In October of 2008 I visited the X party, a Commodore 64 demo scene event in the Netherlands. This sparked my interest in actively doing C64 programming again, something I haven't really done in the 15 years before that, even though I still kept up with the developments in that community.
At this point I actively started looking for a C64 project, and a game conversion seemed like a good idea, given my background as a game developer. I was rusty and needed something bordering on the trivial to get my 6502 assembly juices flowing again.
|Prince of Persia preview by Poharai Attila and Varga Viktor from Hungary|
So in May of 2009 I found the Prince of Persia preview on CSDb. It's an attempt from the 1990s at converting Prince of Persia, that unfortunately only produced a slide show of some of the level graphics. But the comment beneath, by Mikael "Twoflower" Backlund got my attention: "A C64 version of this game would sure be something.". I immediately had the same feeling. And then he wrote: "Furthermore, the sourcecode to one of the 8-bit versions is out there.".
That made me all warm and fuzzy inside. But soon I had to find out that what he meant was the version of Prince of Persia for the SAM Coupe computer. Although an outstanding technical achievement, it was not a direct port of the original game, but rather a fan-made recreation, which was close to Jordan Mechner's classic, but also different in lots of ways. And it was for a Z80-based architecture, which I personally never worked on and have little passion for.
But the idea stuck with me. Is there someone who has done the reverse-engineering work of Prince of Persia? Something that I could just take and port to the C64 quickly?
I kept digging and immediately came across the FreePrince project. This is your typical open-source version of a classic game, created by a group of enthusiasts, using the trusted SDL framework. I thought that I had struck gold. Although it seemed unmaintained and it was hard to find the source code, it was a somewhat working version of Prince of Persia using the original game's graphics and level data. It had a complex animation state machine and lots of things working, so I assumed that it was based on disassembly of the PC or Amiga version. Plus it pointed me towards the Princed project and their tool "PR", a Prince of Persia resource extraction tool.
The latter can be used to unpack the data files that came with the DOS version of Prince of Persia to get the individual images, sounds and palettes that make up the assets of the game.
So, without further ado, I started hacking away. I took the 216 animation frames of the Prince extracted by PR, and first converted them to C64 resolution and colors.
The C64 features a multi-color mode that is using two bits to represent one pixel. This means you can use four colors, which you can select from the sixteen available colors. Since the memory storage size for graphics is the same in single-color and in multi-color mode, the graphics chip has to halve the horizontal resolution of 320 pixels in order to use two bits per pixel instead of one. So the effective resolution for multi-color graphics is 160x200, which is also half of the resolution of the DOS game. And since one of the 4 colors has to be the transparent color that meant that only 3 colors are available for the Prince character. I quickly settled on white for the clothes, light red for the skin and brown or orange for the hair. Using semi-automatic conversion tools I came up with this:
|Original image from DOS version|
|Automatic conversion to C64 resolution and colors|
Doing this conversion for all the 216 animation frames quickly gave me a complete set to be used on the C64.
So at this point I fired up my source code editor. I took at look at the source of Freeprince, and started to implement its animation state machine in assembly code. I laid out the Prince animation frames as sprites in memory and immediately noticed that it's gonna be tight. Those animations sure need a lot of memory. But that would be for later to worry about.
Within a couple of days I had a simple working version. Basic animations states were implemented, such as running, jumping, crouching, etc. On June 15th 2009 I captured a video and showed it to one or two people.
Download a C64 .prg file of the work in progress version from 15-Jun-2009.
The response was a bit muted. Not enough of the game there yet. Also the animations looked dodgy. The positioning of the frames used in FreePrince was not based on the original game. Someone must've redone that manually, and so the locations were wrong.
So I kinda went off in the wrong direction. FreePrince certainly wouldn't be good enough for a faithful conversion. It was also incomplete and not fully playable.
Back to square one. I had to look for better source material. Disassemble the DOS version?
In the meantime I found Jordan Mechner's blog. He had the courage and insight to post all of his old journals from the 1980s. He meticulously kept a log of his daily work. What a great read that was. Just a few days before I started looking for Prince of Persia information Jordan also posted this article on his blog. It contained a link to a PDF, which turned out to be the Prince of Persia source code documentation.
I was amazed. The source was lost on Apple II disks, but the document written just a few days after the release in 1989 was there, with all kinds of juicy little details about the graphics engine, the data structures, lists of images, and more. It was like someone had handed me the key to a long lost treasure. I didn't even know that there was an Apple II version of the game, and that it was the original version. I learned that all others were ports (or ports of ports) of that one original. And it was all in perfect 6502 code, which I could read and which would run fine on the 6510 (a 6502 variant) of the Commodore 64. Sure it said it would need 128KB of RAM, but at that time that seemed like a small challenge. I imagined it to be very inefficient code and easy to optimize. Piece of cake, right?
So how do you go about reverse-engineering 128K of code and data? It's all just a big pile of binary data. It's like one really big crossword puzzle.Where do you start?
I'll cover that in my next post.