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. 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 (which did not exist yet during the time Infocom was active). 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. But 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.
Popular interpreters include Nitfol and Frotz. Nitfol makes use of the Glk API, and supports versions 1 through 8 of the Z-machine, including the version 6 graphical Z-machine. Save files are stored in the standard Quetzal save format. Binary files are currently available for several different operating systems, including Macintosh, Linux, MS-DOS, and Windows.
Frotz is perhaps the most well-known and popular Z-machine implementation available. Its advantages over other Z-machine interpreters are twofold: firstly, though it was not the first non-Infocom interpreter to be released, it was one of the early ones -- its initial release by Stefan Jokisch was in 1995. Secondly, because the program is written in highly portable C, it has been possible to port the original DOS version to most modern computer formats, including not only Unix and Windows but even palmtops and mobile phones. Various extensions have since been added, such as sound effects and graphics.
In 2002, the Frotz core codebase was picked up by David Griffith, who continues to develop it. The codebase was then distinctly split between the virtual machine and the user interface portions such that the virtual machine became entirely independent from any user interface. This allowed some clever programmers to create some of the stranger ports of Frotz. One of the strangest is also one of the simplest: an instant messenger bot is wrapped around a version of Frotz with the bare minimum of IO functionality creating a bot with which one can play most Z-machine games using an instant messenger.