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Leisure Suit Larry, by Al Lowe, was widespread.  Nearly everybody has played or at least heard of it!

AGI, or the Adventure Game Interpreter, was developed by Sierra Online for the development and release of their games in the late 1980's.  Originally released for the PCjr to take advantage of it's new features, it was later ported to many other systems like the PC, Amiga and Macintosh.  Sierra was hailed for creating "3D" games, which meant the player could move their character around the screen and behind objects.  Games using the interpreter have very distinctive blocky graphics and a text parser interface where the player could type in their commands.  Players on the PC could control their characters using the keyboard or the joystick while other systems also had mouse support.

The AGI interpreter was the starting point for many of Sierra's classic adventure game lines:

and also had many original games like Goldrush, Manhunter, The Black Cauldron and Mixed Up Mother Goose.

Time Quest, a fan game by Chad Goulding, follows the Sierra graphical style very closely.

The file formats for the interpreter have been well documented in the Unofficial AGI Specifications.  This has been the foundation to create a variety of tools to help people create games to run under the original interpreter. There are patches available to add more functionality to the original interpreter, newly coded interpreters by fans which run under Windows or under other operating systems and a small community of developers who chat on the AGI messageboard.

Many fan made games have been released or are in development:

Kings Quest IV was the last King's Quest in the series to use the AGI interpreter.

Backtrack for a moment to 1983. Home computers were still a hot topic as major companies jockeyed for a forward position in the market. IBM gave Sierra On-Line a PC one full year before releasing them to the business world. With this head start, Sierra On-Line developed the first game for the new platform: The Wizard and The Princess. Then IBM began development on a personal computer for the home called the PCjr (nicknamed "Peanut"). In order to showcase this new product, IBM asked Sierra On-Line to come up with a game that would take advantage of the PCjr's 16-color palette, three-channel sound, and whopping (for the times) 128K of memory. Working with a small team of programmers and artists, Roberta lived up to the challenge. She designed a game in which the player would take on the persona of Sir Graham, a knight in the land of Daventry. The ailing King Edward sends Graham on a quest to recover three lost treasures. Should Graham succeed, he will become the heir to the throne. With its release in the summer of 1983, King's Quest I: Quest for the Crown becomes the first animated, three dimensional "interactive cartoon."

Using the keyboard arrow keys, the player could now actually control the main character's movement, walking him around rocks and in front of buildings. Simple sentence commands input by way of the keyboard controlled the character's actions. Accompanying music and sound effects greatly enhanced the game.

Unfortunately, the PCjr was not the success that IBM had hoped. Its incompatibility with the IBM PC and its user-unfriendly "chiclet" keyboard not only doomed the PCjr but almost spelled disaster for Sierra On-Line as well.

Then suddenly, in 1984, riding into the home computer market like a knight on a white horse, the Tandy Corporation introduced the Tandy 1000. Otherwise known as "what the IBM PCjr should have been," this MS-DOS (and PCjr) compatible proved a lifesaver. King's Quest I sales skyrocketed as the Tandy 1000 became the leader in the home computer industry. Sierra On-Line dramatically regained its corporate footing and, using the momentum generated by the success of King's Quest I, prepared to propel computer game development to new heights.

- History of Kings Quest, All text from the King's Quest Collection © Sierra Online