Adventure Game Interpreter

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Adventure Game Interpreter
Original author(s) unknown
Developer(s) Sierra On-Line
Initial release May 1984 (32 years ago) (1984-05)
Stable release
3.002.149 / 17 August 1989 (27 years ago) (1989-08-17)
Development status Discontinued
Operating system DOS, Apple SOS, ProDOS, Macintosh System, Atari TOS
Platform Intel 8088, x86, Apple II, Apple IIGS, Macintosh, Amiga, Atari ST, TRS-80 Color Computer
Available in English
Type Game engine
License Proprietary software

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


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, which became King's Quest. Although none of Sierra's earlier games were animated, the companies agreed that this new flagship game should be, and IBM supplied Sierra with a prototype PCjr. The project was too complicated to write easily in assembly language, so Sierra began developing a scripting engine that would greatly simplify programming of the game. In a review by Donald B. Trivette in Compute! magazine, this scripting engine is referred to as the "Game Adaptation Language."[1] It is not clear when "Adventure Game Interpreter" became the standard term.

Roberta Williams worked on the game design, and Chuck Tingley and Ken MacNeill are credited with the programming. In later releases, Sierra programmer Chris Iden is also listed as contributing to the game.

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.[2] King's Quest caused a sensation in the burgeoning market of PC-compatible computers, and Sierra sold more than half a million copies. After this point, Sierra made the PC platform their primary development focus.

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 it to other computing platforms, such as the Apple II, Apple IIGS, Apple Macintosh, Amiga and Atari ST.

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.

Technical design[edit]

The AGI engine is 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 consisted 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 (1980), 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.

Beginning with the AGI v2, 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.

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).

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.

AGI-based games published by Sierra On-Line[edit]

Game PC Apple II Atari ST Amiga Apple IIGS Mac TRS-80 CoCo
King's Quest 1984 1984 1986 1987 1987 1987 No
King's Quest II: Romancing the Throne 1985 1985 1985 1987 1987 No No
The Black Cauldron 1986 1986 1986 1987 1987 No No
Donald Duck's Playground 1986 1986 1986 1986 No No No
King's Quest III: To Heir Is Human 1986 1988 1986 1986 1988 No 1988
Space Quest: The Sarien Encounter 1986 1986 1986 1987 1987 1987 1986
Leisure Suit Larry in the Land of the Lounge Lizards 1987 1987 1987 1987 1987 1988 1988
Mixed-Up Mother Goose 1987 1990 1987 1988 1988 No No
Police Quest: In Pursuit of the Death Angel 1987 1987 1987 1987 1987 1987 No
Space Quest II: Vohaul's Revenge 1987 1987 1987 1988 1988 1988 No
Gold Rush! 1988 1988 1989 1989 1989 1989 No
Manhunter: New York 1988 1988 1988 1988 1988 No No
King's Quest IV: The Perils of Rosella 1988 1990 1990 1990 1989 No No
Manhunter 2: San Francisco 1989 No 1990 1990 No 1989 No

See also[edit]


  1. ^ Trivette, Donald (February 1985). "Inside King's Quest". Compute!. Retrieved 2016-03-26. 
  2. ^


External links[edit]

Contemporary game development[edit]