... I thought the BIOS excutes the POST and then passes control to the OS.....
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To the hard drive or whatever bootable media is selected. Then
the OS is launched.
The BIOS doesn't check for validity.... A certain section of It
is checked! This is why an invariably simply cracked BIOS allows a fully pre-validated OEM installation of Vista, Seven and Eight without a (Largely detectable?) "loader".
This is a necessarily simplistic description and I don't think that I'd be allowed to be any more in depth on the subject here.