INT 33,21 - Reset Mouse Software

	AX = 21h

	on return:
	AX = 0021  mouse driver not installed
	     FFFF  mouse driver installed
	BX = 2	mouse driver installed

	- similar to INT 33,0 but does not reset the mouse hardware or
	  display variables
	- both AX and BX must be correct for successful reset
	- interrupt routines set through INT 33,18 are preserved
	- resets the mouse to the following defaults:

	  .  mouse is positioned to screen center
	  .  mouse cursor is reset and hidden
	  .  no interrupts are enabled (mask = 0)
	  .  double speed threshold set to 64 mickeys per second
	  .  horizontal mickey to pixel ratio (8 to 8)
	  .  vertical mickey to pixel ratio (16 to 8)
	  .  max width and height are set to maximum for video mode