INT 13 - Future Domain SCSI CONTROLLER - GET POINTER TO FREE CONTROLLER RAM
        AH = 1Ch
        DL = hard drive ID (any valid SCSI hard disk)
Return: CF set on error
            AH = status code (see AH=01h)
        CF clear if successful
            ES:BX -> first byte of free RAM on controller
Notes:  the Future Domain TMC-870 contains 1024 bytes of RAM at offsets 1800h
          to 1BFFh on-bard the controller for storing drive information and
          controller status; ES:BX points to the first byte available for other
          uses
        ES contains the segment at which the controller resides; the
          controller's two memory-mapped I/O ports are at offsets 1C00h, 1E00h