Adventure Game Interpreter

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Adventure Game Interpreter
Original author(s)Unknown
Developer(s)Sierra On-Line
Initial releaseMay 1984 (38 years ago) (1984-05)
Stable release
3.002.149 / August 17, 1989 (33 years ago) (1989-08-17)
Operating systemDOS, Apple SOS, ProDOS, Macintosh System, Atari TOS, AmigaOS
PlatformIntel 8088, x86, Apple II, Apple IIGS, Macintosh, Amiga, Atari ST, TRS-80 Color Computer
Available inEnglish
TypeGame engine
LicenseProprietary 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 (1984), an adventure game which 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, 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.[1][2] They discussed some requirements for the game, and IBM supplied Sierra with a PCjr prototype.[1] They both agreed that the game should be animated—a first for Sierra.[citation needed] A team of six designers and developers, led by designer Roberta Williams, worked on the game that was eventually titled King's Quest.[1] Among the developers were Chuck Tingley and Ken MacNeill (later releases[which?] also credit Chris Iden. An Apple II version credits Arthur Abraham).[citation needed]

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.[citation needed] They ported it to other computing platforms, including the Apple II, Apple IIGS, Macintosh, Amiga, and Atari ST, but the PC 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: 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 (320×200 rather than 160×200), a point-and-click interface, and support for additional sound card hardware.

Technical design[edit]

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,[1] a compiler, and a bytecode interpreter (the Adventure Game Interpreter).[3] The Game Adaptation Language was a high-level programming language that resembled C.[3] This was compiled into bytecode, which was executed by the interpreter.[3]

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.[1] Instead, King's Quest drew polygons on the screen, and then colored them.[1] 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[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[note 1] 1986 No 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 No
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

Table Notes[edit]

  1. ^ Donald Duck's Playground was released for the Apple II and Tandy Color Computer platforms but did not use the AGI engine for those ports, nor did the game's release for the Commodore 64

See also[edit]


  1. ^ a b c d e f Trivette, Donald B. (February 1985). "Inside King's Quest". Compute!. Retrieved March 26, 2016.
  2. ^ a b "History of King's Quest". King's Quest Realm. 2006. Archived from the original on March 31, 2008. Retrieved September 28, 2016.
  3. ^ a b c Kelly, Peter (January 21, 2000). "Making adventure games with AGI". Adventure Classic Gaming. Retrieved September 25, 2016.

External links[edit]

Further reading[edit]