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INT 33h,  0Ch (12)       Set User-Defined Subroutine Input Mask

    Installs a user defined subroutine.

       On entry:      AX         0Ch
                      CX         Call Mask
                      ES:DX      Offset of subroutine

       Returns:       Nothing.

  --------------------------------------------------------------------------

    Function 0Ch sets the call mask and subroutine address for the mouse
    software interrupts.

    The mouse software interrupts automatically stop execution of Your
    program and call the specified subroutine whenever one or more of the
    conditions defined by the call mask occur. When the subroutine finishes,
    Your program continues execution at the point of interuption.

    The call mask is a value that defines wich conditions cause an
    interrupt. Each bit in the call mask corresponds to a specific condition
    as shown in the following table:

         Bit  15-5 4 3 2 1 0    Meaning
        ---------------------------------------------------
                .  x . . . .    Right Button Released
                .  . x . . .    Right Button Pressed
                .  . . x . .    Left Button Released
                .  . . . x .    Left Button Pressed
                .  . . . . x    Corsor Position Changes
                x  . . . . .    Not used

    To enable an interrupt for a given condition, set the corresponding call
    mask bit to 1 and pass the mask in CX.

    To disable an interrupt for a given condition, set the corresponding
    call mask bit to 0 and pass the mask in CX. Function 0 automatically
    disables all interrupts.

    When the mouse software makes a call to the subroutine, it loads the
    following information into the registers.

              Register  Information
      ------------------------------------------------------------------
                AX      Condition mask (similar to the call mask except that
                        a bit is set only if the condition has occurred)

                BX      Button state

                CX      Horizontal cursor coordinate

                DX      Vertical cursor coordinate