Color Graphics Adapter

Color Graphics Adapter
Malvineous · CC BY-SA 4.0 · source
Name	Color Graphics Adapter
Caption	An original IBM CGA card with an RCA connector for composite video output.
Manufacturer	IBM, Plantronics, and others
Type	Graphics card
Generation	First generation
Released	1981
Discontinued	1987
Predecessor	Monochrome Display Adapter
Successor	Enhanced Graphics Adapter, Professional Graphics Controller
Memory	16 KB

Color Graphics Adapter. The Color Graphics Adapter was a groundbreaking graphics card introduced by IBM alongside the original IBM Personal Computer in 1981. It was the first color graphics solution for the IBM PC platform, providing a significant visual upgrade from the text-only Monochrome Display Adapter. While its color capabilities were limited by modern standards, it established a foundational standard for PC-compatible graphics and spurred the development of a vibrant ecosystem of early computer games and business software.

History and development

The development of the Color Graphics Adapter was a direct response to the growing market for personal computers with color capabilities, a feature already popularized by systems like the Apple II and the Commodore 64. IBM engineers, led by teams at the company's Boca Raton facility, designed the CGA to be a cost-effective color option for the new IBM Personal Computer XT. Its release was strategically timed to compete with other color-capable machines in the burgeoning home computer market. The card was manufactured not only by IBM but also licensed to third-party companies like Plantronics, which marketed its own compatible version known as the Colorplus.

Technical specifications

The CGA was built around the Motorola 6845 CRTC (Cathode Ray Tube Controller) chip, which was a common component in many contemporary graphics systems. It featured 16 KB of dedicated video memory (DRAM), which was mapped directly into the IBM PC's memory space. The card could generate a color signal for an RGBI monitor using a digital 4-bit interface (3 bits for red, green, and blue, plus an intensity bit) or a composite video signal for connection to standard NTSC televisions or monitors. Its maximum pixel resolution was 640x200 in monochrome or 320x200 with four colors. The character cell for text modes was 8x8 pixels, providing a standard 80-column by 25-row display.

Display modes

The adapter supported several distinct display modes, which were selected via BIOS interrupts or direct hardware programming. Its primary graphics modes included a high-resolution 640x200 monochrome mode (with foreground color selectable from a 16-color palette) and a 320x200 medium-resolution mode with four colors from predefined palettes. A rarely used 160x100 16-color mode was achievable through undocumented techniques. For text, it offered both 40-column and 80-column color text modes, displaying 16 colors of text on a background of 8 colors. The composite video output created additional artifact colors, famously exploited by early games like Microsoft Flight Simulator 1.0 and the original King's Quest to display more hues than the digital RGB mode officially supported.

Hardware compatibility and adoption

The CGA was the standard color graphics card for the IBM PC and IBM PC/XT, and it was widely cloned by manufacturers of IBM PC compatible computers such as Compaq and Tandy Corporation. Its primary competitor was IBM's own Monochrome Display Adapter, which offered sharper text for business applications. For users requiring higher performance, the Enhanced Graphics Adapter (EGA) was introduced in 1984. The CGA's composite output allowed it to connect to inexpensive NTSC monitors and televisions, broadening its appeal. However, its digital RGB signal required a specific IBM 5153 Color Display or compatible monitor, which was a significant additional expense for many early PC buyers.

Software support and legacy

A vast library of early DOS software was written specifically for the CGA's capabilities, including landmark titles from Sierra On-Line, Broderbund, and MicroProse. The distinctive 4-color palette of CGA Mode 4 (cyan, magenta, white, black) became an iconic visual signature of 1980s PC gaming. While quickly superseded by the Enhanced Graphics Adapter and later the Video Graphics Array (VGA), the CGA standard persisted in budget systems and embedded applications for years. Its hardware registers and memory map became a permanent part of IBM PC architecture, ensuring backward compatibility in later graphics adapters. The CGA is remembered as a pivotal technology that brought color graphics to the IBM PC compatible platform, fundamentally shaping the visual experience of early personal computing. Category:IBM PC compatibles Category:Computer display standards Category:Graphics cards Category:1981 in computing