User Tools

Site Tools


ZXT Extension Format Specification

Version 1.0.0

By Adrian “asie” Siekierka

Special thanks: endgame, GreaseMonkey, Lancer-X, Noser, The Mysterious KM, WiL, zzo38

The specification follows a MAJOR.MINOR.PATCH numbering scheme, where:

  • Changes to the MAJOR number are expected to introduce breaking changes, resulting in a change of the extension header magic number;
  • Changes to the MINOR number are expected to introduce changes which don't break backwards compatibility, such as giving meaning to reserved values or defining undefined behaviour;
  • Changes to the PATCH number are expected to introduce changes which don't impact the meaning of the specification, such as fixing typos or clarifying used language.

For example, a ZXT 1.1.x world which doesn't utilize functionality specific to ZXT 1.1.x would be expected to work as-is in a ZXT 1.0.x engine implementation; there is no such requirement placed on a ZXT 2.0.x world.


Extending and tweaking the functionality of the ZZT game engines 1) has always been an undercurrent in their world development community. While many games relying on edited executables or TSRs 2) have been released in the past, they ran into key problems, keeping their count small and the overall idea unpopular:

  • The lack of source code greatly increased the difficulty of performing non-trivial modifications.
  • The requirement for special batch scripts or modified executables, typically made specifically for a given game, created additional hassle for the end user when trying to play such a game.
  • No consistent means of signaling required extensions was devised. For most worlds, this made them indistinguishable from ZZT 3.2-compatible ones for the end user or reimplementations, hurting compatibility.

However, the release of the Reconstruction of ZZT in 2020 greatly lowered the bar for creating such modified engine versions, solving the first problem. With it, interest in creating game-specific forks and providing enhanced functionality has reappeared. Nonetheless, the remaining issues persisted and, as such, a standardized way to declare engine extensions utilized by a given world was deemed necessary.

The following use cases were considered:

  • Creating forks and source ports of ZZT which can support any set of extensions at will,
  • Mixing and matching extensions from various creators within worlds,
  • Usage of extensions within typical ZZT game operations (playing, saving, editing).

The following categories of worlds were considered:

  • Completely custom experiences derived from ZZT game engines. Example: “The King in Yellow Borders”, WiL.
  • Creating games which can benefit from enhanced functionality, but are playable in unmodified ZZT game engines. Example: “Variety”, WiL.
  • Standardizing pre-fork games which modify ZZT or Super ZZT in an incompatible way. Example: “ZZT Enhancer”, Craig Boston; “Banana Quest”, WiL; 64K intros by bitbot.
  • Standardizing pre-fork games which modify ZZT or Super ZZT in ways which are technically compatible. Example: “Angelis Finale: Episode 1”, Commodore (custom character set); “Daedalus' Obelisk”, Darren Hewer (optional ZZT modification to remove certain sounds and messages).


The specification covers:

  • The format of extension header data and resulting world files,
  • Rules for processing extension data and handling identified edge cases,
  • Expectations placed upon game engines implementing the specification.

Notably, the specification currently does NOT cover:

  • The format of save (.SAV) files. We have decided not to mandate this at this time.
  • The format of board (.BRD) files. This is due to the niche nature of the subject, and is likely to be expanded upon in a future version of the specification.


These definitions may be used freely in extension standards without re-introduction.

  • The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this and following documents are to be interpreted as described in RFC 2119.
  • The following types, in data blob specifications, all little-endian (where applicable):
    • u8 - unsigned byte (8 bits),
    • s8 - signed byte,
    • u16 - unsigned word (2 bytes),
    • s16 - signed word,
    • u32 - unsigned dword (4 bytes),
    • s32 - signed dword,
    • u64 - unsigned qword (8 bytes),
    • s64 - signed qword,
    • bool - a special case of an u8, where zero means “false”, and non-zero means “true”,
    • pstring[X] - a Pascal-style string; a length u8 (N, of range between 0 and X) followed by X characters, of which the first N are considered valid.
    • pstring - a Pascal-style string; a length u8 (N, of range between 0 and 255) followed by N characters.
    • plstring - a Pascal-style long string; a length u16 (N, of range between 0 and 65535) followed by N characters.

