The Dreamcast BIOS, also known as the "GD-ROM BIOS," was developed by Sega in collaboration with Hitachi, who manufactured the GD-ROM (Gigabyte Disc Read-Only Memory) drives used in the console. The BIOS is stored on a dedicated chip on the motherboard and is executed when the console is powered on.
Here is a breakdown of what you need to know: bios dreamcast