Define interrupt subroutine parameters

        Entry:
          AX = 000Ch
          CX = call mask
               bit 0 call if mouse moves
               bit 1 call if left button pressed
               bit 2 call if left button released
               bit 3 call if right button pressed
               bit 4 call if right button released
               bit 5 call if middle button pressed (Mouse Systems mouse)
               bit 6 call if middle button released (Mouse Systems mouse)
          ES:DX -> FAR routine

        Note:   when the subroutine is called, it is passed the following
                values:

          AX = condition mask (same bit assignments as call mask)
          BX = button state
          CX = cursor column
          DX = cursor row
          DI = horizontal mickey count
          SI = vertical mickey count