FSUBR           Subtract real reversed               Exceptions: I D O U P
FSUBRP          Subtract real reversed and pop       C3 C2 C1 C0: ? ? * ?

FSUBR [[destination,] source]
FSUBRP [destination, ST]

        Logic   ; FSUBR source                  ; FSUBR and FSUBRP
                ST ← source - ST                ST(1) ← ST - ST(1)
                                                pop ST

                ; FSUBR dest, source            ; FSUBRP dest, ST
                ST(dest) ← ST(src) - ST(dest)   ST(dest) ← ST - ST(dest)
                                                pop ST

    FSUBR/FSUBRP subtract the destination operand from the source
    operand and return the result to the destination operand.

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


    Opcode      Format
    D8 /5       FSUBR m32
    DC /5       FSUBR m64
    D8 E8 + i   FSUBR ST,ST(i)
    DC E0 + i   FSUBR ST(i),ST
    DE E0 + i   FSUBRP ST(i),ST
    DE E1       FSUBR
    DE E1       FSUBRP


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