Boot file

Port-Dreamcast,

I would like to write some code my Dreamcast could boot from and use for 
further boot options.  The part I need help on is actually getting the 
Dreamcast to boot to the extent that it will run my code.  Does anyone 
know where I could find the code I need and possibly an explanation on 
how to edit it/implement it, I'm fairly new to Dreamcast programming.

Thanks,
Michael