Where the specification mentions the terms ZZT game engine or .ZZT, one may substitute Super ZZT game engine and .SZT respectively.

File Format


The extension data can be provided in one of two ways:

  • Providing a .ZAX file with the same filename as a world file (for instance, TOWN.ZZT and TOWN.ZAX) or board file, containing only the Extension Header.
  • Providing a .ZXT file, being formed as a concatenation of an Extension Header and a world file (cat TOWN.ZAX TOWN.ZZT > TOWN.ZXT is a valid operation) or board file.

The two approaches are made with specific intents in mind, but may be used in any way the creator sees fit:

  • The intent of .ZAX files is to extend games with additional functionality (keybind shortcuts, metadata, etc.), while preserving compatibility with the ZZT game engine. (They can also serve as patches on top of an otherwise unmodified .ZZT file.)
  • The intent of .ZXT files is to contain a modified ZZT game within one world file. This is especially useful for games which cannot be supported by unmodified versions of the ZZT game engine.

If a .ZXT file is being loaded, a .ZAX file should be ignored - .ZAX files apply for .ZZT files, not .ZXT files.

The behaviour of multiple concatenated extension headers contained within one .ZXT or .ZAX file is undefined; distributed .ZXT or .ZAX files MUST NOT rely on it.

Extension Header

The extension header's format is as follows:

Offset Type Name Description
0 u16 magic 0xF227 for ZZT-style worlds, 0xF527 for Super ZZT-style worlds, 0xB227 for ZZT-style boards, 0xB527 for Super ZZT-style boards.
2 u32 block_count The number of extension blocks which immediately follow.

After the header follow block_count extension blocks, in sequence:

Offset Type Name Description
0 u16 flags Extension flags; defined below.
2 u32 owner_id Extension owner ID.
6 u16 selector_id Extension selector ID.
8 u8 reserved_0 Reserved; must be set to 0.
9 u16 field_length Field length. Values between 0 and 65534 refer to the length in bytes; if set to 65535, an u32 containing the 32-bit field length in bytes follows.
11 u8[field_length] field_data Field data. Extension-defined.

An extension being “understood” is defined as one for whose ID pair the engine provides an implementation compliant with its standard.

The extension flags are as follows:

Bit Name Description Behaviour Layman's terms
0 parsing_must Required for parsing. If set, an implementation which does not understand this extension MUST NOT continue parsing of the extension block. Generally, this one will be set to 0.
1 reading_must Required for reading. If set, an implementation which does not understand this extension MUST NOT read the world file, but may continue parsing the extension block. If you're changing the world format in a breaking way, set this to 1.
2 writing_must Required for writing. If set, an implementation which does not understand this extension MUST NOT try to write the world file. If you're changing the world format in a partially-breaking way, set this to 1. (For instance, Variety redefines the flag section of a .ZZT file in a way which breaks reading only if at least two flags are set; however, a written world could have modified these flags, so we must prevent that. Another example would be re-using the padding fields.)
3 playing_should Recommended for playing. If set, an implementation which does not understand this extension SHOULD signal this to the end user if an attempt at playing the world is made. Non-strictly-breaking gameplay changes. For example, modified messages or sound effects. Sometimes, modified charsets or palettes.
4 playing_must Required for playing. If set, an implementation which does not understand this extension MUST signal this to the end user and prevent an attempt at playing the world. Breaking gameplay changes. New OOP commands if required, new element IDs, etc.
5 editing_should Recommended for editing. If set, an implementation which does not understand this extension SHOULD signal this to the end user if an attempt at editing the world is made. An engine does not have to make any effort to support such a world further. Non-format-breaking (format-breaking go into writing_must, reading_must or both) editor-side changes. Unknown element IDs count, for example.
6 preserve_should Recommended to preserve on resave. If set, an implementation SHOULD preserve the extension unchanged upon resave; if cleared, an implementation MUST NOT do so, and MUST discard the extension. Generally, you want this set to 1 - unless the field data relies on other board, world or extension data outside itself. Meant in particular for metadata.
7 vanilla_behavior Is this extension something ZZT does anyway? If set, an unmodified implementation of ZZT 3.2 or Super ZZT 2.0 may be assumed to support the playing_must and playing_should conditions without further additional support. This is meant for extensions which either (a) impose additional restrictions on the engine implementation not present in regular ZZT, or (b) depend on ZZT 3.2/Super ZZT 2.0 implementation details which are not required for an otherwise ZXT-compliant implementation. The intent is for external tooling to be able to declare a world ZZT 3.2-compatible.
8 .. 15 reserved Reserved. If set, an implementation MUST NOT continue parsing of the extension block.

