Rise_Run_Angle ($F593)

Given a (rise,run) pair, this routine calculates the angle which corresponds to that (rise,run) pair. The returned angle is relative to the x-axis (+ is CCW), so to convert it to a Vectrex angle (relative to the y-axis, + is CCW), you must subtract the number 0x10 (90 degrees) from the returned value.

ENTRY:

DP = $C8

A-reg = rise value

B-reg = run value

EXIT:

A-reg = the angle from the x-axis

B-reg = the angle from the x-axis

All other registers preserved.

Source