View source for Z-machine

{{Short description|Virtual machine for interactive fiction games}}
{{about|the Infocom virtual machine|the x-ray generator informally known as the "Z machine"|Z Pulsed Power Facility|the IBM mainframe architecture|IBM Z|operating signals used in radio communication|Z code|intermediate language used in ALGOL 68C |ZCODE}}

{{Infobox CPU architecture
| name       = Z-machine
| designer   = [[Infocom]]
| bits       = 16
| introduced = 1979
| version    = 1.1 (2014)
| design     = [[Complex instruction set computer|CISC]]
| type       = 
| encoding   = 
| branching  = 
| endianness = Big
| page size  = 
| extensions = 
| open       = Yes
| registers  = 
| gpr        = 
| fpr        = 
}}

The '''Z-machine''' is a [[virtual machine]] that was developed by Joel Berez and [[Marc Blank]] in 1979 and used by [[Infocom]] for its [[interactive fiction|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 [[ZIP file format|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.<ref name="version">{{cite web|title=The Z-Machine Standards Document|url=https://inform-fiction.org/zmachine/standards/z1point0/appd.html|website=inform-fiction.org|access-date=26 March 2018}}</ref> 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)|MDL]] programming language was derived from [[Lisp (programming language)|Lisp]] at the [[Massachusetts Institute of Technology]] by the Dynamic Modeling group of the [[MIT Computer Science and Artificial Intelligence Laboratory|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.<ref>{{cite journal |doi=10.1109/MC.1979.1658697 |title=Zork: A Computerized Fantasy Simulation Game |first1=P. David |last1=Lebling |first2=Marc S. |last2=Blank |first3=Timothy A. |last3=Anderson |date=April 1979 |volume=12 |pages=51–59 |journal=Computer |publisher=IEEE Computer Society}}</ref><ref name=Infocom-story>{{cite web |url=http://infocom.elsewhere.org/mit-class-history/infocom-paper.pdf |title=Down From the Top of Its Game: The Story of Infocom, Inc. |first1=Hector |last1=Briceno |first2=Wesley |last2=Chao |first3=Andrew |last3=Glenn |first4=Stanley |last4=Hu |first5=Ashwin |last5=Krishnamurthy |first6=Bruce |last6=Tsuchida |publisher=MIT Course 6.933J/STS.420J (Structure of Engineering Revolutions) |date=December 15, 2000 |access-date=3 January 2025}}</ref>{{rp|5–6}} Like ''Adventure'', ''Zork'' was programmed for the [[Digital Equipment Corporation|DEC]] [[PDP-10]]; the finished version occupies 1&nbsp;MB of MDL code and requires 512&nbsp;KB of RAM to run. Because contemporary home computers did not have these resources, considerable effort was needed to port the game.<ref name=Infocom-story/>{{rp|11}} Eventually, the developers split ''Zork'' into two games for personal computers.<ref>{{cite magazine |url=https://archive.org/details/byte-magazine-1980-12/page/n173/mode/2up |title=Zork and the Future of Computerized Fantasy Simulations |first1=P. David |last1=Lebling |date=December 1980 |magazine=BYTE |pages=172–182}}</ref>

The compression required to run ''Zork'' from [[floppy disk]]s with approximately 80&nbsp;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.<ref name=Infocom-story/>{{rp|11}} The resulting Z-machine used an [[Tree (abstract data type)|object tree structure]] for in-game items, locations, characters, and weapons.<ref name=Infocom-story/>{{rp|11}} For comparison, the Z-machine parser occupies 3&nbsp;[[Byte|kB]] of storage, while the original PDP-10 parser occupies 10&nbsp;[[Word (computer architecture)|Kwords]] ([[36-bit computing|36-bit]]).<ref name=CrCo-1980/>

===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,<ref name=CrCo-1980>{{cite magazine |url=https://archive.org/details/creativecomputing-1980-07/page/n81/mode/2up |title=How to Fit a Large Program Into a Small Machine or How to fit the Great Underground Empire on your desk-top |first1=Marc S. |last1=Blank |first2=S. W. |last2=Galley |date=July 1980 |magazine=Creative Computing |pages=80–87 |access-date=7 January 2025}}</ref> 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.<ref name=Infocom-story/>{{rp|12–13}} ZIP consists of a [[compiler]] (ZILCH, short for ZIL Compiler Hack) and an [[Assembly language#assembler|assembler]] (ZAP, the Z-machine Assembler Program).<ref>{{cite web |url=https://eblong.com/infocom/ |title=The Obsessively Complete INFOCOM Catalog |first=Andrew |last=Plotkin |website=eblong.com |access-date=3 January 2025}}</ref>

