3-188
INSTRUCTION SET REFERENCE
FDECSTPDecrement Stack-Top Pointer
Description
This instruction 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
THEN TOP
<
7;
ELSE TOP
<
TOP 1;
FI;
FPU Flags Affected
The C1 flag is set to 0; otherwise, cleared 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.
Opcode
Instruction
Description
D9 F6
FDECSTP
Decrement TOP field in FPU status word.