Inform 7 and Z-Machine Interpreters
January 2006

This note accompanies the public beta release of Inform 7, a new
design system for interactive fiction based on natural language.

Inform 7 is built on Inform 6 as a foundation, and consequently it
compiles story files which comply with the Z-Machine Standards
Document (ZMSD). All the same, minor improvements to interpreters
might make them deal much better with Inform 7-written games. This
may be a minor nuisance to interpreter writers and maintainers, for
which I apologise, but I hope the effort will be thought worthwhile.
The changes described have been made to two interpreters thus far:
Windows Frotz and Zoom for OS X (and Unix).


1. The Z-machine stack

I7 is a rules-based system, and complex I7 code makes heavy use of
the Z-machine stack. At one stage, one of the "Test Suite" of games
used in unit testing of I7 crashed dumb-frotz, the dumb-terminal
interpreter we use for unit testing: it developed that the 4K of
stack allocated by dumb-frotz had overflowed. Optimisation of I7's
stack usage enabled us to bring the Test Suite back into working
condition even on dumb-frotz, but the problem is likely to recur
with more complicated I7 games in the future, and we would prefer
to avoid this if possible.

The ZMSD does not formally specify a size for the stack, only a
minimum likely to make Infocom story files playable. Moreover,
the ZMSD does not specify how the stack is to be stored: on most
interpreters it holds both explicitly stacked values and also
stack frames of routines, but Zoom keeps its stack frames
elsewhere, for instance. This unfortunately means that story
files crashing out on one interpreter may work fine on another.

We ask that interpreter-writers raise the stack size as far as
sensibly possible. On genuinely small machines, this could be a
waste of memory, but at any rate please ensure a stack of no
lower than 16K in size, and if memory is not a concern then
please allow 64K or more, just in case.


2. Blorb and Babel

Inform 7 produces blorbed Z-code story files with the file extension
".zblorb" by default: inside are ".z5", ".z6" or ".z8" story files.

The blorbs comply with v2 of the Blorb specification,

    http://eblong.com/zarf/blorb/blorbtmp.html

and contain bibliographic data and, often, also cover art compliant
with the Treaty of Babel:

	http://babel.ifarchive.org/

Note that Babel provides an API capable of extracting the contents
of a Treaty-compliant blorb, and reading any useful information from
its bibliographic data (e.g. the title and author), for very little
coding overhead.