Z-Machine#

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Z-Machine Specifications#

Lifespan

1979-present (Z-machine versions 1-8)

Media

Story files (.z1-.z8), typically distributed on disk

CPU

Virtual machine (16-bit, stack-based), implemented in software

Memory

128K-512K address space (version-dependent)

Display

Text-based (virtual terminal), typically 80x24

Sound

None (some interpreters support beeps)

Input

Text parser (keyboard)

Best-known games

Zork, The Hitchhiker’s Guide to the Galaxy, Planetfall

History#

The Z-Machine is a virtual machine created by Infocom for its line of interactive fiction games. It was designed in 1979 by Joel Berez and Marc Blank, who wanted a portable architecture so that games could run on many different computers (the name “Z” comes from the original codename “Zork”).

Each game is a “story file” containing a program for the Z-Machine. The story file is interpreted by a “Z-machine interpreter” (called ZIP in Infocom terminology), which was written for dozens of platforms: the Apple II, Commodore 64, IBM PC, TRS-80, Atari 8-bit, Amiga, and many more. The same story file runs unchanged on all of them.

The Z-Machine standard defines versions 1 through 8. Versions 1-5 were used for Infocom’s early and mid-period games; versions 6 and 7 added graphics and sound support (used by Journey and other later titles); version 8 was used for the later games like Beyond Zork and Zork Zero.

Today the Z-Machine lives on through the Inform compiler, which compiles modern interactive fiction into Z-machine story files, and through interpreters like Frotz and the web-based Quixe. The 8bitworkshop IDE supports development in Inform 6 and Dialog.

The Z-Machine Architecture#

The Z-Machine is a stack-based virtual machine with the following components:

  • 16-bit words - the basic unit of data (and 8-bit bytes)

  • Program counter - points into the story file’s code

  • Stack - holds subroutine arguments, return values, and local variables

  • Variables - 16-bit registers: 240 global variables, 15 local variables per routine

  • Object tree - a hierarchy of objects with properties (used for room/object modeling)

  • Dictionary - the game’s vocabulary, used by the text parser

  • Text compression - Z-character encoding packs text into 5-bit characters with abbreviations and ZSCII escape sequences

The standard Z-machine has a 128K address space (versions 1-5) or 512K (version 8), plus a 64K “high memory” area for the game’s code and a “low memory” area for global variables and the object tree.

Memory Layout#

Region

Contents

$0000-$003F

Header (magic number, version, flags, etc.)

Low memory

Globals, object tree, dictionary, abbreviations

High memory

Code, strings, arrays

(Version 6+)

Graphics, sound resources

The Object Model#

Interactive fiction is built around the Z-machine’s object model:

  • Objects represent rooms, items, and actors

  • Objects have properties (descriptions, state) and parent/sibling/child links forming a tree

  • The parser matches player input against the dictionary and invokes routines that manipulate the object tree

The Inform Compiler#

Inform is the primary language for writing Z-machine games. The 8bitworkshop IDE supports two flavors:

Inform 6#

Inform 6 is the “classic” Inform language, a procedural language with syntax derived from C and Pascal:

! Hello world in Inform 6
[ Main;
  print "Hello, world!^";
  quit;
];

Inform 6 programs are compiled with the inform6 compiler to produce Z-machine story files. The 8bitworkshop IDE includes several Inform 6 example programs, including the “House Tutorial” series that walks through building a complete game.

Dialog#

Dialog is a modern declarative language for interactive fiction that compiles to Z-machine story files. It emphasizes rules and relations rather than procedural code. The IDE includes Cloak of Darkness as a Dialog example.

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