Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-302 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
to 6 bits). See the summary chart at the beginning of this section for encoding data
and limits.
Operation
IF (In 64-Bit Mode and REX.W = 1)
THEN COUNT ← COUNT MOD 64;
ELSE COUNT ← COUNT MOD 32;
FI
SIZE ← OperandSize;
IF COUNT
= 0
THEN
No operation;
ELSE
IF COUNT > SIZE
THEN (* Bad parameters *)
DEST is undefined;
CF, OF, SF, ZF, AF, PF are undefined;
ELSE (* Perform the shift *)
CF ← BIT[DEST, COUNT – 1]; (* Last bit shifted out on exit *)
FOR i ← 0 TO SIZE – 1 – COUNT
DO
BIT[DEST, i] ← BIT[DEST, i + COUNT];
OD;
FOR i ← SIZE – COUNT TO SIZE – 1
DO
BIT[DEST,i] ← BIT[SRC, i + COUNT – SIZE];
OD;
FI;
FI;
Flags Affected
If the count is 1 or greater, the CF flag is filled with the last bit shifted out of the desti-
nation operand and the SF, ZF, and PF flags are set according to the value of the
result. For a 1-bit shift, the OF flag is set if a sign change occurred; otherwise, it is
cleared. For shifts greater than 1 bit, the OF flag is undefined. If a shift occurs, the AF
flag is undefined. If the count operand is 0, the flags are not affected. If the count is
greater than the operand size, the flags are undefined.
Protected Mode Exceptions
#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.