Building Infocom Disk Images for CoCo

In the previous posts in this series, I have discussed pairing original Infocom Z interpreter software with new Z code story files in order to enable new interactive fiction games to run on the CoCo. In this post, I intend to tie-up some loose ends and to detail the actual process involved.

Image Issues

The outcome of this whole process will be to build a diskette image. The resulting image can be used as-is in MAME or another emulator, it can be loaded on a real CoCo with a CoCoSDC device or using DriveWire, or it can be used to write an actual physical diskette. Choosing and following the appropriate process for any of the above will be left as an exercise or the reader.

This post will focus on producing what is known as a "JVC" diskette image. In its simplest form, this format is merely a sequential arrangement of sector data arranged sequentially by track, yielding a 161280 byte file representing a single-sided disk with 35 tracks, 18 sectors per track, and 256 bytes per sector. This simple, sequential data layout makes reading and writing specific track data by offset into the disk image very simple.

Filesystem Concerns

The Disk Extended Color BASIC (DECB) code on the CoCo reserves track 17 of the disk for storing the disk's directory. Depending on how a disk is used (i.e. how programs are started), a valid directory may not be required. However, at least some versions of the Infocom Z interpreter rely on being loaded via LOADM and therefore require a valid directory to be present on track 17 of the disk image.

Many CoCo games (including some versions of the Infocom Z interpreter) depend on being started by the DOS command in DECB. The DOS command loads data from track 34 of the disk into a fixed location in memory, checks for a simple signature at the head of that data, and then jumps into the loaded program if the proper signature is found. Care must be taken to preserve the contents of track 34 if use of the DOS command is required to start the game.

Interpreter Differences

I loaded each of the Infocom games from the disk images at the Color Computer Archive. After starting each game, I issued the $VERSION command in order to collect version information from each interpreter. As shown in the table below, at least three interpreter versions were shown to be used by Infocom to package their games for the CoCo!

Result of $VERSION Command by Game

Game	Version
Ballyhoo	COCO VERSION D
Cutthroats	COCO 2 VERSION C
Deadline	COCO 2 VERSION C
Enchanter, The	COCO VERSION D
Hitchhiker's Guide to the Galaxy	COCO 2 VERSION C
Infidel	COCO 2 VERSION C
Lurking Horror, The	COCO 2 VERSION C
Planetfall	COCO 2 VERSION C
Sea Stalker	VERSION A
Sorcerer	VERSION A
Spellbreaker	COCO 2 VERSION C
StarCross	COCO 2 VERSION C
Stationfall	COCO 2 VERSION C
Suspect	VERSION A
Suspended	VERSION A
Wishbringer	COCO 2 VERSION C
Witness, The	COCO VERSION D
Zork I	COCO 2 VERSION C
Zork II	COCO 2 VERSION C
Zork III	COCO 2 VERSION C

Oddly enough, no interpreter found reported anything like "VERSION B". Was a version "B" ever released? What about a version "E" or other later version? Also, if we relate the alphabetical version order to the original release dates of the games above, the release order of the games does not seem to match the version order for the CoCo Z machine interpreters. Were the CoCo versions released in a different order than the original game release order? Or perhaps the diskettes imaged on the Color Computer Archive were purchased in random order over a period of time, while the diskettes sold were sent with whatever version of the CoCo Z machine interpreter was current at the time?

If you have an original Infocom game disk for the CoCo, please boot it up and issue the "$VERSION" command. If you get different results than shown in the table above, then _PLEASE_ let me know in the comments below or however else you may know to reach me!

Given the above, a decision must be made as to which interpreter version to use. The "VERSION A" interpreters are not started via the DOS command, but instead require either the standard "LOADM : EXEC" combo or a BASIC program to do the same. My personal preference for starting via the DOS command and my (baseless?) presumption that higher versions are later and hopefully better (i.e. more features and fewer bugs) causes me to select to use the "COCO VERSION D" interpreter taken from the Ballyhoo disk image.

