Re: Linux and Initrd used to access disk : how does it work ?

From: Karl Essinger
Date: Tue May 24 2005 - 14:32:07 EST

On Tuesday 24 May 2005 10:19 am, Paul Rolland wrote:

> Ok, basically, the trick is that the BIOS knows how to access the
> disk, and Linux doesn't because it doesnt use the BIOS ? Or is it
> some more subtle (though I doubt) thing in which only a part of the
> disk can be accessed by the BIOS.

I'm not an expert in this area, but . . . For lilo I think the boot loader
just records the physical address where the initrd image resides and loads it
directly into memory thru the bios. It doesn't need to understand the device
or filesystem -- it's just retrieving a chunk of data -- as long as the bios
understands how to read raw (real-mode) data from the device. That would be
why you have to rerun lilo after changing any boot files -- because the
addresses probably changed.

I believe grub understands filesystems but I've also had trouble using grub to
boot from devices which need module loading, i.e. USB flash drives. I don't
use grub much though.
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