别运行，写结果
unsigend int a = 256;
int b = a >> 32;
b = ?;
[解决办法]
REX + C0 /5 ib SHR r/m8**, imm8 MI Valid N.E. Unsigned divide r/m8 by 2, imm8 times.
D1 /5 SHR r/m16, 1 M1 Valid Valid Unsigned divide r/m16 by 2, once.
D3 /5 SHR r/m16, CL MC Valid Valid Unsigned divide r/m16 by 2, CL times
C1 /5 ib SHR r/m16, imm8 MI Valid Valid Unsigned divide r/m16 by 2, imm8 times.
D1 /5 SHR r/m32, 1 M1 Valid Valid Unsigned divide r/m32 by 2, once.
REX.W + D1 /5 SHR r/m64, 1 M1 Valid N.E. Unsigned divide r/m64 by 2, once.
D3 /5 SHR r/m32, CL MC Valid Valid Unsigned divide r/m32 by 2, CL times.
REX.W + D3 /5 SHR r/m64, CL MC Valid N.E. Unsigned divide r/m64 by 2, CL times.
C1 /5 ib SHR r/m32, imm8 MI Valid Valid Unsigned divide r/m32 by 2, imm8 times.
REX.W + C1 /5 ib SHR r/m64, imm8 MI Valid N.E. Unsigned divide r/m64 by 2, imm8 times.
NOTES:
* Not the same form of division as IDIV; rounding is toward negative infinity.
** In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.
***See IA-32 Architecture Compatibility section below.
Op/En Operand 1 Operand 2 Operand 3 Operand 4
M1 ModRM:r/m (r, w) 1 NA NA
MC ModRM:r/m (r, w) CL NA NA
MI ModRM:r/m (r, w) imm8 NA NA
SAL/SAR/SHL/SHR—Shift
INSTRUCTION SET REFERENCE, M-Z
Vol. 2B 4-399
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.
IA-32 Architecture Compatibility
The 8086 does not mask the shift count. However, all other IA-32 processors (starting with the Intel 286 processor)
do mask the shift count to 5 bits, resulting in a maximum count of 31. This masking is done in all operating modes
(including the virtual-8086 mode) to reduce the maximum execution time of the instructions.
Operation
IF 64-Bit Mode and using REX.W
THEN
countMASK ← 3FH;
ELSE
countMASK ← 1FH;
FI
tempCOUNT ← (COUNT AND countMASK);
tempDEST ← DEST;
WHILE (tempCOUNT ≠ 0)
DO
IF instruction is SAL or SHL
THEN
CF ← MSB(DEST);
ELSE (* Instruction is SAR or SHR *)
CF ← LSB(DEST);
FI;
IF instruction is SAL or SHL
THEN
DEST ← DEST ? 2;
ELSE
SAL/SAR/SHL/SHR—Shift
INSTRUCTION SET REFERENCE, M-Z
4-400 Vol. 2B
IF instruction is SAR
THEN
DEST ← DEST / 2; (* Signed divide, rounding toward negative infinity *)
ELSE (* Instruction is SHR *)
DEST ← DEST / 2 ; (* Unsigned divide *)
FI;
FI;
tempCOUNT ← tempCOUNT – 1;
OD;
(* Determine overflow for the various instructions *)
IF (COUNT and countMASK) = 1
THEN
IF instruction is SAL or SHL
THEN
OF ← MSB(DEST) XOR CF;
ELSE
IF instruction is SAR
THEN
OF ← 0;
ELSE (* Instruction is SHR *)
OF ← MSB(tempDEST);
FI;
FI;
ELSE IF (COUNT AND countMASK) = 0
THEN
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 non-zero count, the AF flag is undefined.
