The display adapters and the video monitor work together. The video adapter is a small card that fits into a spare slot of the microcomputer. In some microcomputers, the electronics for this card are integrated into the motherboard.
In conjunction with the monitor, the display adapters the data stream of the system bus and formats it into a data stream that the monitor can understand and display.
Graphical display adapters
The basic type of video adapter used now is VGA (Video Graphics Array) and SVGA (Super VGA). They can display information in text form or in graphical form.
The graphical standards for display adapters developed over the years are MDA, HGA, MGA, CGA, EGA, VGA, MCGA, SVGA, XGA, 8514/A, etc. the full description for these display standards is:
- MDA: Monochrome Display Adapter
- HGA: Hercules Graphics Adapter
- CGA: Colour Graphics Adapter
- EGA: Enhanced Graphics Adapter
- VGA: Video Graphics Array
- MCGA: Multi-Colour Graphics Array
- SVGA: Super VGA
- XGA: Extended Graphics Array
The SVGA includes higher resolution and better color vibrancy than VGA and is the best display standard for high-resolution graphics applications.
The 8514/A monitor and display adapters are primarily used for medical and seismic imaging, CAD/CAM presentations, and other applications that require a very high resolution.
The characteristics of common display adapters are given in Table. The present-day display adapter supports all the modes of the preceding display adapters.
|Adapter||Maximum Resolution / Colours|
|CGA||640 x 200/2|
|MDA||720 x 350/3|
|EGA||640 x 350/64|
|HGA||720 x 350/3|
|MCGA||640 x 480/2|
|VGA||640 x 480/262, 144|
|SVGA||up to 1024 x 768/256|
|8514/A||1024 x 768/262, 144|
|XGA||1024 x 786/262, 144|
The above picture (a) shows a display system without using a graphics coprocessor and figure 4.10(b) shows the scheme using a graphics coprocessor. The speed of the graphics coprocessor limits the speed at which the monitor can be refreshed.
Video controller cards also include video RAM(or VRAM). VRAM is dual-ported, that is can send a screenful of data to the monitor while at the same time receiving the next screen full of data from the CPU.
The CPU sends information about every pixel on the screen to the video display adapter. At the resolution of 1024 x 768, there are 786432 pixels to control. For 256 colors, each pixel requires one byte of information.
But for the maximum amount of color each pixel requires 24 bits(millions of colors possible) and the CPU must send 1024 x 768 x 24 bits, which is 1024 x 768 x 24 bytes which is actually 2,359,295 bytes, to the monitor for each screen.
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