> So, a little bird told me that some systems used to use "segment
> 0, offset 7c00", and newer systems use "segment 7c, offset 0" for
> booting from CD.  I don't know if this is true (or if I've got it
> right), but if it is, might this explain why I can't make a
> bootable NetBSD CD that works on my Thinkpad, even though some
> other CD's boot fine?

This is not exactly what you asked, but:

When I wrote a boot selector several years ago, I put the following
comments in the source:

; We know that this program will be loaded at absolute address 07C00h,
; but we don't know if it will be segment 0 offset 7C00,
; or segment 7C0 offset 0, or some other combination.  We do know
; that the CPU will be in real mode (not protected mode).

--apb (Alan Barrett)