A.5.94 FPATAN, FPTAN: Arctangent and Tangent
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.