Lock

        Entry:
          AH = 12h
          AL = drive number or 00h
          ES:SI = Ethernet address or 0000h:0000h
          DS:BX -> 31-byte ASCIZ semaphore name
        Return:
          AL = status (see also AH=11h)
            01h semaphore currently logged

        Note:   Unlike function 11h, this function returns immediately