Dreamcast System Controller reset
Hold A + B + X + Y and press Start in most games to do a soft reset.
Delete internal memory settings
Hold A + B + X + Y and turn on the Dreamcast. Then, release the buttons after the date/time prompt appears. Note: This will not affect your VMU memory and will only reset internal information, such as date, time, and Dreamcast Network settings.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original