I have a vague memory of reading about this, but I can't find it.

When GRUB is installed to--eg--the MBR, how is it configured to look at the correct disk/partition for its config file, etc.? By implication, it sets some code based on the location from where it was installed.

As just one example, if you were trying to rescue an existing install, I assume you could not install GRUB from a live CD and expect it to work with the existing setup.

Heres the long answer http://www.gnu.org/software/grub/manual/grub.html , short answer is if you were to re-install grub in the same position as the original ie grub-install /dev/hda and the /boot/grub/menu.lst still exists, grub will boot up again as before.

It knows the drives from the /boot/grub/device.map .

If I wanted to restore my Fedora grub, I would run the command:
grub

Then, I would tell grub where to look for the grub.conf ( which is located on the second drive, second partition ) with the command: root (hd1,1)

Then, I would tell grub to use the mbr of the first drive with the command: setup (hd0)

type: quit to exit the grub utility.


If I wanted to use my Suse grub.conf ( which is located on /dev/sda6 ) I would use the command: root (hd0,5)

If I wanted to use the Mandriva grub.conf ( which is located on /dev/sda2 ) I would use the command root (hd0,1)

Makes perfect sense...
Ergo, if I dump out the GRUB code on the mbr, there will be some bytes that are unique to the installation. Anyone know the details of where this is found?

There isn't really anything unique in the mbr.

I've installed grub from one distro, then used the same instillation of grub, and physically copied a new distro the the root partition (in this case it was (hd0,3) and rewrote the /boot/grub/menu.lst and it worked fine.

It's really the location of the menu.lst file that's important, and grub just needs to know where it is, which, as stated above, can be fixed with a simple command.

I know this is a bit esoteric, but the question is HOW grub knows where menu.lst is. there's no config file with that info---ergo it must be actually embedded in the mbr code.

It is not in MBR code, it's in stage 1.5, which is can be located in several places outside of filesystems.

Edit: Stage1.5 location is recorded into MBR code, usually stage1.5 is located in empty area right after MBR.

OK---so how does ***stage 1.5*** know where to go for menu.lst?

Stage1.5 can read the filesystem it is made for and stage2 from it. This means you can move your stage2 around and stage1.5 will find it as long as the path remains unchanged. You can guess from here. Stage2 can read the filesystem too and reads menu.lst.