BIOS Drive Numbering

Hi there.

I was wondering if anyone could tell me how the system BIOS determines
what number to assign to ATA/ATAPI devices. I've read, for example,
that hard disks are assigned numbers starting at 80h, but I'm unsure
if, say, the primary master ATA/ATAPI device ALWAYS gets 80h, or if
there are scenarios where that isn't the case; and I don't know if
numbers are assigned in series or if they can skip around. Any
information about this that anyone can spare will be greatly
appreciated, and I thank all of you in advance.

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