It is important to note that the flags can be distinct from the ID pair; for instance, the same ZZT-OOP extension can be defined as “recommended for playing” if optional for gameplay, but “mandatory for playing” if required for gameplay. However, an extension standard may require you to set or clear certain bits for compliance.

Extension IDs

Extension IDs are allocated on a per-owner ID basis. Within one owner ID, selector IDs SHOULD be used in a “one per extension” manner.

Owner ID Ranges

  • The range 00000000 - 000000FF is reserved for standard extensions - types of changes everyone agrees upon, as well as potential expansions to this standard.
  • The range 00000100 - FFFFFEFF is available for public extensions - you are free to claim them via signaling intent on the ZXT Owner IDs page.
  • The range FFFFFF00 - FFFFFFFF is reserved for private use extensions - prototypes and/or experiments within internal/development versions of worlds and engine implementations. Publicly released worlds and engine implementations SHOULD NOT use this range.


Cross-Extension Interactions

  • Extension Order: Extensions MAY depend on their order of definition in the file. Implementations MUST, as such, preserve the order of extension definition when writing a ZXT file.
  • Repeated Extensions: Extensions MAY allow being defined multiple times within a single list of extension blocks. Implementations MUST apply the extensions in order; that is to say, a later extension MAY override the settings of an earlier extension.

Any interactions not explicitly defined and not part of any enabled extension's specification are undefined behaviour and MUST NOT be relied upon. For example:

  • One extension is defined as “increasing the number of Keys from 7 to 16”,
  • Another extension is defined as “redefining the Keys to allow holding multiple at a time”,
  • It is not certain what should happen if both extensions are in place, unless additional detail is provided. One could interpret it as only allowing the seven vanilla ZZT Keys to be held multiple at a time, or as allowing all sixteen to do so.

World Format

  • Header/Data Expansion: If an extension extends the board header, the world header, the tile data, the stat data, or any other existing in-file structure outside of the extension block, the order of parsing MUST be in ascending order of the 32-bit owner ID, then the 16-bit selector ID.
  • Incompatible Format Changes:
    • If any change which sets the reading_must flag to 1 is present, the implementation SHOULD set the world ID to 0xE227. Rare omissions are permitted, if the set of world format changes corresponds 1:1 to another non-standard world format ID (such as FreeZZT) - however, the list of extensions MUST reflect the same format changes. Extensions derive only from ZZT and Super ZZT's vanilla world formats.
    • If a ZXT or ZAX file is present, the implementation MUST ignore the world ID present in the file itself, and trust the ZXT/ZAX file instead.

Engine Extension Scope

  • ZXT standard compliance concerns only the game engine's operation. The user interface and other external factors not impacting gameplay or game presentation are not in scope.
  • An implementation MAY choose to be compatible only with ZXT worlds containing certain extensions. Compatibility with un-extended ZZT worlds for a ZXT implementation is OPTIONAL.

