Trackball

Controller type used only by The Irritating Maze.

Sort of an upside-down mechanical mouse, uses two optical encoders handled by the SIT board.

Special BIOS: 236-bios.sp1. X counter in REG_P1CNT when bit 0 of REG_POUTPUT is 0, otherwise Y counter. Refreshed every vblank unconditionnaly by this BIOS.