Adventure Game Interpreter (AGI)

Last updated: January 12, 2026

Overview

The Adventure Game Interpreter (AGI) was Sierra On-Line’s first standardized game engine, developed in 1984 to power King’s Quest: Quest for the Crown. Originally created as a showcase for IBM’s PCjr computer, AGI established the template for graphic adventure games and powered Sierra’s output through the late 1980s. The engine’s ability to display animated characters moving through graphical environments was revolutionary for its time, effectively creating the graphic adventure genre.

AGI games are characterized by their 160×200 resolution, 16-color graphics, text parser interface, and distinctive pseudo-3D perspective that allowed characters to walk in front of and behind background objects.

Technical Specifications

Specification	Details
Resolution	160×200 pixels
Color Depth	16 colors (EGA palette)
Graphics Modes	EGA, CGA, VGA, Hercules, MCGA, Tandy
Interface	Text parser (typed commands)
Audio	PC Speaker, Tandy 3-voice, Apple IIgs 15-voice
Memory Required	256KB RAM
Distribution	Floppy disk (typically 3-6 disks)

Graphics System

Pseudo-3D Environment

AGI created a distinctive visual style where characters could walk both horizontally and vertically, with the engine automatically determining whether objects appeared in front of or behind the character based on their Y-position on screen. This created an illusion of depth that was groundbreaking for 1984.

Drawing Priority

The engine used a priority screen system where different colors represented different depth planes. Artists drew separate “visual” and “priority” screens, with the priority screen controlling which objects appeared in front of others.

Character Animation

Sprites were limited to 3 frames of animation for each direction (left, right, front, back), creating the characteristic “puppet walk” motion familiar to players of classic Sierra games.

Audio Capabilities

AGI’s audio support varied significantly by platform:

Platform	Audio Capability
IBM PC	Single-voice PC Speaker (beeps and tones)
Tandy 1000	3-voice audio chip
Apple IIgs	15-voice stereo soundtrack
Amiga	4-channel stereo
Atari ST	3-voice audio
Macintosh	Single-channel audio

The Apple IIgs versions are often considered definitive due to their significantly enhanced soundtracks composed specifically for that platform’s superior audio hardware.

Interface Evolution

Early Text Parser (1984-1986)

Players typed verb-noun combinations to interact:

LOOK TREE
GET SWORD
TALK MAN
OPEN DOOR

The parser understood approximately 1,000 words per game, with responses ranging from helpful to humorously sarcastic.

Pull-Down Menus (1987)

Later AGI games added optional pull-down menus for common commands like Save, Restore, and Inventory, reducing reliance on memorizing keyboard shortcuts.

Games Using AGI

King’s Quest Series

1984 - King’s Quest - Quest for the Crown - AGI debut
1985 - King’s Quest II - Romancing the Throne
1986 - King’s Quest III - To Heir Is Human
1988 - King’s Quest IV - The Perils of Rosella - Dual AGI/SCI release

Space Quest Series

1986 - Space Quest - The Sarien Encounter
1987 - Space Quest II - Vohaul’s Revenge - Last major AGI release

Other AGI Games

Technical Innovations

First Digitized Speech

AGI was capable of basic digitized audio, used sparingly due to memory constraints. Space Quest’s alarm klaxon was among the first digitized sounds in a Sierra game.

Cross-Platform Architecture

The AGI interpreter was designed for portability, allowing Sierra to release games on multiple platforms with consistent gameplay. This business model became standard practice for Sierra throughout the 1980s and 1990s.

Save/Restore System

AGI introduced Sierra’s signature save game system, essential given the games’ high difficulty and frequent player deaths.

Limitations

Resolution Constraints

The 160×200 resolution (doubled horizontally to display at 320×200) limited detail in character art and backgrounds, though artists like Mark Crowe and Douglas Herring achieved remarkable results within these constraints.

Parser Frustrations

The text parser often failed to recognize synonyms or alternate phrasings, leading to the infamous “I don’t understand” responses that frustrated many players. Sierra’s SCI engine would address these issues with point-and-click interfaces.

Memory Management

With only 256KB RAM available, AGI games couldn’t store large amounts of data simultaneously, requiring frequent disk swapping and limiting soundtrack complexity.

Transition to SCI

By 1988, AGI’s limitations became apparent as competitors released more visually sophisticated games. Sierra developed the Sierra Creative Interpreter (SCI) to address these shortcomings:

Feature	AGI	SCI
Resolution	160×200	320×200
Colors	16	256
Interface	Text parser	Point-and-click
Audio	Limited	Sound card support
Sprites	3 frames	Smooth animation

King’s Quest IV (1988) was released in both AGI and SCI versions, marking the transition period. After 1989, Sierra abandoned AGI for all new development.

Legacy

Fan Development

The AGI engine has been thoroughly documented by fans, leading to:

AGI Studio - Fan tool for creating AGI games
NAGI - Open-source AGI interpreter
ScummVM - Full AGI support for modern systems

Preservation

All AGI games are playable today through ScummVM, which provides accurate emulation across all modern platforms including Windows, macOS, Linux, iOS, and Android.

Historical Significance

AGI established Sierra’s dominance in the adventure game market and created design conventions that influenced the entire genre. The engine’s text parser, death mechanics, and save system became industry standards.

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