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

See also: 1512

1512 - TopView - SEND MESSAGE - "LOCK" - REQUEST EXCLUSIVE ACCESS TO RESOURCE