Entry 2: The Problem

A project log for Fixing 37-year-old software: Three Mile Island

I'm an Apple ][ fan. One game from my childhood keeps crashing on me... and it's time to finally fix it!

jorj-bauerJorj Bauer 01/09/2017 at 18:390 Comments

The problem is multivariate.

This is a disk image that I got online. I have no idea what its provenance is. I don't know if someone had to crack it (there's no crack notice, which was a point of pride back in the day, so I suspect not).

The software is old. Not just 36-years-old-from-today. It's also from 1980, which was early days. The Apple ][+ had just been released; words like "language card" were on the way out. When the Apple //e was released the Apple world took big steps forward again. Everything I know about the internals of Apple ][s comes from 1982 and later. This is, to me, from Apple pre-history.

And then I'm also running this in emulators. No emulator is perfect. I expect glitches.

So when 36-year-old "old" software in an emulator crashes, where do you look for the problem? Well, for 14 years (I'm guessing I downloaded this along with many other titles around 2002), the answer is "nowhere." When you forget that '7' is the keystroke of death and accidentally push it, you stop playing.

What makes now different, then?

I found a copy of Merlin Pro 2.58 - a ProDOS version of the assembler that I used through High School.

This was the first piece of serious software that I bought for myself. It was expensive, IIRC. But it opened so many doors. I spent a lot of high school programming nonsense in my free time - mucking around with the internals of ProDOS, creating new commands; mucking around with the internals of floppy drives, learning what prologs and epilogs were and how bits were encoded on the disk; generally breaking things and learning how they worked. And a lot of the most advanced work I did was powered by Merlin Pro 2.58.

I had occasionally looked for this online before. Merlin 2.4* (2.43, maybe?) has been available as an image for a long time. But this is DOS-only, and my work disks were all ProDOS (unreadable from DOS 3.3).

So when I found Merlin Pro 2.58, I jumped at the chance to go look at the assembly that I wrote back in the late 1980s. And wow, such nostalgia! I remember vividly the printouts of fan-fold continuous feed sprocket dot matrix assembly. Marking them up during math class. Trying out new things in our Apple lab in my high school (they didn't have assemblers; the classes were all in BASIC, but that's another story). Running home to code changes after school, print it back out in the morning, and repeat. I loved those days.

After perusing the code I wrote as a teenager, I looked at some of the other disk images in my archive. I played Ali Baba and the Forty Thieves with my 11-year-old son (who is now addicted to it, mwahahaha). I found Temple of Apshai images online and spent a few minutes remembering how I wanted to love this game but hated its controls. And I found my image of Three Mile Island, in its semi-defunct state.

Curious, I thought I'd troubleshoot it a bit.

Was it perhaps designed for a specific ROM? Virtual ][+, my emulator of choice, comes with ROMs for the ][ and the ][+. A quick test showed that it failed the same way there.

Is this an emulator bug? I downloaded every emulator I could find for the Mac, and tried a few JavaScript online emulators. They all do the same thing, too.

Is this a bad disk image? I downloaded every copy of TMI that I could find - some with different names - just to find that they're all exactly the same disk image. So either this is exactly the original image, or they're all copies of the same single disk.

Which leaves little to do except to poke in the binary itself and see what the heck it's doing.