INT 41 - Acorn BBC Master 512 - "OSGBPB" - MULTI-BYTE GET/PUT
        AL = function
            01h put bytes sequentially
            02h put bytes, ignoring sequential pointer
            03h get bytes sequentially
            04h get bytes, ignoring sequential pointer
            05h get media title and boot option
            06h get current device and directory
            07h get current library and device
            08h search directory
        DS:BX -> control block (see below)
Return: CF clear if successful
        CF set on error
        AL = 00h if operation attempted
        AL unchanged if unsupported function
SeeAlso: INT 42"Acorn",INT 43"Acorn"

Format of control block:
Offset  Size    Description
 00h    BYTE    file handle
 01h    DWORD   pointer to data in either I/O processor or Tube processor
 05h    DWORD   number of bytes to be transferred
 09h    DWORD   transfer address