DW      ARC             ; Draw an elliptical arc

  Input:
     AX         The starting angle of the arc in degrees (0-360)
     BX         The ending angle of the arc in degrees (0-360)
     CX         X radius of the elliptical arc
     DX         Y radius of the elliptical arc

  Return:
     Nothing

ARC draws an elliptical arc using the (CP) as the center point of the
arc, from the given start angle to the given end angle. To get circular
arcs the application (not the driver) must adjust the Y radius as follows:

     YRAD := XRAD * (ASPEC / 10000)

where ASPEC is the aspect value stored in the DST.