fdecstp

Decrement Stack-Top Pointer

FDECSTP

Decrements the TOP field in the FPU status word.

Details

Decrements the TOP field (bits 13:11) of the x87 FPU status word, rotating the register stack pointer backward by one position. This is a zero-operand instruction that does not modify CPU EFLAGS. It is used to manually adjust the FPU stack pointer without pushing data.

Pseudocode Operation

TOP ← (TOP - 1) mod 8;

Example

FDECSTP

Encoding

Binary Layout
D9
+0
F6
+1
 
Format Legacy
Opcode D9 F6
Extension x87 FPU

Operands

Reference (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