11. The format of the header

11.1

The header table summarises those locations in the Z-machine's header which an interpreter must deal with. (For further notes on traditional usage, see Appendix B.) "Hex" means the address, in hexadecimal; "V" the earliest Version to which the rule is applicable; "Dyn" means that the byte or bit may legally be changed by the game during play; "Int" means that the interpreter may change it; "Rst" means that the interpreter must set it correctly after loading the game, after a restore or after a restart.

Some early Version 3 files do not contain length and checksum data, hence the notation 3+.

11.1.1

It is illegal for a game to alter those fields not marked as "Dyn". An interpreter is therefore free to store values of such fields in its own variables.

11.1.2

The state of the transcription bit (bit 0 of Flags 2) can be changed directly by the game to turn transcribing on or off (see S 7.3, S 7.4). The interpreter must also alter it if stream 2 is turned on or off, to ensure that the bit always reflects the true state of transcribing. Note that the interpreter ensures that its value survives a restart or restore.

11.1.3

Infocom used the interpreter numbers:

   1   DECSystem-20     5   Atari ST           9   Apple IIc
   2   Apple IIe        6   IBM PC            10   Apple IIgs
   3   Macintosh        7   Commodore 128     11   Tandy Color
   4   Amiga            8   Commodore 64

(The DECSystem-20 was Infocom's own in-house mainframe.) An interpreter should choose the interpreter number most suitable for the machine it will run on. In Versions up to 5, the main consideration is that the behaviour of 'Beyond Zork' depends on the interpreter number (in terms of its usage of the character graphics font). In Version 6, the decision is more serious, as existing Infocom story files depend on interpreter number in many ways: moreover, some story files expect to be run only on the interpreters for a particular machine. (There are, for instance, specifically Amiga versions.)

11.1.3.1

Interpreter versions are conventionally ASCII codes for upper-case letters in Versions 4 and 5 (note that Infocom's Version 6 interpreters just store numbers here).

Modern games are strongly discouraged from testing the interpreter number or interpreter version header information for any game-changing behaviour. It is rarely meaningful, and a Standard interpreter provides many better ways to query the interpreter for information.

11.1.4

*** The use of bit 7 in 'Flags 1' to signal whether timed input is available is new in this document: see the preface.

11.1.5

*** If an interpreter obeys Revision n.m of this document perfectly, as far as anyone knows, then byte $32 should be written with n and byte $33 with m. If it is an earlier (non-standard) interpreter, it should leave these bytes as 0.

11.1.6

The file length stored at $1a is actually divided by a constant, depending on the Version, to make it fit into a header word. This constant is 2 for Versions 1 to 3, 4 for Versions 4 to 5 or 8 for Versions 6 and later.

11.1.7

The header extension table provides potentially unlimited room for further header information. It is a table of word entries, in which the initial word contains the number of words of data to follow.

11.1.7.1

If the interpreter needs to read a word which is beyond the length of the extension table, or the extension table doesn't exist at all, then the result is 0.

11.1.7.2

If the interpreter needs to write a word which is beyond the length of the extension table, or the extension table doesn't exist at all, then the result is that nothing happens.

11.1.7.3

*** Words in the header extension table have been allocated as follows:

Remarks

In the Infocom period, the larger Version 3 story files would not entirely fit on a single Atari 800 disc (though they would fit on a single Apple II, or a single PC disc). Atari versions were therefore made which were identical to the normal ones except for having Flags 1 bit 2 set, and were divided into the resident part on one disc and the rest on another. (This discovery was announced by Stefan Jokisch on 26 August 1997 and sees the end of one of the very few Z-machine mysteries left when Standard 1.0 was first published.)

See the "Infocom fact sheet" for numbers and letters of the known interpreters shipped by Infocom. Interpreter versions are conventionally the upper case letters in sequence (A, B, C, ...). At present most ports of Zip use interpreter number 6, and most of ITF use number 2.

The unusual behaviour of 'Beyond Zork' concerns its character graphics: see the remarks to S 16.

The Macintosh story file for 'Zork Zero' erroneously does not set the pictures bit (Flags 2, bit 3).

The bit in the header described as "requesting screen redraw" may be set by modern interpreters after, for example, resizing the "screen"; games should ideally redraw the screen if they see this bit set. This will usually mean the game clears the screen contents and rearranges borders, etc, so the bit should not be set except when necessary.

The (Version 6) sound and picture bits in Flags 1 indicate general availability of sound and graphics - ie whether the associated opcodes are available and functional.

The bits in Flags 2 should ideally be set reflecting current availability, rather than general support. In other words, if no Blorb (or other) resources for this story file have been found, or if the Blorb file contains no graphics or no sound, the corresponding bits should be cleared.

Also, it is recommended that interpreters that would prompt for an auxiliary Blorb file should do so immediately on start up if any of the "game wants to use sound/music/graphics" bits are set; this allows the bits to be cleared if no file is forthcoming, before the game starts execution. The game can then take appropriate action.

Contents / Preface / Overview

Section 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16

Appendix A / B / C / D / E / F