Command Performance

Starting here, I will presume that you have a "version 3" Z machine story file available called "newgame.z3" and an image of the original Infocom Ballyhoo diskette called "ballyhoo.dsk". So, let's start by using the Toolshed decb command to create a clean disk image called "newgame.dsk":

rm -f newgame.dsk
decb dskini newgame.dsk

Next, copy the Z machine interpreter from Ballyhoo to the new disk image. This is done by copying the first 9216 bytes (i.e. two tracks, eighteen 256-byte sectors each) from one disk image to the other:

dd if=ballyhoo.dsk of=newgame.dsk conv=notrunc bs=1 count=9216

While pillaging the Ballyhoo image, also copy over the boot track code. This is done by copying the contents of track 34 (at byte offset 156672) from one disk image to the other. The size could be as much as an entire track (4608 bytes):

dd if=ballyhoo.dsk of=newgame.dsk conv=notrunc bs=1 \
     count=4608 skip=156672 seek=156672

The final missing piece is the new story file. But there is a problem. While there are now 142848 unused bytes on the new disk image, track 17 (i.e. the directory track) is roughly in the middle of the unused space. So, any story file above a certain size (i.e. exactly 64512 bytes) will need to be written to the disk image in two parts. Given that, two commands are needed to write a large story file:

dd if=newgame.z3 of=newgame.dsk conv=notrunc bs=1 \
     seek=9216 count=64512

dd if=newgame.z3 of=newgame.dsk conv=notrunc bs=1 \
     skip=64512 seek=82944

Mischief Managed

Well, that's about it. The directory track remains empty -- enterprising hacker's could copy the directory track from the original Ballyhoo disk, enabling users to see a LOADM'able BIN file for the game (in case they don't have the DOS command available). Or, the directory track could be modified to show other messages as desired by the hacker. Such directory modifications will also be left as an exercise for the reader.

Beyond that, there remains a moral issue -- technically, doing the project as described amounts to software piracy. While I doubt that any Infocom lawyers will come knocking on your door, those who feel strongly about the issue may have objections to "borrowing" the Infocom code in this way (or at all). You'll have to resolve that issue for yourself, of course. But if nothing else, this is a fun way to consume exising Interactive Fiction on the CoCo!

Writing a new Z machine interpreter from scratch is not an unreasonable task, even for an old machine like the CoCo. Maybe if some people start consuming Inform content as described above, that will encourage someone to write a more modern Z machine interpreter for the CoCo? Then we could all be happy and our consciences could be free. Who is up for the challenge? Let me know! I'll stay tuned...

Using Infocom's ZIP on the CoCo

In the previous post, I discussed some background information relating to the system used by Infocom to publish their interactive fiction games across a variety of competing computer systems that existed when their games were commercially viable. Beyond that I suggested that there were tools available which people today can still use to develop games for that same system. Moreover, I hinted that Infocom's own code (i.e. their Z interpreter or "ZIP") could be "borrowed" to enable running those new games on the CoCo. Let's explore those latter options now...

(At the end of that same post, I suggested that you take a look at Michael Sternberg's paper and the video of his KansasFest 2017 presentation on An Apple II Build Chain for Inform. If you have not done so already, then now would be an excellent time for you to check-out that source of information. Much of the information presented there is specific to the Apple II, but I believe that it nevertheless provides a good background for understanding the material presented below.)

Get Informed

Inform is the tool for compiling game descriptions into the form necessary to feed the Z interpreter. Teaching people to program in Inform is well beyond the scope of this blog post, so I will just touch on a few important points and leave the rest as an exercise for the reader...

The first version of Inform appeared in 1993, and versions progressed until a major rewrite in 1996 brought Inform 6. The Inform 6 language is largely procedural with object-oriented features. Inform 7 dates from around 2006 and features a declarative syntax with a high level of domain specificity (i.e. it is highly geared toward interactive fiction). While an Inform 6 compiler is used "under the covers" as a back-end compiler in Inform 7, the two languages are highly diverged and quite different to use.

It so happens that there are different versions of the Z machine interpreter (ZIP). Not surprisingly, the older ZIPs (such as what Infocom used on the CoCo) only support older ZIP standards. It turns-out that the last versions of Inform to effectively support such old ZIPs were somewhere in the Inform 6 range. For writing games to run on the CoCo, use of Inform version 6.1.5 as suggested in Sternberg's paper is recommended.

