On a standard computer you get the following sequence

The video card bios loading
The System board bios - Ami or Award usually
Then it start's the POST
One of the first checks is to count through the ram.

OK on an old machine it can take forever, and on the newer machine you can blink and miss it.

After the ram has been checked, any ide devices set to auto then are reported on the screen...

This is followed by any other devices with their own bios. Finally you get a screen showing all the pci devices and their irqs and address used. At this point, if it's set to boot from CDrom you will get the message - press any key to boot from cdrom. You have about 3 seconds to hit a key. Failure to hit a key will result in cdrom being ignored and it will move on to the next boot device in the list in bios.