When the computer loads a boot sector on a normal Linux system, what it loads is actually a part of lilo, called the ``first stage boot loader''. This is a tiny program who's only job in life is to load and run the ``second stage boot loader''.
The second stage loader gives you a prompt (if it was installed that way) and loads the operating system you choose.
When your system is up and running, and you run lilo, what you are actually running is the ``map installer''. This reads the configuration file /etc/lilo.conf and writes the boot loaders, and information about the operating systems it can load, to the hard disk.
There are lots of different ways to set your system up to boot. What I have just explained is the most obvious and ``normal'' way, at least for a system who's main operating system is Linux. The Lilo Users' Guide explains several examples of ``boot concepts''. It is worth reading these, and trying some of them out.
The configuration file for lilo is /etc/lilo.conf. There is a manual page for it: type man lilo.conf into a shell to see it. The main thing in lilo.conf is one entry for each thing that lilo is set up to boot. For a Linux entry, this includes where the kernel is, and what disk partition to mount as the root filesystem. For other operating systems, the main piece of information is which partition to boot from.
DANGER: take care with these exercises. It is easy enough to get something wrong and screw up your master boot record and make your system unuseable. Make sure you have a working rescue disk, and know how to use it to fix things up again. See below for a link to tomsrtbt, the rescue disk I use and recommend. The best precaution is to use a machine that doesn't matter.
Set up lilo on a floppy disk. It doesn't matter if there is nothing other than a kernel on the floppy - you will get a ``kernel panic'' when the kernel is ready to load init, but at least you will know that lilo is working.
If you like you can press on and see how much of a system you can get going on the floppy. This is probably the second best Linux learning activity around. See the Bootdisk HOWTO (url below), and tomsrtbt (url below) for clues.
Get lilo to boot unios (see section hardware exercises for a URL). As an extra challenge, see if you can do this on a floppy disk.
Make a boot-loop. Get lilo in the master boot record to boot lilo in one of the primary partition boot sectors, and have that boot lilo in the master boot record... Or perhaps use the master boot record and all four primary partitions to make a five point loop. Fun!
lilo-u-21.ps.gz (or a later version). You may already have this document though. Check /usr/doc/lilo or there abouts. The postscript version is better than the plain text, since it contains diagrams and tables.