Tandy Graphics Adapter

Lore & Background

The Tandy 1000 series began in 1984 as a clone of the IBM PCjr, offering support for existing PCjr software. Its graphics subsystem is largely compatible with the PCjr, which was built around IBM's Video Gate Array and an MC6845 CRTC. TGA extends CGA by increasing the number of colors: CGA's 2-color mode can display four colors, and its 4-color mode can display all 16 colors. The Tandy 1000 was much more successful than the PCjr, so their shared hardware became more associated with the Tandy brand. Common parlance referred to it as TGA, though Tandy's documentation called it 'Video System Logic'.

TGA offers several CGA-compatible modes (320×200 in 4 colors, 640×200 in 2 colors, and text modes) plus enhanced modes: 160×200 with 16 colors, 320×200 with 16 colors, and 640×200 with 4 colors. Later Tandy 1000 SL/TL/RL models (Tandy Video II) added a 640×200 16-color mode. Unlike other IBM PC video standards, TGA uses some main system RAM as video RAM, sharing the first 128 KB of base RAM. This allowed double-buffering and saved the cost of dedicated video RAM.

The TGA programmable palette allows remapping of colors in CGA modes, and this palette logic was carried over to the IBM EGA and later VGA. However, software written for TGA is not compatible with EGA cards. The PCjr and Tandy 1000 are not identical: the Tandy makes 32 KB of video RAM available at address 0xB8000, while the PCjr only makes 16 KB, causing a 'venetian-blinds' effect in some software.

Reader's Guide

The Tandy Graphics Adapter became a significant standard in the IBM PC-compatible market due to its enhanced graphics and sound capabilities. With built-in joystick ports, 16-color graphics, and multichannel sound, the Tandy 1000 was considered the best platform for PC-compatible games before VGA. By 1988, games mentioning 'Tandy' on packaging was common, and Radio Shack was likely to sell such games in stores. In 1989, 28 of 66 games tested by Computer Gaming World supported Tandy graphics. Titles such as Indiana Jones and the Last Crusade, Loom, Prince of Persia, The Secret of Monkey Island, and SimCity supported PCjr/Tandy graphics. A display driver for Tandy graphics was supplied with Windows 2.0 and could be used on Windows 3.0. The TGA's use of shared system RAM was unique among IBM-designed PC video standards, offering cost savings and enabling smooth animation through double-buffering. While EGA eventually delivered a superset of TGA graphics, software written for TGA is not compatible with EGA cards, cementing TGA as a distinct standard in its own right.

Did You Know?

• TGA uses some of the main system RAM as video RAM, unlike every other IBM-designed PC video standard.
• The Tandy 1000 was much more successful than the IBM PCjr, so their shared graphics capabilities became more associated with Tandy.
• TGA's programmable palette was carried over to the IBM EGA, where it was extended to 6-bit entries for 64 colors.
• Some games detect Tandy hardware and display enhanced graphics even when their CGA display mode is selected.