image/svg+xml FDECSTP—Decrement Stack-Top Pointer 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; 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 #NMCR0.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. OpcodeInstruction64-Bit Mode Compat/ Leg Mode Description D9 F6FDECSTPValidValidDecrement TOP field in FPU status word. This UNOFFICIAL reference was generated from the official Intel® 64 and IA-32 Architectures Software Developer’s Manual by a dumb script. There is no guarantee that some parts aren't mangled or broken and is distributed WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE .