PSX mods with mkpsxiso

Before going digital, you might scribbling down some ideas in a sketchbook.

By Kion August 3, 2022, 2:20am EDT

One of the largest painpoints i had for Miku Legends 2 was working with the CD image to be able to swap out the files. Originally I wanted to be able to extract files from the disk, replace them and then rebuild the disk. I wasn’t able to figure out how to do this, and I ended up slicing up the files into chunks of 0x800 bytes and then doing a search and replace for each one of the individual chunks.

This was until I found Hilltop’s video on How to Romhack: Mega Man Legends 2, where he mentions the tool mkpsxiso. Unfortunately by the time I had found the thing I was looking for, I was almost complete with the mod and ended up pushing forward with the chunk approach. Now that Miku Legends 2 version 1.0 is out as a patch, I can go back and look into other approaches that should make the process easier.

Installing

First note that the project is over at: https://github.com/Lameguy64/mkpsxiso. And first we’ll install the dependencies.

sudo apt update
sudo apt install -y cmake build-essential git

Then we will clone, compile and install.

cd /tmp
git clone https://github.com/Lameguy64/mkpsxiso.git
cd mkpsxiso
git submodule update --init --recursive
cmake -B build -DCMAKE_BUILD_TYPE=Release .
cmake --build build
sudo cmake --install build

After that, the binaries will be copied to /usr/local/bin/. You can confirm this with the which command.

which mkpsxiso
/usr/local/bin/mkpsxiso

And you can also run mkpsxiso --help to see the options.

Unpacking

Now that we have the tool installed, we can export the files from the MegaMan Legends 2 rom into a folder along with an xml file in order to build it later.

In terms of pathing, I’m in /home/kion/Documents/psx/ with a rom named Mega Man Legends 2 (USA) (Track 1).bin. We can export the files along with with the xml file with the following command.

dumpsxiso -x files/ -s mml2.xml Mega\ Man\ Legends\ 2\ \(USA\)\ \(Track\ 1\).bin

This will export the extracted game files to /home/kion/Documents/psx/files (dynamically created), and give us a mml2.xml file that we can use to extract it later.

Decompressing

As a proof of concept, we’re going to start with COMMON/PL00T.BIN. This contains MegaMan’s body face texture that is used for most of the game. Technically it’s all of the game, except when the game patches in from a different file after a cutscene. Once the system is started, this will be the default texture that gets loaded into RAM. And it is compressed, which makes it very annoying to mod.

Since the decompressed version of this file is in PL00T2.BIN, it looks like we can be lazy and simply splice these two together. The code for this is as follows.

import { readFileSync, writeFileSync } from "fs";
const PL00T = readFileSync("PL00T.BIN");
const PL00T2 = readFileSync("PL00T2.BIN");

const pop = PL00T.subarray(0x3000);
const shift = PL00T2.subarray(0, 0x8800);

const merged = Buffer.concat([shift, pop]);
writeFileSync("files/COMMON/PL00T.BIN", merged);

Now that we have that written, we can go ahead and rebuild the CD image to see if it worked or not.

Rebuilding

From there we can try to rebuild the ROM with the following command. And then we can open up the rebuilt rom in an emulator.

mkpsxiso -o MML2.bin mml2.xml

And once we try it, the game crashes after the Capcom logo with a black screen. My experience with this error is that it tends to be and issue with reading an archive. Considering that we updated the body texture, it wouldn’t surprise me if that was being read for something shortly after loading the game.

Which means that right now, this is a failure since i don’t have the option to debug I highly doubt that the game has per-archive code for loading specific archives in terms of where it expects files, or there formats to be within the archive. But at the same time i wouldn’t put it passed the devs.

Another option is that the game has the offsets to each one of the files hard coded into the EXE, and that i might need to shift down the sectors of where a file is expected to be. To sanity check, i rebuilt the image without loading in my updated PL00T.BIN file. Which indicates that there isn’t an issue with mkpsxiso itself.

For now I’ve confirmed that while this approach is appealing, it means that i took the correct approach with respect to replacing files in the Miku mod. I have been able that while I can’t change the length of the files, this does seem to be a better mechanism for extracting and rebuilding a PlayStation rom.