# 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;
1
2
3
2
3
# 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. |