A.5.75 FDECSTP: Decrement Floating-Point Stack Pointer
FDECSTP ; D9 F6 [8086,FPU]
FDECSTP decrements the "top" field in the floating-point status word.
This has the effect of rotating the FPU register stack by one, as if the
contents of ST7 had been pushed on the stack. See also FINCSTP (section
A.5.85).