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Graphic Adapters

Tracy Pettigrue
Tracy Pettigrue
Oct 3, 2009
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CGA:

The standard IBM CGA graphics card is equipped with 16 kilobytes of video memory, and could be connected either to a NTSC-compatible monitor or TV via an RCA jack, or to a dedicated RGBI CRT monitor.

The CGA's maximum color depths of four bits results in a palette of 16 colors. The lower three bits, representing red, green and blue, corresponded to the three cathode rays, with black meaning all rays were almost off. The remaining 8 colors were achieved by turning on a fourth "intensifier" bit, giving a brighter version of each color, although the dark gray color was indistinguishable from black with many monitors.

CGA offered two text modes:

  1. 40×25 characters in up to 16 colors.
  2. 80×25 characters in up to 16 colors.

It also offered two commonly-used graphics modes:

  1. 320×200 pixels, as with the 40×25 text mode. In the graphics mode, however, each pixel could be addressed independently. The tradeoff was that only 4 colors could be displayed at a time.
  1. 640×200 pixels, as with the 80×25 text mode. All pixels could be addressed independently. This mode was monochrome with a pixel aspect ratio of 1:2.4. By default the colors were black and white, but the foreground color (white) could be changed to any other color of the CGA palette.

VGA:

Video Graphics Array (VGA) is an analog computer display standard first marketed in 1987 by IBM. The term VGA is often used to refer to a resolution of 640×480, regardless of the hardware that produces the picture.

VGA is referred to as an "array" instead of an "adapter" because it was implemented from the start as a single chip.

 

The VGA specifications are as follows:

 

 

1.           256 KB Video RAM

2.           16-color and 256-color modes

3.           262144-value color palette

4.           Selectable 25 MHz or 28 MHz master clock

5.           Maximum of 720 horizontal pixels

6.           Maximum of 480 lines

7.           Refresh rates at up to 70 Hz

8.           Vertical Blanking interrupt

9.           Planar mode: up to 16 colors

10.        Packed-pixel mode: 256 colors

11.        Hardware smooth scrolling support

12.        Some "Raster Ops" support

13.        Barrel shifter

14.        Split screen support

15.        Soft fonts

 

The VGA supports both All Points Addressable graphics modes, and alphanumeric text modes.

Standard graphics modes are

640×480 in 16 colors

640×350 in 16 colors

320×200 in 16 colors

320×200 in 256 colors (Mode 13h)

SVGA:

SVGA is the short form for Super VGA. It is a set of graphics standards designed to offer greater resolution than VGA. SVGA supports 800 x 600 resolution, or 480,000 pixels.

The SVGA standard supports a palette of 16 million colors. One SVGA system might display only 256 simultaneous colors while another displays the entire palette of 16 million colors.

While the output of a Super VGA video card is analog, the internal calculations the card performs in order to arrive at these output voltages are entirely digital. To increase the number of colors a Super VGA display system can reproduce, no change at all is needed for the monitor, but the video card needs to handle much larger numbers and may well need to be redesigned from scratch.

AGP:

AGP is a short for




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