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The 'X' device driver is an 80-column display device driver for all ATARI computers. When loaded into memory it installs itself between addresses $2B00 and $35D0. Since it is not yet relocatable this places it high enough to allow room for normal DOS users (DOS/XL, OS/A+, DOS2.0s) and leaves space for the 'R' handler if the 850 Interface Module is used.