46 What is VGA, and how does it work?
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This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
46 What is VGA, and how does it work?
OK, the answer to this one could easily be a book (actually, see the references because it _is_ a book or several). I'll give a very cursory overview of what the VGA is capable of.
The Video Graphics Array is a standard established by IBM to provide higher pixel addressability, colour graphics than are available with EGA. In fact, VGA is a superset of EGA, incorporating all EGA modes.
The VGA consists of seven sub-systems, including: graphics controller, display memory, serializer, attribute controller, sequencer and CRT controller. Basically, the CPU performs most of the work, feeding pixel and text information to the VGA.
Graphics Controller: Can perform logical functions on data being written to display memory.
Display Memory: A bank of 256k DRAM divided into 4 64k colour planes. It is used to store screen display data.
Serializer: Takes display data from the display memory and converts it to a serial bitstream which is sent to the attribute controller.
Attribute Controller: Contains the colour LUT (Look Up Table) which determines what colour will be displayed for a given pixel value in display memory.
Sequencer: Controls timing of the board and enables/disables colour planes.
CRT Controller: Generates syncing and blanking signals to control the monitor display.
It is beyond the scope of this FAQ to describe the functionality of these components in detail, so for further reading consult Sutty & Blair (see References).
VGA provides very low-level graphics commands. This, combined with the fact that a VGA card has a frame buffer but no real processing power, means that the PC's CPU has to do most of the graphics number crunching. As a result, the VGA speed of a given computer is highly dependent on the CPU speed, and the two cannot be uncoupled. Basically this renders VGA speed comparisons between video cards installed in systems which use different processors meaningless. Also, the VGA performance of a video card _can not_ be used to estimate how fast that card will be in another video mode (i.e. SVGA, Windows 3.1, etc).
VGA is really an outdated standard, but in fact, all PC's today boot in VGA text mode 7 (see table below) and there is no indication that this will change in the near future. Most DOS games still use it because of its universality. While most GUI users think that 800x600 is a minimum pixel addressability, most DOS games only use a 320x200 pixel mode. Now, a number of SVGA games (640x480 with >16 colours or higher resolutions) are being released. However, the larger number of pixels which are being displayed require a faster processor and sometimes even a fast Pentium can appear sluggish.
The VGA modes are:
Mode Type Resolution Chars Colours (Hex) 0,1 text 360x400 40x25 16 2,3 text 720x400 80x25 16 4,5 gfx 320x200 40x25 4 6 gfx 640x200 80x25 2 7 text 720x400 80x25 mono D gfx 320x200 40x25 16 E gfx 640x200 80x25 16 F gfx 640x350 80x25 mono 10 gfx 640x350 80x25 16 11 gfx 640x480 80x30 2 12 gfx 640x480 80x30 16 13 gfx 320x200 40x25 256
The next 'standard' (and hopefully it will be widely adopted), is VESA SVGA, and provides standard SVGA modes (pixel addressabilities & colour depths), registers and refresh rates.
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