[lug] boot loader mystery

[David L. Anselmi]

karl horlen wrote:
[...]
> So disk locations would seem to be necessary.
> I think that might be what that map file is for in
> lilo?  And that's probably why you need to rerun lilo
> every time you make a change to it.  But I'm wondering
> how grub knows the physical location of /boot?

The BIOS provides a service that reads the disk using block addresses. 
They used to be specified as cylinder, head, sector triplets but modern 
stuff may be different.

As stated, the BIOS (built in or extension from the controller) knows 
enough to load the MBR at boot time.  The boot loader talks to the disk 
using the BIOS routines.

LILO stores the block addresses of the kernel and initrd.  GRUB knows 
how to read a filesystem, so it reads the superblock for the appropriate 
partition, decodes the directory entries, and finds the files.  I'd 
guess it does all that using the BIOS routines so in some ways it 
duplicates the FS driver functions.  The kernel doesn't use the BIOS 
routines once it has smarter drivers loaded.

Dave