Zork Implementation Language

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 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.

History and design

Nomenclature and conventions

The "Z" of Z-machine stands for ''Zork'', Infocom's first adventure game. Infocom used file extensions of .dat (Data) and .zip (ZIP = Z-machine Interpreter Program), but the latter clashed with the widespread use of .zip for PKZIP-compatible archive files starting in the 1990s, after Activision had closed 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 majority of Infocom's released games. Later versions had more capabilities, culminating in some graphic support in version 6.

The modern convention for 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. As previously noted, the Infocom games used the equivalent of .z1 through .z6; .z7 and .z8 were proposed and adopted after Infocom had shut down.

Before Z-machine

The MDL programming language was derived from Lisp at the Massachusetts Institute of Technology by the Dynamic Modeling group of the Laboratory for Computer Science (LCS) in the 1970s; inspired by '' Colossal Cave Adventure'' (1977), members of that group went on to write ''Zork'' in MDL, completing the initial version two weeks later. Like ''Adventure'', ''Zork'' was programmed for the DEC PDP-10; the finished version occupies 1 MB of MDL code and requires 512 KB of RAM to run. Because contemporary home computers did not have these resources, considerable effort was needed to port the game. Eventually, the developers split ''Zork'' into two games for personal computers.

The compression required to run ''Zork'' from floppy disks with approximately 80 KB of storage seemed like an insurmountable barrier to Blank. Berez realized that UCSD Pascal used a virtual machine (VM) model to generate executable files that could be readily ported across platforms, and together with Blank, they devised requirements for a lightweight VM optimized for text adventure games, which would retrieve data and execute instructions as needed from storage to compensate for the relatively small RAM sizes in typical microcomputers. The resulting Z-machine used an object tree structure for in-game items, locations, characters, and weapons. For comparison, the Z-machine parser occupies 3  kB of storage, while the original PDP-10 parser occupies 10  Kwords ( 36-bit).

ZIL and ZIP, ZILCH and ZAP

To complement the Z-machine, Infocom developed the high-level computer language Zork Implementation Language (ZIL) by streamlining MDL, and the Z-language Interpreter Program (ZIP), which compiles ZIL into Z-machine instructions in a two-stage process; this made text adventure development platform-independent and enabled porting to different systems simply by writing an appropriate Z-machine interpreter. ZIP consists of a compiler (ZILCH, short for ZIL Compiler Hack) and an assembler (ZAP, the Z-machine Assembler Program).

ZILCH has never been released, although documentation of ZIL still exists, and an open-source replacement "ZILF" has been written. After Mediagenic moved Infocom to California in 1989, ''Computer Gaming World'' stated that "ZIL ... is functionally dead", and reported rumors of a "completely new parser that may never be used".

Graham Nelson and ''Inform''

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.

Inform has become popular in the interactive fiction community. A large proportion of interactive fiction is in the form of Z-machine story files. Demand for the ability to create larger game files led Nelson to specify versions 7 and
8 of the Z-machine, though version 7 is 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 elaborate games.

During the 1990s, Graham Nelson drew up a Z-Machine Standard based on detailed studies of the existing Infocom files. The standard also includes extensions used by his newer versions, as well as links to the "Blorb" resource format used by Infocom, and a "Quetzal" savefile format. In 2006, Nelson expanded Z-machine to the 32-bit Glulx format for Inform 7. The Interactive Fiction Technology Foundation, founded 2016, manages all these standards.

ZIL (Zork Implementation Language)

The ZIL is based on MDL from MIT. Here is the definition of Zork I's brass lantern in ZIL, with added comments to illustrate the meaning of each line: