Mode X is a 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''. 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, 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 Usenet (), a portmanteau of User's Network, is a worldwide distributed discussion system available on computers. It was developed from the general-purpose UUCP, Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Elli ...

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.

Variants

In addition to unchained being called Mode Y, Mode Q (short for "cube") is sometimes used to refer to a 256-color mode.Bas van Gaalen. 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

External links

Graphics Programming Black Book
by Michael Abrash, chapter
474849

Mode X tutorial at GameDev.net (archived copy)

Computer display standards {{Compu-graphics-stub