a2zip - Infocom ZIP interpreters for Apple II, Z-Machine architectures 1 through 4, partially reverse-engineered

z2zip development is hosted at the a2zip Github repository.

Introduction

Infocom games are written in ZIL and compiled to Z-code, which is interpreted on a ZIP interpreter. ZIP interpreters were written for many computers, including the Apple II. In 1983 to 1984 I reverse-engineered a substantial portion of the Apple II version 1 through 3 ZIP interpreters.

More recently I've partially reverse-engineered a few newer Apple II ZIP and EZIP interpreters.

ZIP (Z-Machine architectures v1 through v3)

The Apple II ZIP intepreters for v1, v2, v3, v3 A, and v3B can be built from the source file "zip.asm".

Z-Machine architectures v1 through v3 have only fairly minor differences. The interpreters for these versions are known as ZIP. The v1 and v2 architectures were only used for early releases of Zork I and Zork II, and were quickly obsolted by v3. which was the most common architecture for Infocom games that did not require more than 128 KiB of virtual machine memory.

Z-Machine architecture v1 is only known to have been used for Zork I release 2 (which might not have been released for the Apple II), and Zork I release 5. Zork I release 5 was provided for the Apple II on a 13-sector disk, while no later releases or other Infocom games were available on 13-sector disks.

Z-Machine architecture v2 is only known to have been used for Zork I release 15 and Zork II release 7.

The earliest v3 interpreter for the Apple II, like those for v1 and v2, did not have any more specific interpreter revision identification. Later v3 interpreter revisions had a revision letter, with revisions A, B, F, H, K, and M known to exist. Of the lettered revisions, only A and B are currently represented here.

Revision A added support for the Apple IIe 80-column text mode, with upper and lower case display, and allowed the user to select a slot number for the printer interface card, which previously was required to be slot 1.

Revision B added support for splitting the screen into two windows.

Between Revision B and revision F, a substantial rearrangement of the code occurred. These and later changes to the v3 interpreter have not yet been analyzed, and are not currently represented here.

EZIP

Z-Machine architecture v4 doubled the available virtual machine memory size to 256 KiB, allowed a game to have more objects, allowed vocabulary words to have up to nine significant characters, and added some improvements to I/O capabilities. The v4 interpreters are known as EZIP. There were five revisions of the Apple II EZIP interpreter, designated 2A through 2D, and 2H.

Early work on reverse-engineering these is present in the source file "ezip.asm".

XZIP and YZIP

Eventually Infocom added even more capabilities, resulting in Z-Machine architectures 5 and 6, with interpreters known as XZIP and YZIP. These are not currently represented here.

Archive of earliest reverse-engineered source files

When I originally reverse-engineered the early interpreters in 1983-1984, I assembled the code using Microsoft ALDS, which is not reasily available or easy to use today. The version of the sources for that assembler are in the "alds" subdirectory.

The current source code assembles using the Macro Assembler AS: http://john.ccac.rwth-aachen.de:8000/as/

Acknowledgements

Special thanks to:

Richard Turk, for saving the printed listing from the work I did in 1983-1984, which was otherwise lost
4am, for preserving disk images of huge amounts of Apple II software, including early, now-rare Infocom games (e.g., Zork I release 5, Zork II release 7)

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