FDECSTP
Decrement Stack
| Opcode | Mode | Leg Mode | Description | |
|---|---|---|---|---|
| D9 F6 | Decrement TOP field in FPU status word. |
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.
Compatibility Mode Exceptions
Same exceptions as in protected mode.
64-Bit Mode Exceptions
Same exceptions as in protected mode.
This UNOFFICIAL, mechanically-separated, non-verified reference is provided for convenience, but it may be incomplete or broken in various obvious or non-obvious ways. Refer to Intel® 64 and IA-32 Architectures Software Developer’s Manual for anything serious.