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

["Richard B. Johnson" <linux-os@analogic.com>]

On Tue, 24 May 2005, Paul Rolland wrote:

> Hello Dick,
>
> > initrd means Initial RAM disk.
> >
> > The boot image is put onto your Hard disk. The hard disk needs
> > to be accessible from the BIOS to boot from it. Most controllers
> > have a BIOS module that lets it be accessed during boot so you
> > don't need a chicken before the egg to start the boot.
>
> 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.

Linux needs to load a driver before it can access the disk. The
BIOS only works for 16-bit real-mode access.

> > Then, when booting, LILO or grub starts linux which uncompresses
> > a RAM disk and mounts it instead of your hard disk. A special
> > version of `init` (called nash) gets started which reads a script
> > called linuxrc. It contains commands like:
> Yes, I've seen that in my .img file...
>
> > The module(s) are in the /lib directory of the RAM disk.
> > They are put there by a build-script that installs initrd.
> > This script gets executed when you do 'make install' in
> > the kernel directory. If you have private modules, you
> > can copy them to the appropriate /lib/modules/`uname
> > -r`/kernel/drivers
> > directory. You put the module(s) name(s) you need to boot
> > in /etc/modprobe.conf or /etc/modules.conf (depends upon the
> > module tools version) as:
> >
> > alias scsi_hostadapter aic7xxx
> > alias scsi_hostadapter1 ata_piix
> >
> > ... any scsi_hostadapter stuff will be put into initrd when
> > you execute the script, /sbin/mkinitrd.
>
> Hmmm... I didn't know this part, thanks for the details.
>
> So, it seems that I'm stuck with my binary module unless I can find a
> way to tell the kernel to use the BIOS to access the disk ;-)))

No. You are "stuck with a driver" just like everybody else. You
either have drivers built-in (getting rare, because there are so
many), or you load a driver while booting. This is the usual
way. From the boot perspective, it doesn't matter if the module
is a 'binary' one or a GPL one.

> Cheers,
> Paul

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11.9 on an i686 machine (5537.79 BogoMips).
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