Adventure Game Interpreter#AGI-based games published by Sierra On-Line

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.{{Cite journal |last=Trivette |first=Donald B. |date=February 1985 |title=Inside King's Quest |url=https://archive.org/stream/1985-02-compute-magazine/Compute_Issue_057_1985_Feb#page/n137/mode/2up |journal=Compute! |access-date=March 26, 2016}}{{cite web |url=http://members.aol.com/KQswst104/history.html |title=History of King's Quest |date=2006 |website=King's Quest Realm |archive-url=https://web.archive.org/web/20080331033134/http://members.aol.com/KQswst104/history.html |archive-date=March 31, 2008 |access-date=September 28, 2016}} 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.{{cite web | title=Meet Roberta Williams, The Queen of Graphic Adventure Video Games | author=Joanna Goodrich | date=28 September 2020 | url=https://spectrum.ieee.org/meet-roberta-williams-the-queen-of-graphic-adventure-video-games#:~:text=The%20game%20that%20made%20Roberta,fictional%20royal%20family%20of%20Daventry | website=IEEE Spectrum | publisher=IEEE | access-date=4 May 2025}} 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 releases{{which|date=September 2016}} also credit Chris Iden. An Apple II version credits Arthur Abraham).{{citation needed|date=September 2016}}

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.{{citation needed|date=September 2016}} They ported it to other computers, including the Apple II, Apple IIGS, Mac, 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.

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).{{cite web |url=http://www.adventureclassicgaming.com/index.php/site/features/143/ |title=Making adventure games with AGI |last=Kelly |first=Peter |date=January 21, 2000 |website=Adventure Classic Gaming |access-date=September 25, 2016}} 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.

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• SCUMM
• ScummVM

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Retrieved August 15, 2014

• [http://www.oldskool.org/shrines/pcjr_tandy/#sierra Excerpt from a 1984 interview with Roberta Williams]

Category:1984 software

Category:Adventure game engines

Category:Sierra Entertainment

Category:Video game development software

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