FDECSTP
Decrement Stack-Top Pointer
stableVMJITAOTinstruction
Encodings
| Opcode | Instruction | Op/En | 64-bit | Compat/Legacy | Description |
|---|---|---|---|---|---|
D9 F6 | FDECSTP | Valid | Valid | Decrement TOP field in FPU status word. |
Measured cost
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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.
Exceptions
Protected mode
#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
| Same exceptions as in protected mode. |
Virtual-8086 mode
| Same exceptions as in protected mode. |
Compatibility mode
| Same exceptions as in protected mode. |
64-bit mode
| Same exceptions as in protected mode. |