FDECSTP         Decrement FP stack pointer           Exceptions: None
                                                     C3 C2 C1 C0: ? ? 0 ?
FDECSTP

        Logic   TOP ← (TOP - 1) AND 07h

    FDECSTP substracts one from the three-bit TOP field of the status
    word. In effect, FDECSTP rotates the stack; it is not equivalent to
    pushing the stack. If TOP is 0, this instruction makes it 7.


    Opcode      Format
    D9 F6       FDECSTP


    Timing
    Operand       8087         287        387      486     Pentium
    -             6-12         6-12       22        3       1      NP