Adventure Game Interpreter

The Adventure Game Interpreter (AGI) is a game engine developed by Sierra On-Line. The company originally developed the engine for King's Quest (1984), an adventure game that Sierra and IBM wished to market in order to attract consumers to IBM's lower-cost home computer, the IBM PCjr.

AGI was capable of running animated, color adventure games with music and sound effects. The player controls the game with a keyboard and, optionally, a joystick.

After the launch of King's Quest, Sierra continued to develop and improve the Adventure Game Interpreter. They employed it in 14 of their games between 1984 and 1989, before replacing it with a more sophisticated engine, the Sierra Creative Interpreter.

History

In late 1982, IBM began work on the PCjr, a lower-priced variant of the IBM Personal Computer with improved graphics and sound. The PCjr's Video Gate Array video adapter could display up to 16 colors at a time—a major improvement over the Color Graphics Adapter's four-color limit. The new sound chip, too, could output a wider range of tones than the PC speaker.

IBM commissioned Sierra to produce a game that could showcase these new capabilities. They discussed some requirements for the game, and IBM supplied Sierra with a PCjr prototype. They both agreed that the game should be animated—a first for Sierra. A team of six designers and developers, led by designer Roberta Williams, worked on the game that was eventually titled King's Quest. Among the developers were Chuck Tingley and Ken MacNeill (later releasesTemplate:Which also credit Chris Iden. An Apple II version credits Arthur Abraham).

IBM premiered the PCjr in 1984; it did not sell well and, therefore, neither did King's Quest. However, later that year Tandy Corporation released the Tandy 1000, an IBM PC compatible that succeeded where the PCjr failed. King's Quest caused a sensation in the burgeoning market of PC-compatible computers, and Sierra sold more than half a million copies. They ported it to other computers, including the Apple II, Apple IIGS, Macintosh, Amiga, and Atari ST, but IBM PC compatibles remained the primary platform for their games.

In 1988, with the release of King's Quest IV: The Perils of Rosella, Sierra debuted a more sophisticated game engine: the Sierra 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 (320×200 rather than 160×200), a point-and-click interface, and support for additional sound card hardware.

Technical design

The technical complexity of King's Quest made it a burden to write in assembly language, so the programmers created a game engine to simplify development. The engine comprised a bespoke programming language called the Game Adaptation Language, a compiler, and a bytecode interpreter (the Adventure Game Interpreter). The Game Adaptation Language was a high-level programming language that resembled C. This was compiled into bytecode, which was executed by the interpreter.

Like Sierra's earlier adventure titles, such as Wizard and the Princess (1980), AGI games used vector graphics. The PCjr accepted floppy disks with a capacity of 360 kilobytes, and raster graphics would have consumed an excessive amount of disk space. Instead, King's Quest drew polygons on the screen, and then colored them. Beginning with AGI version 2, the game engine drew graphics in an off-screen data buffer, then blitted them into video memory. This approach was not just to economize use of system resources; it also prevented the game from revealing hidden objects while it drew the screen.

AGI was principally developed for 16-bit computer architectures, which were the state of the art in home computers at the time. These included the IBM PC compatible, the Atari ST, Commodore's Amiga series, and Apple's Macintosh computers. In addition, Sierra ported AGI to three 8-bit computer models: the TRS-80 Color Computer, the Apple IIe, and the Apple IIc.

AGI-based games published by Sierra On-Line

Game	MS-DOS	Apple II	Atari ST	Amiga	Apple IIGS	Mac	CoCo 3
Template:Rh | King's Quest	1984	1984	1986	1987	1987	1987	No
Template:Rh | King's Quest II: Romancing the Throne	1985	1985	1985	1987	1987	1987	No
Template:Rh | The Black Cauldron	1986	1986	1986	1987	1987	No	No
Template:Rh | Donald Duck's PlaygroundTemplate:Refn	1986	No	1986	1986	No	No	No
Template:Rh | King's Quest III: To Heir Is Human	1986	1988	1986	1986	1988	1988	1988
Template:Rh | Space Quest: The Sarien Encounter	1986	1986	1986	1987	1987	1987	No
Template:Rh | Leisure Suit Larry in the Land of the Lounge Lizards	1987	1987	1987	1987	1987	1988	1988
Template:Rh | Mixed-Up Mother Goose	1987	1990	1987	1988	1988	No	No
Template:Rh | Police Quest: In Pursuit of the Death Angel	1987	1987	1987	1987	1987	1987	No
Template:Rh | Space Quest II: Vohaul's Revenge	1987	1987	1987	1988	1988	1988	No
Template:Rh | Gold Rush!	1988	1988	1989	1989	1989	1989	No
Template:Rh | Manhunter: New York	1988	1988	1988	1988	1988	No	No
Template:Rh | King's Quest IV: The Perils of Rosella	1988	1990	1990	1990	1989	No	No
Template:Rh | Manhunter 2: San Francisco	1989	No	1990	1990	No	1989	No

See also

• SCUMM
• ScummVM

External links

Retrieved August 15, 2014

Further reading

• Excerpt from a 1984 interview with Roberta Williams