Hardware :: Booting From CDs And USB Sticks

When first a regular HDD boots up, the BIOS passes control over to the first 512 bytes of the disk, so long as the disk has the magic number (correct signature) at the end of this first sector. The BIOS doesn't 'care' what is in this first sector; it just passes control over to the code within it and hopefully boot-up is underway. Other boot instructions may commonly lay elsewhere on the disk, which is fine, since the boot sector code will point to them and the process goes on.

Now, what happens when you boot from a CD or a USB stick? Not quite the same thing happens, does it? What I need to know is, when booting from alternative media, what is the BIOS 'looking for' and whereabouts on these new media does it expect to find the boot code? I've looked at the directory structures of CDs and sticks and there doesn't seem to be any common factor in the files and directories there that I can identify, unlike when I examine a regular HDD and its partitions with a hex editor.