At this point you have a kernel and a compressed root filesystem. If you are making a boot/root disk, check their sizes to make sure they will both fit on one disk. If you are making a two disk boot+root set, check the root filesystem to make sure it will fit on a single diskette.
You should decide whether to use LILO to boot the bootdisk kernel. The
alternative is to copy the kernel directly to the diskette and boot without
LILO. The advantage of using LILO is that it enables you to supply some
parameters to the kernel which may be necessary to initialize your hardware
(Check the file /etc/lilo.conf on your system. If it exists and
has a line like ``append=...'', you probably need this feature). The
disadvantage of using LILO is that building the bootdisk is more complicated
and takes slightly more space. You will have to set up a small separate
filesystem, which we shall call the kernel filesystem, where you transfer
the kernel and a few other files that LILO needs.
If you are going to use LILO, read on; if you are going to transfer the kernel directly, skip ahead to section Without using LILO.
The first thing you must do is create a small configuration file for LILO. It should look like this:
boot =/dev/fd0
install =/boot/boot.b
map =/boot/map
read-write
backup =/dev/null
compact
image = KERNEL
label = Bootdisk
root =/dev/fd0
For an explanation of these parameters, see LILO's user documentation. You
will probably also want to add an append=... line to this file from
your hard disk's /etc/lilo.conf file.
Save this file as bdlilo.conf.
You now have to create a small filesystem, which we shall call a kernel filesystem, to distinguish it from the root filesystem.
First, figure out how large the filesystem should be. Take the size of your
kernel in blocks (the size shown by ``ls -l KERNEL'' divided by 1024
and rounded up) and add 50. Fifty blocks is approximately the space needed
for inodes plus other files. You can calculate this number exactly if you
want to, or just use 50. If you're creating a two-disk set, you may as well
overestimate the space since the first disk is only used for the kernel
anyway. Call this number KERNEL_BLOCKS.
Put a floppy diskette in the drive (for simplicity we'll assume /dev/fd0) and create an ext2 kernel filesystem on it:
mke2fs -i 8192 -m 0 /dev/fd0 KERNEL_BLOCKS
The ``-i 8192'' specifies that we want one inode per 8192 bytes.
Next, mount the filesystem, remove the lost+found directory,
and create dev and boot directories for LILO:
mount /dev/fd0 /mnt
rm -rf /mnt/lost+found
mkdir /mnt/{boot,dev}
Next, create devices /dev/null and /dev/fd0.
Instead of looking up the device numbers, you can just copy them from your
hard disk using -R:
cp -R /dev/{null,fd0} /mnt/dev
LILO needs a copy of its boot loader, boot.b, which you can take from
your hard disk. It is usually kept in the /boot directory.
cp /boot/boot.b /mnt/boot
Finally, copy in the LILO configuration file you created in the last section, along with your kernel. Both can be put in the root directory:
cp bdlilo.conf KERNEL /mnt
Everything LILO needs is now on the kernel filesystem, so you are ready to run
it. LILO's -r flag is used for installing the boot loader on some other
root:
lilo -v -C bdlilo.conf -r /mnt
LILO should run without error, after which the kernel filesystem should look something like this:
total 361 1 -rw-r--r-- 1 root root 176 Jan 10 07:22 bdlilo.conf 1 drwxr-xr-x 2 root root 1024 Jan 10 07:23 boot/ 1 drwxr-xr-x 2 root root 1024 Jan 10 07:22 dev/ 358 -rw-r--r-- 1 root root 362707 Jan 10 07:23 vmlinuz boot: total 8 4 -rw-r--r-- 1 root root 3708 Jan 10 07:22 boot.b 4 -rw------- 1 root root 3584 Jan 10 07:23 map dev: total 0 0 brw-r----- 1 root root 2, 0 Jan 10 07:22 fd0 0 crw-r--r-- 1 root root 1, 3 Jan 10 07:22 null
Do not worry if the file sizes are slightly different from yours.
Now leave the disk in the drive and go to section Setting the ramdisk word.
If you are not using LILO, transfer the kernel to the bootdisk with the
dd command:
% dd if=KERNEL of=/dev/fd0 bs=1k
353+1 records in
353+1 records out
In this example, dd wrote 353 complete records + 1 partial record, so the
kernel occupies the first 354 blocks of the diskette. Call this number
KERNEL_BLOCKS and remember it for use in the next
section.
Finally, set the root device to be the diskette itself, then set the root to be loaded read/write:
rdev /dev/fd0 /dev/fd0
rdev -R /dev/fd0 0
Be careful to use a capital -R in the second rdev command.
Inside the kernel image is the ramdisk word that specifies
where the root filesystem is to be found, along with other options. The
word can be accessed and set via the rdev command, and its contents
are interpreted as follows:
bits 0-10: Offset to start of ramdisk, in 1024 byte blocks
bits 11-13: unused
bit 14: Flag indicating that ramdisk is to be loaded
bit 15: Flag indicating to prompt before loading rootfs
If bit 15 is set, on boot-up you will be prompted to place a new floppy diskette in the drive. This is necessary for a two-disk boot set.
There are two cases, depending on whether you are building a single boot/root diskette or a double ``boot+root'' diskette set.
KERNEL_BLOCKS). Bit 14 will be set
to 1, and bit 15 will be zero.
For example, say you're building a single disk and the root filesystem will
begin at block 253 (decimal). The ramdisk word value should be 253
(decimal) with bit 14 set to 1 and bit 15 set to 0. To calculate the value
you can simply add the decimal values. 253 + (2^14) = 253 + 16384 = 16637.
If you don't quite understand where this number comes from, plug it into a
scientific calculator and convert it to binary,
After carefully calculating the value for the ramdisk word, set it with
rdev -r. Be sure to use the decimal value. If you used LILO,
the argument to rdev here should be the mounted kernel path,
e.g. /mnt/vmlinuz; if you copied the kernel with dd, instead
use the floppy device name (e.g., /dev/fd0).
rdev -r KERNEL_OR_FLOPPY_DRIVE VALUE
If you used LILO, unmount the diskette now.
The last step is to transfer the root filesystem.
dd with the seek option, which specifies
how many blocks to skip:
dd if=rootfs.gz of=/dev/fd0 bs=1k seek=KERNEL_BLOCKS
dd if=rootfs.gz of=/dev/fd0 bs=1k
Congratulations, you are done! You should always test a bootdisk before putting it aside for an emergency! If it fails to boot, read on.