x86 Instruction Set Reference

FDECSTP

Decrement Stack-Top Pointer

Opcode	Mnemonic	Description
`D9 F6`	`FDECSTP`	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.

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.

Operation
if(Top == 0) Top = 7; else Top = Top - 1;

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 EM or TS in CR0 is set.

Real-Address Mode Exceptions

#NM EM or TS in CR0 is set.

Virtual-8086 Mode Exceptions

#NM EM or TS in CR0 is set.