ZILCH has never been released, although documentation of ZIL still exists, and an open-source replacement "ZILF"<ref name="ZILF">{{cite web |url=https://foss.heptapod.net/zilf/zilf|title=ZILF|first=Tara|last=McGrew|website=foss.heptapod.net|access-date=22 August 2025}}</ref> 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".<ref name = "CGW">{{cite magazine |date = September 1989| issue=63 | magazine = [[Computer Gaming World]] | title = Inside the Industry: Infocom's West Coast Move Stirs Controversy | pages = 10 | url=https://archive.org/details/Computer_Gaming_World_Issue_63/page/n9/mode/1up}}</ref>

===Graham Nelson and ''Inform''===
{{main|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 and the "[[Quetzal file format|Quetzal]]" savefile format.<ref name="standard">{{cite web|title=Inform - ZMachine - Standards|url=https://inform-fiction.org/zmachine/standards/index.html|website=inform-fiction.org|access-date=26 March 2018}}</ref> 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.<ref>{{cite web |title=Glk, Glulx, and Blorb Specifications |url=https://github.com/iftechfoundation/ifarchive-if-specs |publisher=Interactive Fiction Technology Foundation |date=16 November 2022}}</ref>

==ZIL (Zork Implementation Language)==

The ZIL is based on [[MDL (programming language)|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:<ref>{{cite web |url=https://www.filfre.net/2012/01/zil-and-the-z-machine/ |title=ZIL and the Z-Machine |first=Jimmy |last=Maher |website=The Digital Antiquarian |date=January 7, 2012 |access-date=3 January 2025}}</ref>

<syntaxhighlight lang="clojure">
<OBJECT LANTERN                 #_Defines the LANTERN object
  (LOC LIVING-ROOM)             #_Defines the initial object location
  (SYNONYM LAMP LANTERN LIGHT)  #_Defines synonyms that can be used instead of LANTERN
  (ADJECTIVE BRASS)             #_Optional adjective to distinguish this lantern from other lanterns
  (DESC "brass lantern")        #_Short description in inventory list
  (FLAGS TAKEBIT LIGHTBIT)      #_What is this object capable of? It can be TAKEN. It provides LIGHT.
  (ACTION LANTERN-F)            #_Subroutine which defines special actions with this object
  (FDESC "A battery-powered lantern is on the trophy case.")  #_Description during first encounter
  (LDESC "There is a brass lantern (battery-powered) here.")  #_Later description in other locations
  (SIZE 15)>                    #_Defines weight to limit inventory capacity
</syntaxhighlight>

The equivalent object in MDL is defined as:

<syntaxhighlight lang="lisp">
<OBJECT ["LAMP" "LANTE" "LIGHT"]
        ["BRASS"]
        "lamp"
        <+ ,OVISON ,TAKEBIT ,LIGHTBIT>
        LANTERN
        ()
        (ODESC0 "A battery-powered brass lantern is on the trophy case."
         ODESC1 "There is a brass lantern (battery-powered) here."
         OSIZE 15
         OLINT [0 >])>
</syntaxhighlight>

A more complex example involving combat, along with its MDL Zork equivalent, is presented in a 2019 blog post by Andrew Plotkin. Notably, the Z-machine has no support for [[Garbage collection (computer science)|garbage collection]] and ZIL has no concept of Lisp's list system.<ref>{{cite web |last1=Plotkin |first1=Andrew |title=What is ZIL anyway? |url=https://blog.zarfhome.com/2019/04/what-is-zil-anyway.html |website=Zarf |language=en |date=April 17, 2019}}</ref>