The point of getting an Inform 6 compiler up and running is to be able to produce story files containing Z code. In some cases these might be found pre-compiled at the Interactive Fiction archive or other sites. In still more cases, example Inform 6 source files might be found for you to compile on your own. Ideally, you will write your own games in Inform 6! But again, teaching you that skill is outside the scope of this blog post. I will suggest that you acquire a copy of the Inform Designer's Manual: 4th Edition, which apparently remains the definitive reference for the language.

Grab the ZIP!

So, the simple nugget of realization that I gleaned from the Sternberg paper was that Infocom wasn't making any use of the filesystem on the Apple II diskettes. So, why would they do things any differently on the CoCo? Applying that nugget of realization along with a hex dump of some Infocom disk images for the CoCo provided the missing information. Each of the Infocom disk images I looked at for the CoCo shared the same data for the boot track and for the initial two tracks on the disk. The data in between those tracks varied with each game, leading me to surmise that copying the boot track and the first two tracks (which I presumed to contain the Z interpreter) to a new disk along with copying a different game's Z machine story file in between would result in a disk that could boot and play that other game.

Continuing with the above theory, I created an image of an empty CoCo disk. I used the Zork I CoCo disk image as a source for it's boot track and its first two tracks (i.e the interpreter). I used the 'dd' command on Linux to retrieve those tracks from the Zork I disk image and to write their data to the appropriate locations on the new disk image. I continued by using the 'dd' command to write a different story file to the space in between the interpreter tracks and the boot track. I then tested the resulting disk image by loading it in MAME and booting it with the 'DOS' command on the emulated CoCo. I was rewarded by seeing the other game start and run.

Admittedly, the above is a bit light on some details. Moreover, there were some complicating facts that arose as I continued to investigate. For example, it turns out that there are at least 3 versions of the Z interpreter for the CoCo.  But that info and a few more details on the above are going to have to wait for now. If you want to learn more, then you will just have to stay tuned... ;-)

Building CoCo Games with Inform

Often we see people do retro computing projects that really are only a piece of the puzzle. Such projects include hardware gadgets with cool capabilities but no hardware to use them, or media players with no stash of media content to consume. Hopefully these projects "scatch an itch" for the people doing them, but they often do little for the community other than providing an entertaining read or a cool "look at that" moment.

One type of retro computing project that can be fun and rewarding involves enabling an existing pile of content to be consumed on a computer where it works perfectly well, but where it has never been published. Such a situation presents itself where some game producers of the past had standardized on platforms that presented games on multiple computers using identical source materials. These platforms packaged custom software for each computer with the computer-agnostic game data in order to reach the largest market possible. Sierra On-Line used this sort of strategy with their AGI game engine, and Guillaume Major has exploited that system to add a number of fan-made AGI games to the Color Computer Archive.

Infocom Games

A contemporary of Sierra On-Line, Infocom is often considered as a pioneering company in the "text adventure" or "interactive fiction" genre of games. The lack of graphics in such games probably led to their eventual loss of popularity, but it did make them especially suitable to the sort of "write once, publish anywhere" strategy described above. Not surprisingly, Infocom implemented just this sort of strategy by targeting their game designs at what they called the "Z machine".

By targeting their games to the Z machine, Infocom did not just simplify their own task of releasing to the large variety of machines that were commercially viable in that era. Infocom also enabled their games to be played on any machine to follow, provided only that someone implement a Z machine interpreter for the target machine. Portable code for such interpreters has been available since the mid-1990's, and such interpreters are available for nearly any machine of merit today.

Inform Us

Hot on the heels of the portable Z machine interpreters came a Z machine compiler called Inform. This tool allows individuals that want to write interactive fiction games to do so in a way that allows users to consume their games, almost as if the games had been released by Infocom itself years ago. All that is required is a Z machine interpreter. Fortunately for us, the CoCo already happens to have one of those... :-)

The inspiration for this endeavour was a paper presented at KansasFest 2017 by Michael Sternberg, available via his blog in a post entitled An Apple II Build Chain for Inform. I recommend reading his paper and viewing the video from KansasFest (linked from his blog) as background information before continuing to my next post on this subject.

The next post on this topic will provide some technical information and describe the steps necessary to "borrow" the Infocom interpreter from an existing CoCo Infocom game and to make it use a different Z-machine game file. If you have been desperate to demonstrate your skills at writing an interactive fiction game, then this may be the best way for you to get such a game released to the CoCo world. If that sounds good to you, then you had better stay tuned...