Weirdly didn't the Dreamcast not have any OS on it? It relied on the OS being on the game disk, the cleverly designed normal CD-ROM run at half speed for double density (or something of that nature).
Hmm, there had to be something, cause it ran without a disc. More menus then the PS2. Plus it says powered by windows CE right on the front.
Maybe it was the Dreamcast equivalent of the BIOS before the OS was loaded from the disc? I know that I have machines that say powered by Windows XP that really aren't! :-)