SEGA SATURN DEVELOPER'S MANUAL

■ Relative coordinates: Set the relative coordinate in the relative coordinate register. The coordinates specified by the drawing command are added to the relative coordinate values to become the drawing coordinates. The part is drawn in the frame buffer based on the drawing coordinates.
When the relative coordinate is (0,0), the part is drawn at the coordinates specified by the drawing command with the upper left corner of the screen as (0,0). To set the center of the screen to (0,0), specify the relative coordinates as (160,112).
∙ Relative coordinates are held in the register until they are reset, so when you want to move the coordinates of several parts together and draw them, set them before those drawing commands.
∙ Coordinate comparison in clipping is processed with the value obtained by adding this relative coordinate to the drawing specified coordinates of the part.
Also, since relative coordinates are not added to the clipping coordinates, the clipping area will not move.
The value of the relative coordinate register becomes undefined after power-on or reset, so be sure to set it before starting drawing.