Dreamcast GD-ROM layout is unlike a regular CD-ROM as it contains multiple data tracks with specific requirements to how they must be arranged. This makes the task of modifying the ROM image somewhat difficult without proper tools. In this tutorial I shall demonstrate the steps and tools necessary to do this.
Extracting the files is easily done with GD-ROM Explorer. In this example I am going to use the DoTF Beta from November 1999 to modify the Levels.txt to include a map into the game which isn’t accessible otherwise. We start with a directory containing the initial GDI file and several other files, one per each track of the disc.
We are now satisfied the files have been extracted, now it is time to make the changes needed to make things work.
In this version of DoTF the order the levels are to be loaded is set inside Levels.txt. While there already is a file 190_LOBNOE.GDE on the disc it isn’t on the list just yet so this is what needs to be changed. Simply open the file in the text editor of your choice and add the missing map using the neighbouring lines as an example.
I have added an apostrophe in front of the name to make it stand out in the list.
For this we are going to need a piece of software called GDI Builder which is designed to automate the task of building a bootable Dreamcast disc image.
Both pieces of software used in this tutorial are available here: https://projects.sappharad.com/tools/gdibuilder.html
So why there are two data tracks with a CD Audio one in the middle? The Dreamcast GD-ROM can be laid out in two different ways. Both of them start with a pair of single density (SD) tracks at the centre of the disc, the first one being a data track (which sometimes may contain the bonus content for the game) and the second one being an audio track with a warning not to play the disc on a regular audio player. After this begins the high density (HD) area which is about 1 GB in size and contains the actual game files, and this is where things may be different. Depending on the layout this can be a single big track from beginning to the end like shown here:
Most of the Dreamcast games use this kind of layout. However there is a second option which allows games to make use of CD Audio tracks put into the high density area. In this case Track 3 is merely a small Table of Contents (TOC), telling machine the names of the files and where on the disc they are located. After that there can then be up to 95 CD-Audio tracks to be used within the game, and finally there is another data track which contains the actual files themselves.
With this beta of DoTF the developers have apparently been experimenting with the idea of putting in-game music onto the GD-ROM as audio tracks, thus using the second option described. Even though there is no actual audio on this disc yet, the program is already built with this structure in mind and had we not told GDI Builder about there being a dummy Track 4, all the data would have ended up in Track 3, making the whole ROM un-bootable.