FSUB            Subtract real                        Exceptions: I D O U P
FSUBP           Subtract real and pop                C3 C2 C1 C0: ? ? * ?

FSUB [[destination,] source]
FSUBP [destination, ST]

        Logic   ; FSUB source                   ; FSUB and FSUBP
                ST ← ST - source                ST(1) ← ST(1) - ST
                                                pop ST

                ; FSUB dest, source             ; FSUBP dest, ST
                ST(dest) ← ST(dest) - ST(src)   ST(dest) ← ST(dest) - ST
                                                pop ST

    FSUB/FSUBP subtract the source operand from the destination operand
    and leave the difference in the destination operand.

    The one-operand form of FSUB subtracts a (single or double real)
    memory operand from ST.
    The two-operand form of FSUB calculates the difference between two
    register operands (ST must be one of these) and returns the result
    to destination.
    FSUBP works like the two-operand FSUB but requires ST to be the
    source operand; it also pops the stack.
    The no-operand forms FSUB and FSUBP equal 'FSUBP ST(1), ST'.


    Opcode      Format
    D8 /4       FSUB m32
    DC /4       FSUB m64
    D8 E0 + i   FSUB ST,ST(i)
    DC E8 + i   FSUB ST(i),ST
    DE E8 + i   FSUBP ST(i),ST
    DE E9       FSUB
    DE E9       FSUBP


    Timing
    Variations/
    operand       8087         287        387      486     Pentium
    fsub  reg     70-100      70-100     26-37     8-20    3/1     FX
    fsub  m32    (90-120)+EA  90-120     24-32     8-20    3/1     FX
    fsub  m64    (95-125)+EA  95-125     28-36     8-20    3/1     FX
    fsubp reg     75-105      75-105     26-34     8-20    3/1     FX