FISUB           Integer subtract                     Exceptions: I D O P
FISUBR          Integer subtract reversed            C3 C2 C1 C0: ? ? ? ?

FISUB source
FISUBR source

                ; FISUB                 ; FISUBR
        Logic   ST ← ST - source        ST ← source - ST

    FISUB subtracts a word or short-integer source in memory from ST
    and returns the difference in ST.

    FISUBR subtracts ST from a word or short-integer source in memory
    and returns the difference in ST.


    Opcode      Format
    DA /4       FISUB m32
    DE /4       FISUB m16
    DA /5       FISUBR m32
    DE /5       FISUBR m16


    Timing
    Variations/
    operand       8087         287        387      486     Pentium
    fisub  m16   (102-137)+EA 102-137     71-85    20-35    7/4     NP
    fisubr m32   (108-143)+EA 108-143     57-82    19-32    7/4     NP