Color Graphics Adapter

IBM's first graphics card and first color display card for the IBM PC: 640×200, 16 colors.

The Color Graphics Adapter was the first color display card for the IBM PC, introduced in 1981. It was IBM's first graphics card, thus becoming the PC's first color computer display standard.

Built around the Motorola 6845 display controller, the CGA card featured several graphics and text modes. The highest display resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors).

The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory and could be connected either to a dedicated direct-drive CRT monitor using a 4-bit digital (TTL) RGBI interface, such as the IBM 5153 color display, or to an NTSC-compatible television or composite video monitor via an RCA connector. The RCA connector provided only baseband video, so to connect the CGA card to a standard television set required a separate RF modulator unless the TV had a composite video input, although doing so with the former combined with an amplifier was sometimes more practical since one could then hook up an antenna to the amplifier and get wireless video.

Hardware Design

The original IBM CGA graphics card was built around the Motorola 6845 display controller, came with 16 kilobytes of video memory built-in, and featured several graphics and text modes. The highest display resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors).

The CGA card could be connected either to a direct-drive CRT monitor using a 4-bit digital (TTL) RGBI interface, such as the IBM 5153 color display, or to an NTSC-compatible television or composite video monitor via an RCA connector. The RCA connector provided only baseband video, so to connect the CGA card to a television set without a composite video input required a separate RF modulator.

IBM produced the 5153 Personal Computer Color Display for use with the CGA, but this was not available at release and would not be released until March 1983.

Although IBM's own color display was not available, customers could either use the composite output (with an RF modulator if needed,) or the direct-drive output with available third-party monitors that supported the RGBI format and scan rate. Some third-party displays lacked the intensity input, reducing the number of available colors to eight, and many also lacked IBM's unique circuitry which rendered the dark-yellow color as brown, so any software which used brown would be displayed incorrectly.

CGA offered several video modes.

Graphics modes:

  • 160x100 in 16 colors, chosen from a 16-color palette, utilizing a specific configuration of the 80x25 text mode.
  • 320×200 in 4 colors, chosen from 3 fixed palettes, with high- and low-intensity variants, with color 1 chosen from a 16-color palette.
  • 640×200 in 2 colors, one black, one chosen from a 16-color palette.

Some software achieved greater color depth by utilizing artifact color when connected to a composite monitor.

Text modes:

  • 40×25 with 8×8 pixel font (effective resolution of 320×200)
  • 80×25 with 8×8 pixel font (effective resolution of 640×200)

IBM intended that CGA be compatible with a home television set. The 40×25 text and 320×200 graphics modes are usable with a television, and the 80×25 text and 640×200 graphics modes are intended for a monitor.

image
Color Graphics Adapter
acronymn
  • CGA
resources
source
Adapted from content published on wikipedia.org
credit
  • Image By Malvineous - Own work, CC BY-SA 4.0 — from wikimedia.org
Last modified on May 30, 2021, 5:37 pm
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