Last update: January 25, 2001

• It's hard for the cowboy to get up in the morning. In order to do so, he has to pull on his boots.
• It's hard pulling on those boots. In order to do so, he has to pull the straps attached to those boots.

That's where the computer model got its terminology. Your computer is tired. (Maybe it had too much to drink.) It's been turned off all night. At the instant that you turn it on, it can't do very much. It's almost as if it was just borne.

It needs instructions on how to do just about anything.

Asad was just borne. He is a living miracle (far more impressive than a computer).

He needs instructions on how to do just about anything.
(That's what mothers are for.)

He can do only the most Basic of things. Food goes In. Smelley stuff comes Out.

Well that's not really fair. Using his Basic Input Output System, Asad has already learned a tremendous amount from Mom. He's already quite the young man. He'll soon be going to school.

I'm cutting a few corners here, but suffice to say that the computer's BIOS isn't capable of much. However, it is capable of reading in 512 bytes worth of instructions from the very first sector if its hard disk.

That tiny space, the 1st 512 bytes on the disk, is properly called the Master Boot Record. (It's also called "sector 0", but that sounds too much like Star Trek.)

The BIOS is capable of reading that small area and executing the instructions that it finds there. But how do those instructions get there in the first place?

A program called /sbin/lilo, as in LIux LOader, is capable of gathering information from a variety of sources. It can place some of that information into that tiny space. The rest will have to go elsewhere.

/sbin/lilo wants to put a lot of information into the MBR, but those 512 bytes aren't nearly enough room. It wants to say "Here's a large set of instructions that will culminate in your loading of an Operating System. You need to execute those instructions now."

But it aint gonna happen.
(Adding insult to injury, only 446 of those 512 bytes are reserved for instructions. 64 bytes (4 * 16 = 64) are reserved for information regarding the addresses of as many as 4 partitions.)

In that small space of 446 bytes, there's only enough room to say something like "Go to school. Here's the school's address. (Welcome to Grade 1)"

I've called that address "Grade 1" but it might better be called called the boot or install area. In fact, it really is a regular file, but the boot-process doesn't know about files yet.

The instructions necessary to describe a file system (like Linux's ext2 file system) are complex. The computer hasn't learned that much yet. File systems are not covered until Grade 3.

For that reason, /sbin/lilo doesn't store install's filename in the MBR. Instead it stores install's physical address in the MBR, because you don't need a file system to access a physical address.

In the install area, /sbin/lilo will have prepared more instructions for the boot-up process to follow. It's the Grade 1 curriculum.

At the end of the instructions in the install is something like "Congratulations. You may now proceed to Grade 2. Here is its physical address."

I've called that address "Grade 2" but it might better be called called the map area. It too is in fact a regular file, but the boot-process still doesn't know about files. For that reason, /sbin/lilo doesn't store map's filename in the install area. Instead it stores map's physical address in the install area, because you don't need a file system to access a physical address.

By the time the boot-up procedure has reached Grade 2, it has learned enough to be able to ask you a question! It asks you "Which of these several operating systems do you want to initiate (ie: do you want me to learn about) next?"

We are moving up the socio-economic ladder. We're about to move to a new house. We have (perhaps) several to choose from. You have to make that choice. It will determine where you'll be attending Grade 3.

At the end of the instructions in the map are textual descriptions of one or more operating systems and their physical addresses. After a choice has been made, explicitly or by default, the boot-up process will jump to its physical location.

Grade 3 is at that physical location. Grade 3's curriculum is the operating system. Amongst other things, the operating system will be teaching things like file-systems, so we won't have to use physical addresses anymore.

Most of /sbin/lilo's configuration options are in a configuration file called /etc/lilo.conf    Let's take a very preliminary look at a typical /etc/lilo.conf (Don't get too excited. We're only going to look at the first few lines of it. We'll revisit it later.)

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boot=/dev/hda	This indicates the first of several destinations for LILO's output. It's the location of the 512 bytes situated at the beginning of some disk, which is the MBR. In this example, the disk is /dev/hda Note that we've said /dev/hda which is the Unix name for the first IDE disk. We did not say something like /dev/hda1 which would have meant the first partition on that first disk.
install=/boot/boot.b map=/boot/map	Grade 1    Grade 2 LILO will derive, on the basis of other parameters that are also specified in /etc/lilo.conf that we havn't yet discussed, a lot more information necessary for the full boot-up process. It will put that information in these two output files. After creating those 2 files, LILO will calculate the precise physical location on the disk where these 2 files reside in terms of cylinders, heads, tracks and sectors. Grade 1's address (install) will be placed in the MBR Grade 2's address (boot) will be placed in Grade 1. You might think that you can avoid these two parameter settings. Well, yes and no. If either or both of image and boot are not specified, Linux will implicitly behave as if they were specified, ie. it will provide default values for them. The implicit default values are precisely the same as the examples I've chosen. I suggest that you explicitly specify them, if, for no other reason than that you can see those choices in front of you.

