INT 6F - 10-NET - LOCK FCB
        AH = 11h
        AL = mode
            01h sequential
            02h random
            03h random block
                CX = number of records
        DS:DX -> FCB (see INT 21/AH=0Fh)
Return: CF set on error
            AX = error code (see also AH=00h)
                0002h file not found
        CF clear if successful