Engine Accuracy

Engines are expected to be accurate to the ZZT game engine's intended behaviour. However, preserving ZZT's memory and stack layouts - and the respective bugs - are OPTIONAL, with the following exceptions:

  • Implementations MUST emulate the Black Key. The behaviour, including the gem value change and the message duration, MUST be preserved, although the specific text message emitted MAY be changed.
  • Implementations MUST support reading the state of stat -1. While the state of stat -1 in ZZT may change in rare circumstances, it is recommended to assume the following values:
Field Value
X 0
Y 1
Step X 256
Step Y 256
Cycle 256
P1 0
P2 1
P3 0
Follower 1
Leader 1
Under Element 1, Color 0x00
Data Pos 1
Data Len 1

Note that any feature which crashes regular ZZT (or causes double-frees, particularly double #BIND) remains undefined behaviour. ZXT-compliant implementations may choose to fix them.

Of course, any of the assumptions in this section can be overridden by an extension.

Best Practices

This is not a formal part of the specification - it's more of an “advice” section.

World Developers

  • Distributing the world as a single .ZXT allows for greater user-friendliness than having to carry around two files, especially for non-ZZT-compatible worlds.
  • Once you opt into the .ZXT format, ZZT tricks relying on memory/stack layout are not guaranteed to work. If you're relying on such behaviours, consider making an extension standard for them instead! This means in particular, but not limited to:
    • Most out-of-bounds memory reads and writes,
    • The argTile2 trick pioneered in “Wake Up and Unlock The Door“.

Extension Specification Authors

  • As soon as your extension is used by publicly-released worlds or implemented in publicly-released implementations, the expectation is for it not to change in a breaking manner! While undefined or explicitly reserved behaviour may be clarified further, breaking changes should be introduced via new extensions. There are two ways to avoid that issue during development:
    • Use an ID inside the area of private use extensions while the extension's shape is not yet fully formed,
    • Mark your public specification as “Draft” while details are not fully decided upon.
  • If your extension is intended to work only with ZZT or Super ZZT, but not the other, it's a good idea to mention that explicitly in the specification.

Engine Developers

  • Not every ZXT-compliant engine implementation must provide every extension! Don't fall neck-deep into code bloat, especially on your first try. Focus on the ones you care about, or which games you enjoy utilize.
  • Remember that your save files need to be aware of which extensions ought to be enabled in order to correctly restore a gameplay session. An easy way for this is to do what ZZT itself does, and share the code for creating world files and save files.

Side Notes

This is not a formal part of the specification - rather, it provides insight into some contested decisions.

  • I am aware that the “owner/selector ID” system is not ideal. However, other alternatives considered had flaws deemed more important:
    • A linear ID allocation system would most likely cause conflicts;
    • A string-based ID system would increase implementation difficult, particularly on older compilers;
    • The owner/selector divide was introduced after the decision to faciliate 48-bit IDs on older compilers, particularly Turbo Pascal. While 32-bit IDs were considered, they were rejected as potentially being too constraining.
  • Save files are not mandated by the specification for two key reasons:
    • Some extensions may require data specific to save files, and implementations may wish to have the freedom to decide how to store them in this case;
    • Some implementations may store ZZT data in a custom format internally, and removing this requirement can make their code simpler.
  • Multiple concatenated extension headers may be used as a feature in the future - for example, for faciliating multi-patch application for ZZT worlds.


This is not a formal part of the specification.


ZXT 1.0.x

  • ZXTMGR v0.3.1 - Turbo Pascal 5.5-compatible library, intended for using within a ZZT fork.


ZXT 1.0.x

  • ZAXMAKE, PostScript tooling for defining ZXT extension headers.
  • ZXTSPLIT, C utility for extraction, listing and maintenance of ZXT/ZAX files.



ZZT and Super ZZT
Terminate-Stay Resident programs
zxt/format_specification.txt · Last modified: 2021/08/11 06:21 by zzo38