Int 15 Fn 12  - Topview - Send Message - "lock" - Request Exclusive Acces  [Q]

   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: AH=12h/BH=0Dh