Mode X
Mode X is a 320 × 240 256-color graphics display mode of the VGA graphics hardware for IBM PC compatibles. It was first publicized by Michael Abrash in his July 1991 column in Dr. Dobb's Journal and then in chapters 47-49 of Abrash's Graphics Programming Black Book.[1] The term "Mode X" was coined by Abrash. Mode X is a variant of the 320 × 200 Mode 13h with the resolution increased to 320 × 240, giving square pixels instead of the slightly elongated pixels of Mode 13h. It is enabled by entering Mode 13h via an MS-DOS 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,[2] 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 320 × 200 mode, which became known as Mode Y in the Usenet rec.games.programmer group.[3][4]
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.
Variants
In addition to unchained 320 × 200 being called Mode Y, Mode Q (short for "cube") is sometimes used to refer to a 256 × 256 256-color mode.[5][6] The Y coordinate can simply be put in the high byte of the address, and the X coordinate in the low byte, forming the address of the pixel without a multiply.
References
- ^ Abrash, Michael. Michael Abrash's Graphics Programming Black Book Special Edition. The Coriolis Group, Scottsdale Arizona, 1997. ISBN 1-57610-174-6: PDF available online [1] Archived 2007-03-11 at the Wayback Machine
- ^ Abrash, Michael. Michael Abrash's Graphics Programming Black Book Special Edition. The Coriolis Group, Scottsdale Arizona, 1997. ISBN 1-57610-174-6: PDF available online [2] Archived 2007-03-11 at the Wayback Machine pg. 877
- ^ "Mode Y". August 1993. Retrieved 2007-04-05.
- ^ Roberts, Dave. PC Game Programming Explorer. The Coriolis Group, Scottsdale Arizona, 1994. ISBN 1-883577-07-1. Page 106.
- ^ Robert Schmidt."tweak16b". 1993. Archived from the original on 2015-08-11. Retrieved 2015-07-18.
- ^ Bas van Gaalen. "Tweaked 256x256x256 Chained". August 1994.
External links
- Graphics Programming Black Book by Michael Abrash, chapters 47, 48, 49.
- Mode X tutorial at GameDev.net (archived copy)
- Tweaked VGA Modes by Robert C. Pendleton (archived copy)
- Introduction to Mode X by Robert Jambor (archived copy)
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