m (r, w)
B NA NA CL (r) ModRM:r/m (r, w)
C NA NA imm8 ModRM:r/m (r, w)

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'sManual, 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'sManual, 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.

Pseudo Code

IF 64-Bit Mode and using REX.W
	countMASK = 3FH;
ELSE
	countMASK = 1FH;
FI;
tempCOUNT = (COUNT AND countMASK);
tempDEST = DEST;
WHILE (tempCOUNT != 0)
	IF instruction is SAL or SHL
		CF = MSB(DEST);
	ELSE
		(* Instruction is SAR or SHR *)
		CF = LSB(DEST);
	FI;
	IF instruction is SAL or SHL
		DEST = DEST * 2;
	ELSE
		IF instruction is SAR
			DEST = DEST / 2; (* Signed divide, rounding toward negative infinity *)
		ELSE
			(* Instruction is SHR *)
			DEST = DEST / 2 ; (* Unsigned divide *)
		FI;
	FI;
	tempCOUNT = tempCOUNT - 1;
ELIHW;
(* Determine overflow for the various instructions *)
IF (COUNT and countMASK) = 1
	IF instruction is SAL or SHL
		OF = MSB(DEST) XOR CF;
	ELSE
		IF instruction is SAR
			OF = 0;
		ELSE
			(* Instruction is SHR *)
			OF = MSB(tempDEST);
		FI;
	FI;
ELSE
	IF (COUNT AND countMASK) = 0
		All flags unchanged;
	ELSE
		(* COUNT not 1 or 0 *)
		OF = undefined;
	FI;
FI;

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" above); 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 nonzero count, the AF flag is undefined.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#GP(0) If the memory address is in a non-canonical form.
#SS(0) If a memory address referencing the SS segment is in a non-canonical form.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#SS If a memory operand effective address is outside the SS segment limit.
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector.