I believe it's more.... LEADIN>TOC>Bootsector The leading has a bit that signals it's bootable, and the TOC tells where it is on the disc. As for the emulation of the IO.SYS, it's not really neccesary. IO.SYS was used because it was a DOS subsystem. the CDROM boots whatever it needs to. Like the XP CDs, but into the ntloader which then starts setup. Linux boots something, then starts the kernel. the CD's bootcode and what to look for, IS the bootsector. Which is why the bootsector has to have a spec, so it's all compatible across systems.