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I promise that we'll come back to the /etc/lilo.conf configuration file. But for right now, we need to look at how we partition the disk for two different Linux distributions on it.

Assume we want to construct a single disk that will house two versions of Linux: a SuSE and a Red Hat. Ultimately, of course, we'll need the flexibility of selecting, at boot time, which one we want to start up. That issue will take us back to our LILO discussions but, for right now, let's focus on our slicing.

While doing the first install (SuSE), we keep the second install (Red Hat) in mind. While installing SuSE, we define all the partitions -- even the ones we know SuSE doesn't care about.



The SuSE slicing will look something like this. Note in particular: hda1 in SuSE is mounted as /boot hda2 in SuSE is mounted as /RHboot hda2 will also be Red Hat's /boot The SuSE install procedure won't do anything with the strangely named "/RHboot" partition nor will it touch the others. As part of the SuSE installation, - I will have LILO write the MBR When it comes time to do my Red Hat install, - I won't have LILO write the MBR. - I won't change any of the partitions. - (but I will change most of the mount points.) hda2 as Red Hat's /boot hda3 as Red Hat's {swap} hda10 as Red Hat's / hda11 as Red Hat's /usr hda12 as Red Hat's /opt hda13 as Red Hat's /var hda14 as Red Hat's /home

Slice SuSe Red Hat hda1 /boot   hda2 /RHboot /boot hda3 {swap} hda4 (Extended partition) hda5 /   hda6 /usr   hda7 /opt   hda8 /var   hda9 /home   hda10   / hda11   /usr hda12   /opt hda13   /var hda14   /home

As part of the Red Hat install, I will NOT have LILO write the MBR. I want to leave the one that SuSE wrote in tact. That means that the system will only be able to re-boot into SuSE. But that's ok. All I want now is SuSE.

When I re-boot the system, SuSE will come up. Afterall, as part of the SuSE install, I had the installer run LILO which loaded the MBR in such a way as to enable SuSE to boot.

Once SuSE has been booted, we'll modify SuSE's /etc/lilo.conf and run SuSE's /sbin/lilo to enable Red Hat. We will never run Red Hat's /sbin/lilo

We've just rebooted the computer. SuSE is running. We had earlier looked briefly at its /etc/lilo.conf
I've added a couple of reasonably self-explanatory commands to it:

boot=/dev/hda          # write your 512 bytes to the MBR

install=/boot/boot.b   # write the installer code to /boot/boot.b
                       # and put /boot/boot.b's address into the MBR

map=/boot/map          # write the map code to /boot/map
                       # and put /boot/map's address into /boot/boot.b

prompt                 # when the computer boots, give me a boot prompt
timeout=50             # and give me 5.0 seconds to respond to it

default=SuSE           # If I don't respond, use this default label

This code is only the "global" portion of a LILO configuration file. Now we need to write portions that will each pertain to a single operating system.

These portions are called "stanzas". (In a poem or song, each verse may also be called a "stanza". Think of it that way.) The following are the stanzas as originally written by the installation (with only slight editing.)

#######################################
image    = /boot/vmlinuz
  label  = SuSE
  root   = /dev/hda5
  initrd = /boot/initrd
#######################################
image    = /boot/vmlinuz.suse
  label  = OldSuSE
  root   = /dev/hda5
  initrd = /boot/initrd.suse
#######################################

Stanzas are introduced with by

• either the keyword image, which indicates that we're talking about a Linux image here
• or the keyword other which indicates that we're talkin about some other arbitrary system.

Now I'm going to add another stanza for Red Hat. For the most part, the following is lifted directly out of Red Hat's copy of its /etc/lilo.conf -- however there is one very important distinction.

• In Red Hat's /etc/lilo.conf, the device specified for the Red Hat image was /boot
• In the SuSE /etc/lilo.conf, the device specified for the Red Hat image must be /RMboot

Think about it. SuSE's /RMboot and Red Hat's /boot are the very same thing!
Under SuSE, its mount point is /RMboot
Under Red Hat, its mount point is /boot
Since we're going to be running /sbin/lilo from SuSE, we must use /RMboot

#######################################
image    = /RHboot/vmlinuz-2.2.14-5.0
  label  = RedHat
  read-only
  root   = /dev/hda10
#######################################

