fdecstp

Decrement Stack-Top Pointer

FDECSTP

Decrements the TOP field in the FPU status word.

Pseudocode Operation

TOP ← (TOP - 1) mod 8;

Encoding

Binary Layout

D9

+0

F6

+1

 

Format Legacy

Opcode D9 F6

Extension x87 FPU

Operands

Related

More in x87 FPU

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
D9 F6	FDECSTP		Valid Valid		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;

Exceptions

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. FDECSTP-Decrement Stack-Top Pointer Vol. 2A 3-332