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(nasm.info)Section B.4.94

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B.4.94. `FPATAN', `FPTAN': Arctangent and Tangent
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     FPATAN                        ; D9 F3                [8086,FPU]
     FPTAN                         ; D9 F2                [8086,FPU]

   `FPATAN' computes the arctangent, in radians, of the result of
dividing `ST1' by `ST0', stores the result in `ST1', and pops the
register stack. It works like the C `atan2' function, in that changing
the sign of both `ST0' and `ST1' changes the output value by pi (so it
performs true rectangular-to-polar coordinate conversion, with `ST1'
being the Y coordinate and `ST0' being the X coordinate, not merely an
arctangent).

   `FPTAN' computes the tangent of the value in `ST0' (in radians), and
stores the result back into `ST0'.

   The absolute value of `ST0' must be less than 2**63.