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So here's what the entire SuSE /etc/lilo.conf now looks like, with all the pieces brought together:

boot=/dev/hda          # write your 512 bytes to the MBR

install=/boot/boot.b   # write the installer code to /boot/boot.b
                       # and put /boot/boot.b's address into the MBR

map=/boot/map          # write the map code to /boot/map
                       # and put /boot/map's address into /boot/boot.b

prompt                 # when the computer boots, give me a boot prompt
timeout=50             # and give me 5.0 seconds to respond to it

default=SuSE           # If I don't respond, use this default label

#######################################
image    = /boot/vmlinuz
  label  = SuSE
  root   = /dev/hda5
  initrd = /boot/initrd
#######################################
image    = /boot/vmlinuz.suse
  label  = OldSuSE
  root   = /dev/hda5
  initrd = /boot/initrd.suse
#######################################
image    = /RHboot/vmlinuz-2.2.14-5.0
  label  = RedHat
  read-only
  root   = /dev/hda10
#######################################

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• Remember that only making changes to /etc/lilo.conf won't have any affect on your next re-boot.
  /etc/lilo.conf is only a configuration file that the /sbin/lilo program uses when it goes about making changes. So, those changes won't happen until you run the /sbin/lilo program.
• The top of the /etc/lilo.conf configuration file is the global area, which defines parameters relevant to the whole process.
• One or more stanzas follow the global area. Each stanza defines parameters relevant to a particular operating system. Each stanza is introduced by the keyword image=so-and-so
• When /sbin/lilo runs, it calculates and stores the physical address pointers (not filesystem names) of files indicated in the global section as well as the kernel operating system mentioned in each stanza.

  Thus, if you've rebuilt your kernel, -- even if you've given the new kernel the same name as the old one (which is probably not a good idea) -- you must remember to run /sbin/lilo again. Why?- because you're new kernel isn't at the same physical location anymore.

There are many other parameters that may exist in the /etc/lilo.conf configuration file. They go in either the GLOBAL section because they are relevant to the whole thing or in a STANZA section because they are relevant to a particular operating system.

In /etc/lilo.conf, the boot= line indicates where you want /sbin/lilo to say "this boot processes begins here."
- boot=/dev/hda is the MBR of /dev/hda
- boot=/dev/hdaN is the boot sector of /dev/hda's Nth partition.

Why might you wish /sbin/lilo to construct a chain of events that starts at something other than the MBR, say /dev/hda3? Afterall, when the computer starts up it is the MBR that gets accessed, isn't it?

Yes, that's true. But if you have already installed another operating system and it has written the MBR, you might wish to leave the MBR relatively untouched. In your /etc/lilo.conf you could specify boot=/dev/hda3 which would tell /sbin/lilo to write its boot-up instructions there instead.

This tells /sbin/lilo not to touch the MBR at all. But you do need to touch it just a little. Otherwise, the computer will still boot referencing the MBR and won't know of the instructions which you've put in the boot sector of /dev/hda3. You have to go one small step further.

Remember those 64 bytes in the MBR that contain the partition table?
Besides containing the locations of up to 4 partitions, there is also an indicator that says which one of them, if any, is considered active, also known as bootable.

If one of those partitions is indeed active, then the boot-up procedure will jump to the boot sector of that partition in order to continue the boot-up process.

So how do you make a particular parition active/bootable?

The program fdisk is available in both the Windows and Linux worlds. (Several variants are available in Linux). One of the things that fdisk can do is to mark a particular partition as active/bootable.

1. Run Linux's fdisk /dev/hdwhatever and type "m" for "menu" where you'll see a list of its available commands which includes "a" for "activate" a partition.
2. Type "a" to tell fdisk that you wish to activate a partition. It must be one of partitions 1 through 4 where you've told /sbin/lilo to put its stuff.
3. Type "w" to tell fdisk that you wish to write the partition table and exit.
4. There are other commands available such as:
     p to print the partition table.
     q to quit without doing anything

Because this process hardly effects the MBR at all, the original MBR can be put back again without running /sbin/lilo. All that is necessary is to modify the active/bootable partition with fdisk.

To summarize

• You can instruct /sbin/lilo to write its stuff to the boot sector of a particular partition of a particular disk rather than the disk's MBR.
• You would want to do so if you wanted to leave the MBR as untouched as possible
• You must run fdisk and activate that partition.

There are many command line parameters that may be specified when you run the /sbin/lilo program. Let's look at some of them.

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-t -v -t -v -v -t -v -v -v -t

The -t (as in test) command line option tells /sbin/lilo to test things without altering the way things are. /sbin/lilo reports on things and assures you "The boot sector and the map file have *NOT* been altered." Specifying one or more -v's as in verbose, requests /sbin/lilo to progressively report its findings with more and more detailed verbosity.

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1. Examination of other commands in /etc/lilo.conf
2. Using "other" to boot up a non-Unix operating system
3. Discussion of rdev
4. Sources of reference