FPATAN          Partial arctangent                   Exceptions: U P
                                                     (operands not checked)
FPATAN                                               C3 C2 C1 C0: ? ? * ?

        Logic   ST ← arctan(ST(1)/ST)
                pop ST

    FPATAN computes the artangent of ST(1) divided by ST and puts the
    result (in radians) in ST. It then pops the stack. The result has
    the same sign as the operand from ST(1), and a magnitude less than
    PI.

    On the 8087 and 80287, ST and ST(1) must be within certain limits.
    There is no restriction on ST(1) or ST on later FPUs.

    You can also use FPATAN to compute other trigonometric functions.
    For example, arcsin(x) is the arctangent of x divided by
    sqrt(1 - x**2).


    Note
    The 80486+ check for interrupts while executing this instruction. It
    will abort FPATAN to service an interrupt.


    Opcode      Format
    D9 F3       FPATAN


    Timing
    Operand       8087         287        387      486     Pentium
    -           250-800      250-800    314-487  218-303   17-173