Lock Extended Memory Block (Function 0Ch)

        ARGS:   AH = 0Ch
                DX = Extended memory block handle to lock

        RETS:   AX = 0001h if the block is locked, 0000h otherwise
                DX:BX = 32-bit physical address of the locked block

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = A2h if the handle is invalid
                BL = ACh if the block's lock count overflows
                BL = ADh if the lock fails

This function locks an extended memory block and returns its base
address as a 32-bit physical address.  Locked memory blocks are
guaranteed not to move.  The 32-bit pointer is only valid while the
block is locked.Locked blocks should be unlocked as soon as possible.

NOTE: A block does not have to be locked before using Function 0Bh (Move
Extended Memory Block). "Lock counts" are maintained for EMBs.