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Mode X is a 256-color graphics mode of the VGA graphics hardware for IBM PC compatibles. It was first publicized by Michael Abrash in his July 1991 "Ramblings in Realtime" column in Dr. Dobb's Journal. The articles were reprinted in chapters 47-49 of Abrash's Graphics Programming Black Book.Abrash, Michael. Michael Abrash's Graphics Programming Black Book Special Edition. The Coriolis Group, Scottsdale Arizona, 1997. : PDF available online [1] The term "Mode X" was coined by Abrash. Mode X is a variant of the Mode 13h with the resolution increased to , giving square pixels instead of the slightly elongated pixels of Mode 13h. It is enabled by entering Mode 13h via a BIOS system call, then changing the values of several VGA registers.
Additionally, Abrash enabled the VGA's planar memory mode (also called "unchained mode"). Even though planar memory mode is a documented part of the VGA standard and was used in earlier commercial games,Abrash, Michael. Michael Abrash's Graphics Programming Black Book Special Edition. The Coriolis Group, Scottsdale Arizona, 1997. : PDF available online [2] pg. 877 it was first widely publicized in the Mode X articles, leading many programmers to consider Mode X and planar memory synonymous. It is possible to enable planar memory in standard mode, which became known as Mode Y in the Usenet rec.games.programmer group.Roberts, Dave. PC Game Programming Explorer. The Coriolis Group, Scottsdale Arizona, 1994. . Page 106.
Planar memory arrangement splits the pixels horizontally into groups of four. For any given byte in video memory, four pixels on screen can be accessed depending on which plane(s) are enabled. This is more complicated for the programmer, but the advantages gained by this arrangement—primarily the ability to use all 256 KB of VGA memory for one or more display buffers, instead of only one quarter of that (64 KB)—were considered worthwhile by many.
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