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.
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
#NMEM or TS in CR0 is set.
Real-Address Mode Exceptions
#NMEM or TS in CR0 is set.
Virtual-8086 Mode Exceptions
#NMEM or TS in CR0 is set.