Did you know that a Sega Dreamcast game disc is not a CD, and not a DVD either? It's called a GD-Rom...

What is a GD-Rom?

GD-Rom (which stands for Giga Disc - Read Only Memory) is the standard disc format for Sega Dreamcast Games. It's a way to write up a disc that was conceived by Yamaha. It's basically a CD that is written in a certain way that it can hold 1 Gb.

How does it work?

When we read a standard CD, the bits aren't placed equally distant on the surface. When we do a full 360 turn to read data on a CD, at the start of the CD this turn will be smaller, and the more we go to the edge of the disc, the bigger the turn will be. So, on a standard CD, the data bits are getting farther apart when you reach the end of it. This leaves a lot of free space on the CD. The GD-Rom will use this space to put more data... but, you need to have a special reader to read these discs (The Sega Dreamcast!). The GD-rom reader will read at a variable speed specially to reach those extra bits which are now placed at an equidistant spacing on the disc.

What's the structure of a GD-Rom?

It contains 2 sessions. On the inner part of the disc you find a session which is made in a standard CD format (includes a data part and an audio part, where the information that the disc is a GD-rom is embedded). On the outer part, you get the GD session which contains the true data of the game.

So, how does the Dreamcast recognize a GD-Rom from a standard CD?

By having a special boot sequence that goes like this:

1- The Dreamcast looks for an audio track in the first session.
2- If it finds a track, it tries to read the first block of the second session (GD part of the disc), at this moment, it must finds a file named IP.BIN (which contains data related to the peripherals used by the Dreamcast for the game, the OS, and a reference to the executable file of the game, normally called 1ST_READ.BIN).
3- Then, the Dreamcast looks for this executable file on the disc.
4- If the executable file is readable, the game starts...

There you go! You now know a lot more about the format used for the Dreamcast Games, and hopefully you'll find an incentive to handle them with a little bit more care than a standard CD! Give some respect to the GD-rom disc!  ;)