INT 15 - TopView - SEND MESSAGE - "LOCK" - REQUEST EXCLUSIVE ACCESS TO RESOURCE
        AH = 12h
        BH = 14h
        BL = object
            00h mailbox handle is DWORD on top of stack
            02h use given task's mailbox (task's handle on top of stack)
            03h use current task's default mailbox
Note:   release exclusive access by sending CLOSE message to mailbox
        access may be requested multiple times, and requires multiple CLOSEs