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The Z-machine is a virtual machine that was developed by Joel Berez and Marc Blank in 1979 and used by Infocom for its text adventure games. Infocom compiled game code to files containing Z-machine instructions (called story files, or Z-code files), and could therefore port all its text adventures to a new platform simply by writing a Z-machine implementation for that platform. With the large number of incompatible home computer systems in use at the time, this was an important advantage over using native code or developing a compiler for each system.

The "Z" of Z-machine stands for Zork, Infocom's first adventure game. Z-code files usually have names ending in .z1, .z2, .z3, .z4, .z5, .z6, .z7 or .z8, where the number is the version number of the Z-machine on which the file is intended to be run, as given by the first byte of the story file. Version# and specification. This is a modern convention, however. Infocom itself used extensions of .dat (Data) and .zip (ZIP = Z-machine Interpreter Program), but the latter clashes with the present widespread use of .zip for PKZIP-compatible archive files starting in the 1990s, after Activision had shut down Infocom. Infocom produced six versions of the Z-machine. Files using versions 1 and 2 are very rare. Only two version 1 files are known to have been released by Infocom, and only two of version 2. Version 3 covers the vast majority of Infocom's released games. The later versions had more capabilities, culminating in some graphic support in version 6.

The compiler (called Zilch) which Infocom used to produce its story files, has never been released, although documentation of the language used (called ZIL, for Zork Implementation Language) still exists and an open-source replacement (called ZILF) has been written. In May 1993, Graham Nelson released the first version of his Inform compiler, which also generates Z-machine story files as its output, even though the Inform source language is quite different from ZIL. Most files produced by Inform are version 5.

Inform has since become very popular in the interactive fiction community and, as a consequence, a large proportion of the interactive fiction now produced is in the form of Z-machine story files. Demand for the ability to create larger game files led Graham Nelson to specify versions 7 and 8 of the Z-machine, though version 7 is very rarely used. Because of the way addresses are handled, a version 3 story file can be up to 128K in length, a version 5 story can be up to 256K in length, and a version 8 story can be up to 512k in length. Though these sizes may seem small by today's computing standards, for text-only adventures, these are large enough for very elaborate games.

During the 1990s, Graham Nelson drew up a Z-machine standard, based on detailed studies of the existing Infocom files.



Interpreters for Z-code files are available on a wide variety of platforms. The Inform website lists links to freely available interpreters for 15 desktop operating systems (including 8-bit microcomputers from the 1980s such as the Apple II, TRS-80 and Sinclair, and grouping "Unix" and "Windows" as one each), 10 mobile operating systems (including Palm OS and the Nintendo Game Boy), and three interpreter platforms (Emacs, Java and JavaScript). According to Graham Nelson, it is "possibly the most portable virtual machine ever created".[1]

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