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80386 Programmer's Reference Manual -- Opcode SAL

SAL/SAR/SHL/SHR -- Shift Instructions

Operation

(* COUNT is the second parameter *)
(temp) := COUNT;
WHILE (temp <> 0)
DO
   IF instruction is SAL or SHL
   THEN CF := high-order bit of r/m;
   FI;
   IF instruction is SAR or SHR
   THEN CF := low-order bit of r/m;
   FI;
   IF instruction = SAL or SHL
   THEN r/m := r/m * 2;
   FI;
   IF instruction = SAR
   THEN r/m := r/m /2 (*Signed divide, rounding toward negative infinity*);
   FI;
   IF instruction = SHR
   THEN r/m := r/m / 2; (* Unsigned divide *);
   FI;
   temp := temp - 1;
OD;
(* Determine overflow for the various instructions *)
IF COUNT = 1
THEN
   IF instruction is SAL or SHL
   THEN OF := high-order bit of r/m <> (CF);
   FI;
   IF instruction is SAR
   THEN OF := 0;
   FI;
   IF instruction is SHR
   THEN OF := high-order bit of operand;
   FI;
ELSE OF := undefined;
FI;

Description

SAR and SHR shift the bits of the operand downward. The low-order bit is shifted into the carry flag. The effect is to divide the operand by 2. SAR performs a signed divide with rounding toward negative infinity (not the same as IDIV); the high-order bit remains the same. SHR performs an unsigned divide; the high-order bit is set to 0.

The shift is repeated the number of times indicated by the second operand, which is either an immediate number or the contents of the CL register. To reduce the maximum execution time, the 80386 does not allow shift counts greater than 31. If a shift count greater than 31 is attempted, only the bottom five bits of the shift count are used. (The 8086 uses all eight bits of the shift count.)

The overflow flag is set only if the single-shift forms of the instructions are used. For left shifts, OF is set to 0 if the high bit of the answer is the same as the result of the carry flag (i.e., the top two bits of the original operand were the same); OF is set to 1 if they are different. For SAR, OF is set to 0 for all single shifts. For SHR, OF is set to the high-order bit of the original operand.

Flags Affected

Protected Mode Exceptions

Real Address Mode Exceptions

Virtual 8086 Mode Exceptions

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