If you have a Dreamcast console manufactured before October 2000 (model number HKT-3000 or HKT-3010 with a "1" or "2" in the circle on the bottom label), you can play burned CDs without modding:
Technically, the Dreamcast used (Gigabyte Discs), a proprietary format that held 1GB of data—about 50% more than a standard CD-ROM. When we talk about "Dreamcast ROMs," we’re actually referring to disc images (usually in .GDI, .CDI, or .CHD format) that replicate the contents of an original game disc. sega dreamcast roms