I discovered that I have two /boot/grub/menu.lst files. One is on drive a: The other on drive b:

Interesting, the menu.lst file that is read at boot is located on drive b:

However, drive a: is the drive from which the system boots -- according to the bios.

How does the system determine which menu.lst file to reference?

I use Kubuntu Feisty.

Thanks

The grub portion in the mbr tells where to look in what partition for the menu.lst

How is this portion of the MBR updated or edited, I wonder? Clearly the MBR would be binary, so it is not edited directly. Perhaps it happens everytime the menu.lst file is edited/changed?

You edit the mbr by telling it where to look for the root directory. Say you want it to boot from the first partition of drive a and that is where your menu.lst is in terminal as root type grub and you should get a grub prompt then to change the mbr type

root (hd0,0)
setup (hd0)

Now on boot the mbr will point to sda and the first partition. Grub starts counting from 0 not 1, that is why (hd0,0) means the first hd, first partition.

(hd1,0) means second hd or sdb in most cases and the first partition and of course it you change root you need to tell the mbr and you do that with
setup (hd0)

This man has written numerous articles on grub and if you look at his signature there are several links to his articles

http://www.justlinux.com/forum/showthread.php?t=144294

Larry is right.

I just show the variety of it below.

We tell Grub where you want to source its system files from by the "root" statememnt.

The "setup" is to link the Grub's system files to the location you desire.

We can play tunes with it.

Say if you want Grub in the 14th partition of your 3rd disk to

(1) able to boot from a floppy,
(2) in the MBR of the 1st disk so that it boots whenever there is no floppy in the drive,
(3) chainloadable in its root partition so that anther operating system can boot it (like Xp, Vista or another Linux)
(4) put into the MBR of the 3rd disk just in case you may need it one day
(5) Boot it up now

You can all them in one go in a Grub shell or a Grup prompt. The latter is needed if you want Item (5). A Grub shell is the one you get it in the Linux terminal by typing "grub". A Grub prompt is the one you get by pressing the "c" key at the booting screen at which time the kernel has not yet been loaded. Many distros drop the Grub prompt and offer you a graphic screen instead.

The partition becomes chainloadable after it has been "setup (hd2,13)"

What Grub does in the "setup" statement is to dump its stage1 (512 byte large) in the specified destination and hard-codes the hard disk address specified in the "root" statement so that it can get back to it.

There is nothing complicated involved. People just don't play around with a boot loader because it is low-tech.