FDECSTP

Decrement Stack-Top Pointer

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
D9 F6 FDECSTP None Valid Valid Decrement TOP field in FPU status word.

Description

Subtracts one from the TOP field of the FPU status word (decrements the top-ofstack 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.

Pseudo Code

IF TOP = 0
	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.

Exceptions

Floating-Point Exceptions

None.

64-Bit Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#MF If there is a pending x87 FPU exception.
#NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1.