A Brief Explanation of AGI
By Juha Terho
AGI (Adventure Game Interpreter) was an adventure game development system / engine designed by Jeff Stephenson and Ken Williams at Sierra On-Line and used in about a dozen graphic adventures from 1984 to 1989, including the original King's Quest 1, Space Quest 1, Police Quest 1, and Leisure Suit Larry 1 games. The system was originally developed in 1984 for King's Quest I, a game created to showcase the graphics and sound capabilities of the IBM PCjr home computer. AGI featured a then-unique mix of "three-dimensional" movement and text-adventure type parser input. Internally, it ran scripts that determined the actions of the game, allowing for easy porting to other platforms (such as the Macintosh). The graphics were rather crude: because of AGI's PCjr legacy, the games were always limited to a resolution of only 160 × 200 with 16 colors, even on EGA graphics adapters. Still, it is interesting how the relatively primitive AGI could be used to create the illusion of a reasonably immersive "virtual world". With AGI, Sierra created a number of excellent adventure games, Leisure Suit Larry being my favorite. In the late 1980s, AGI was superseded by a much more sophisticated system mostly designed by Jeff Stephenson, called SCI (originally Script Code Interpreter, later renamed Sierra's Creative Interpreter). Sierra used SCI to develop games until 1997.
In their time, Sierra's adventure games were immensely popular, but today the adventure game genre has all but died. Indeed, in a final stroke for the once-lucrative genre, Sierra's original adventure division was shut down in 1999, thanks to corporate consolidations and mergers.
In the late 1980s, I was around 10 years old and a huge fan of these games. Since I had always been fascinated by the technical aspects of the AGI and SCI engines, I was happy to discover in the late 1990s that AGI and SCI had been reverse-engineered and that the details of the inner workings of both interpreters were publicly available on the Internet. Using freely available editors, I created my own AGI game, AGI Quest I.