3-314 Vol. 2A FDECSTP—Decrement Stack-Top Pointer
INSTRUCTION SET REFERENCE, A-M
FDECSTP—Decrement Stack-Top Pointer
Description
Subtracts one from the TOP field of the FPU status word (decrements the top-of-
stack pointer). If the TOP field contains a 0, it is set to 7. The effect of this instruction
is to rotate the stack by one position. The contents of the FPU data registers and tag
register are not affected.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
IF TOP = 0
THEN TOP ← 7;
ELSE TOP ← TOP – 1;
FI;
FPU Flags Affected
The C1 flag is set to 0. The C0, C2, and C3 flags are undefined.
Floating-Point Exceptions
None.
Protected Mode Exceptions
#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1.
#MF If there is a pending x87 FPU exception.
#UD If the LOCK prefix is used.
Real-Address Mode Exceptions
Same exceptions as in protected mode.
Virtual-8086 Mode Exceptions
Same exceptions as in protected mode.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D9 F6 FDECSTP Valid Valid Decrement TOP field in FPU status
word.
