INT 2F - Communicating Applications Specification - GET EXTERNAL DATA BLOCK
        AX = CB0Eh
        DS:DX -> 256-byte buffer
Return: AX = 0000h successful
                buffer filled
        AX < 0     error code (see AX=CB01h)

Format of external data block:
Offset  Size    Description
 00h    BYTE    CAS major version
 01h    BYTE    CAS minor version
 02h 68 BYTEs   ASCIZ path to directory containing CAS software, ends in slash
 46h 13 BYTEs   ASCIZ name of current phonebook (in CAS directory)
 53h 13 BYTEs   ASCIZ name of current logo file (in CAS directory)
 60h 32 BYTEs   ASCIZ default sender name
 80h 21 BYTEs   ASCIZ CCITT identification of fax device
 95h 107 BYTEs  reserved

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