Adventure Game Interpreter

The Adventure Game Interpreter (AGI) is a game engine which Sierra On-Line used through most of the 1980s to create and run animated, color adventure games. AGI-based computer games accept typed commands via keyboard, as well as joystick input.

In late 1982, IBM began work on the PCjr, a variant of the IBM PC that would sell for a lower price and be more amenable to the home and educational markets. They thus commissioned Sierra to produce a game that could showcase its enhanced sound and graphics capabilities. Since Sierra's previous efforts were static screen adventures, the next logical step was to have animation. Because the project was too complicated to write easily in assembly language, Sierra began developing a scripting engine that would greatly simplify programming of the game. With Roberta Williams working on the storyline and basic design of King's Quest, Robert Abraham was in charge of developing the engine. After Sierra dismissed Abraham, IBM contributed to the development of King's Quest's game engine in order to help ensure the project's success.

The AGI engine was an interpreter similar to BASIC. Games were written in a high level syntax similar to C, which the interpreter would convert on the fly into machine language. The bulk of AGI games was comprised of the script data with the rest being the translator and drivers to interface with the hardware.

Sierra had already established a practice of using vector graphics in their earlier adventures like Wizard and the Princess and this continued with AGI. Graphics data was not stored as tiles or pre-rendered bitmaps and instead the game would simply draw the outlines of the screen and paint them in, a method that saved a considerable amount of storage space. Since the Apple II and PC used bitmap graphics, this method was particularly suited to them. It was less suitable for tile-based hardware such as the Commodore 64's and was one reason why Sierra did not port the AGI games to it.

Beginning with the AGI V2 engine, games would draw in an off-screen buffer and blit them into video memory, partially for the sake of tidiness, but also so as to avoid revealing hidden objects and spoiling puzzles.

Because the PCjr was a marketplace flop, King's Quest also failed commercially. But because the AGI engine was designed to allow easy portability, Sierra simply produced a new translator and drivers for standard PCs and the Apple II. Later that year Tandy Corporation released the Tandy 1000, another IBM PC compatible that had originally been designed as an enhanced clone of the PCjr and which succeeded where the PCjr failed. King's Quest caused a sensation in the burgeoning market of PC-compatible computers, and Sierra eventually sold more than half a million copies. After this point, Sierra made the PC platform their primary development focus.

Due to the AGI engine's having been designed around the 16-bit PC hardware, it was not easily portable to 8-bit machines such as the Atari 800 and Commodore 64, and so Sierra ignored those platforms because of memory and graphics limitations (the Apple II versions of AGI games required the 128k IIe and IIc).

Roberta Williams' King's Quest established a new type of interactive adventure game, and Sierra named their new game engine the Adventure Game Interpreter. Following the success of King's Quest, they ported the game (AGI included) to other computing platforms, such as the Apple II, Apple II, Apple Macintosh, Amiga and Atari ST.

The 160x200 resolution of AGI games was designed around PC video hardware, but also proved suitable for other platforms such as the Amiga and Atari ST. However, Sierra adapted the color palette for other video hardware.

In 1988, with the release of King's Quest IV: The Perils of Rosella, Sierra debuted a more sophisticated proprietary game engine: Sierra's Creative Interpreter, or SCI. Since the SCI engine required a more powerful home computer, Sierra released an AGI version of the game at the same time. However, Sierra overestimated consumer demand for the lesser version, and ceased production.

The following year, Sierra published its final AGI-based title, Manhunter 2: San Francisco, then focused exclusively on SCI for new adventure game development. Among SCI's enhancements were a more versatile scripting system, an object-oriented programming model, higher-resolution graphics (320x200 rather than 160x200), a point-and-click interface, and support for additional sound card hardware.