FTST            Test for zero                        Exceptions: I D
                                                     C3 C2 C1 C0: * * * *
FTST

        Logic   CMP ST, 0.0
                set condition codes

    FTST compares the stack top to 0.0. Following the instruction, the
    condition codes reflect the relationship between ST and 0.0.

                C3 C0
                0  0  ST > 0.0
                0  1  ST < 0.0
                1  0  ST = +- 0.0
                1  1  ST is not comparable

    Note
    If ST contains a NaN or has an undefined format, or a stack fault
    occurs, the invalid-operation exception is raised and the condition
    bits are set to 'unordered'.
    The sign of zero is ignored, so that -0.0 = +0.0.


    Opcode      Format
    D9 E4       FTST


    Timing
    Operand       8087         287        387      486     Pentium
    -            38-48        38-48       28        4      4/1     FX