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.
DP = $C8
A-reg = rise value
B-reg = run value
A-reg = the angle from the x-axis
B-reg = the angle from the x-axis
All other registers preserved.