Difference between revisions of "Video Graphics Array"
Latest revision as of 15:37, 18 November 2019
The Video Graphics Array (VGA) is a video graphics standard created by IBM, initially for their IBM PS/2 line of personal computers first released on 1987-04-02, and was then adopted by IBM clones and later IBM personal computers. Expanding upon their earlier Color Graphics Adapter and Enhanced Graphics Adapter, VGA includes a higher resolution, greater color depth, and backward compatibility with the previous two standards. The device is called an "array" rather than an "adapter" because it was directly integrated into the computer's motherboard rather than an adapter, which has caused people to misidentify the name. Too add to the confusion, third party manufacturers did make VGA adapter cards which could be used to give VGA capabilities to computers that didn't have a built-in VGA. The two most popular VGA display modes was 640x480 at 16 colors and 320x200 at 256 colors, although the device supports several more.
VGA was the last IBM graphics standard to see widespread adoption as third party competitors were developing competing Super VGA systems as early as 1988, and, by the time IBM got around to developing their official VGA replacement, the Extended Graphics Array (XGA) in 1990, they were no longer leading the technology, and software developers didn't adopt it as readily.
Even though it's no longer relevant to modern hardware, the term "VGA" is still used to generically describe any display mode used by the VGA, Super VGA, or Extended Graphics Array hardware. It is also used to describe the DE-15 video connector regardless of the signal format.
My family's first computer had a VGA which was already obsolete when we bought it in 1991. I was stuck with its inferior graphics until we got a new computer some time after 1995, long after SVGA had taken over. Most MS-DOS games maxed out at 320x200 with 256 colors and I ran Windows 3 at 640x480 with 16 colors. However, this wasn't too bad since there was no agreed upon VGA+ standard between 1987-1995, so developers were hesitant to use better graphics cards since only a handful of people actually had them, so most software continued to support the obsolete VGA standard. It wasn't until Windows 95 came out that increasing standards became universal.
In 320x200 graphics mode, VGA supports 256 color indexes with the default palette displayed to the right. It was designed to be backward-compatible with both the CGA and EGA palettes, so the first 16 colors match the CGA/EGA standard. After that, there are 16 shades of gray, and then a 24-index color spectrum beginning with blue and rotating toward red, yellow, green, and back to blue. The spectrum is repeated two more times with each subsequent copy having a lower saturation than the previous. Those 72 colors are repeated two more times with each subsequent copy having a lower intensity. There are also 8 indexes at the end left black, I'm presuming because it would take nine indexes to accommodate another color using this scheme, though I don't know why they didn't just pad out the gray scale with the unused values or provide eight more unique colors.
Although this is the default color set, the entire palette could be customized. Each index can be made up of red, green, and blue intensities ranging from 0-63 (6-bit). It was common for games developed for the VGA platform to have palette files among their resources which would be exactly 768 bytes in size, enough bytes to define an RGB intensity for each index. Because of this, it's common for game editors to assume a 768-byte file is a palette lookup.
In the 640x480 graphics mode and 720x400 text mode, VGA uses the first 16 color indexes by default, although they are also user-definable. All other modes are for backward compatibility and use the same colors as their older counterparts.
|Character Resolution||Character Size||Pixel Resolution||Colors||Palette||Mode|
|80x25||9x16||720x400||16||6-bit RGB definable||VGA|
|80x25||9x16||720x400||2||Black & white||MDA|
|80x25||8x16||640x400||16||6-bit RGB definable||MCGA|
|40x25||9x16||360x400||16||6-bit RGB definable||VGA|
|80x50||8x8||640x400||16||definable from 64 colors||EGA|
|80x43||8x8||640x344||16||definable from 64 colors||EGA|
VGA text mode allowed for a fair amount of customization for. You could redefine all of the characters in the font and you could even set the height and width of the text to a custom size. For example, changing the character size from 9x16 to 8x10 would result in a character resolution of 90x40.
Very few programs took advantage of these features. Word Perfect for MS-DOS with VGA support allowed the user to put the text display into 50-row mode, which would display more of the document by sacrificing readability. A program that modified the VGA font is MegaZeux.
|640x480||16||6-bit RGB definable||VGA|
|640x480||2||Black & white||MDA|
|640x350||16||definable from 64 colors||EGA|
|640x200||16||definable from 64 colors||EGA|
|640x200||2||definable from 16 colors||CGA|
|320x240||256 planar||6-bit RGB definable||VGA (Mode X)|
|320x200||256 linear||6-bit RGB definable||VGA (Mode 13h)|
|320x200||256 planar||6-bit RGB definable||VGA (Mode Y)|
|320x200||16||definable from 64 colors||EGA|
|320x200||4||definable from 4 possible options||CGA|
A lot of unofficial display resolutions can also be accomplished by coding directly to the display controller. You could get up to 800 pixels wide (while sacrificing height) and 600 pixels tall (while sacrificing width). You can also get larger than spec resolutions by decreasing the refresh rate. However, since these abnormal resolutions required a monitor that supported them, they were rarely used.
Doom, 320x200, 256 colors.
King's Quest V: Absence Makes the Heart Go Yonder!, 320x200, 256 colors.
Lemmings, 640x350, 16 colors.
Might and Magic III: Isles of Terra, 320x200, 256 colors.
Solitaire, 640x480 16 colors.
SimFarm, 640x480, 16 colors.