==Interpreters==
[[File:Zork on Frotz on iPhone.jpg|right|thumb|An implementation of Frotz running on an [[iPhone]], playing the [[MIT]] version of ''[[Zork]]''.]][[Interpreter (computing)|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 {{nowrap|[[ZX Spectrum]]}}, and grouping "Unix" and "Windows" as one each), 10 mobile operating systems (including [[Palm OS]] and the [[Game Boy]]), and four interpreter platforms ([[Emacs]], Java, [[JavaScript]], and Scratch). According to Nelson, it is "possibly the most portable virtual machine ever created".<ref name="inform-interpreters">{{cite web |url=https://www.inform-fiction.org/zmachine/interpreters.html |first=Graham |last=Nelson |title=About Interpreters |work=Inform website |access-date=2009-11-07}}</ref>

Popular interpreters include Nitfol and Frotz. Nitfol makes use of the [[Glk (software)|Glk]] [[application programming interface|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 file format|Quetzal]] save format. [[executable|Binary]] files are available for several different [[operating systems]], including [[classic Mac OS]], [[Unix-like]] systems, [[Windows]].<ref name="if-archive-infocom-interpreters-nitfol">{{cite web |url=https://www.ifarchive.org/indexes/if-archiveXinfocomXinterpretersXnitfol.html |title=if-archive/infocom/interpreters/nitfol |access-date=2016-10-29}}</ref>

Frotz was written in [[C (programming language)|C]] by Stefan Jokisch in 1995 for DOS. Over time it was ported to other platforms, such as [[Unix-like]] systems,<ref name="frotz-history">{{cite web |url=https://gitlab.com/DavidGriffith/frotz/blob/master/README |title=Frotz README file on Gitlab |access-date=2019-02-19}}</ref> [[RISC OS]],<ref>{{cite web|url=http://www.zen22994.zen.co.uk/musus/frotz/|title=The RISC OS Frotz Home Page|date=1999-09-18|archive-url=https://web.archive.org/web/20211015144705/http://www.zen22994.zen.co.uk/musus/frotz/|archive-date=2021-10-15}}</ref> and [[iOS]].<ref>{{cite web|url=https://apps.apple.com/us/app/frotz/id287653015|title=Frotz on the App Store|website=App Store}}</ref> Sound effects and graphics were supported to varying degrees. By 2002, development stalled and the program was picked up by David Griffith. The code base was split between [[virtual machine]] and [[user interface]] portions in such a way that the virtual machine became independent from any user interface. This allowed more variety in porting Frotz. One of the stranger ports is also one of the simplest: an [[instant messaging]] [[Internet bot|bot]] is wrapped around a version of Frotz with the minimum [[Input/output|I/O]] functionality creating a bot with which one can play most Z-machine games using an instant messaging client.<ref name="frotz-dumb">{{cite web |url=https://gitlab.com/DavidGriffith/frotz/blob/master/DUMB |title=Frotz DUMB file on Gitlab |access-date=2019-02-19}}</ref>

Another popular client for [[macOS]] and other Unix-like systems is Zoom.<ref name="logicalshift-zoom">{{cite web |url=http://www.logicalshift.co.uk/unix/zoom/ |title=Logical Shift Zoom |access-date=2016-10-29}}</ref> It supports the same [[Quetzal file format|Quetzal]] save-format, but the packaging of the file-structure is different.

==See also==
*[[Glulx]] – Similar to the Z-machine, but relieves several limitations
*[[Inform]] – A programming language that can produce Z-machine programs
*[[SCUMM]] – by [[LucasArts]], a graphical system similar to Z-machine
*[[TADS]] – Made to address some of the Z-machine's limitations

== References ==
{{reflist}}

==External links==
*[https://inform-fiction.org/zmachine/standards/ The Z-Machine standards document]
*{{webarchive|url=https://web.archive.org/web/20100807003406/http://xlisp.org/zil.pdf|date=August 7, 2010|title=Learning ZIL}} (PDF) is the Infocom ZIL manual from 1989
*{{webarchive|url=https://web.archive.org/web/20120309172205/http://xlisp.org/zip.pdf|date=March 9, 2012|title=Description of ZIP}} (PDF) the Z-Language Interpreter Program (Infocom Internal Document) from 1989
*[https://www.ifarchive.org/indexes/if-archiveXinfocomXinterpreters.html Interpreters]
*[https://www.mud.co.uk/richard/htflpism.htm How to Fit a Large Program Into a Small Machine] describes the creation and design of the Z-machine

{{Infocom games}}
{{Video game engines}}

[[Category:Interactive fiction engines]]
[[Category:Virtual machines]]
[[Category:Infocom]]
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