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【标准】计算机视频显示标准介绍(英文) - Video Graphics Array (VGA)

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【标准】计算机视频显示标准介绍(英文)
MDA
HGC
CGA
EGA
PGC
8514
VGA
MCGA
SVGA
XGA
SXGA
QXGA
HXGA
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Video Graphics Array (VGA)

With VGA you see a change in the terminology from adapter to array. This was a result of the fact that VGA graphics started to come on the motherboard as a single chip and not as plug-in adapter boards that took up an expansion slot in the computer. While since replaced with other standards for general use, VGA's 640x480 remains a sort of lowest common denominator for all graphics cards. Indeed, even the Windows splash screen logo comes in at 640x480 because it shows before the graphics drivers for higher resolution are loaded into the system.

VGA supports both graphics and text modes of operation and can be used to emulate most (but not all) of the EGA, CGA, and MDA modes of operation). The most common VGA graphics modes include:

  • 640×480 in 16 colors. This is a planar mode with four bit planes. When speaking about VGA, this is the mode most often thought of and is often what is meant when some say "VGA."
  • 640×350 in 16 colors.
  • 320×200 in 16 colors.
  • 320×200 in 256 colors (Mode 13h). This is a packed-pixel mode.

The VGA specification dictated 256KB of video RAM, 16- and 256-color modes, a 262,144 color palette (six bits for each of red, green, and blue), a selectable master clock (25 MHz or 28 MHz), up to 720 horizontal pixels, up to 480 lines, hardware smooth scrolling, split screen support, soft fonts, and more.

Another VGA programming trick essentially created another graphics mode: Mode X. By manipulating the 256 KB video RAM four separate planes could be formed where each used 256 colors. Mode X transferred some of the video memory operations to the video hardware instead of keeping them with the CPU. This sped up the display for things like games and was most often seen in 320x240 pixel resolution as that produced square pixels in 4:3 aspect ratio. Mode X also allowed double buffering; a method of keeping multiple video pages in memory in order to quickly flip between them. All VGA 16-color modes supported double buffering; only Mode X could do it in 256 colors.

Many other programming tweaks to VGA could (and were) also performed. Some, however, caused monitor display problems such as flickering, roll, and other abnormalities so they were not used commercially. Commercial software typically used "safe" VGA modes.

Video memory typically mapped into real mode memory in a PC in the memory spaces...

  • B0000h (used for monochrome text mode)
  • B8000h (used for color text and CGA graphics modes)
  • A0000h (used for EGA/VGA graphics modes)

Note that by using the different memory areas it is possible to have two different monitors attached and running in a single computer. Early on, Lotus 1-2-3 took advantage of this by having the ability to display "high resolution" text on an MDA display along with color (low-resolution) graphics showing an associated graph of some part of the spreadsheet. Other such uses included coding on one screen with debugging information on another and similar applications.

VGA also had a subset called...