Making a Bootable Diskette

We will describe three methods to boot Linux from a diskette.

• Booting the kernel directly from a diskette and mount a root file system on a different diskette.
• Booting the kernel with LILO and mount a root file system on a diskette (possibly the same diskette).
• Booting the kernel with LILO and add a RAM disk for the root file system.

Instead of LILO we could use another boot loader, such as SYSLINUX or GRUB. This is considered to be outside the scope of this text.

For the boot diskettes we have to format 1.44MB floppy disks or take formatted disks that may be erased. In Linux there are two programs to format a diskette. One program is fdformat. A diskette is formatted as follows:

fdformat /dev/fd0H1440

Some distributions have superformat instead. This is used as follows:

superformat --hd /dev/fd0

We assume 3.5'' HD diskettes and we only format them in the standard way (1440kB), so we do not try to get a few more sectors per track or a few more tracks.

All commands in this section should be done as root.

Booting Linux directly from a diskette

We do not have to use a complicated boot loader such a LILO in order to boot a Linux system. The Linux kernel has its own primitive boot loader. When the kernel is transferred directly to a diskette with the dd command, the kernel can boot itself. This way we cannot supply a command line and it works only with diskettes. We also cannot use the initrd feature, but there is a different way of loading a RAM disk, which we will not discuss (it is discussed in the Bootdisk HOWTO).

First we have to create a file system on the root diskette and transfer the files to it. Type the following commands:

mke2fs /dev/fd0
mkdir /home/lennartb/myboot/mnt
mount /dev/fd0 /home/lennartb/myboot/mnt
cp -a /home/lennartb/myboot/rootfs/* \
   /home/lennartb/myboot/mnt
umouunt /dev/fd0

The root file system will be mounted read-only. If this is not desired, add the following line to the file etc/init.d/rcS while the floppy is mounted:

mount -o remount /dev/fd0 /

Next we create the boot disk. With a different diskette in the drive type the following command:

dd if=/home/lennartb/myboot/linux/arch/boot/zImage \
   of=/dev/fd0
rdev /dev/fd0 /dev/fd0

The rdev command selects the device on which the root file system should be mounted by the kernel after booting. We could use the following command to make the diskette mount the root file system on the first hard disk partition.

rdev /dev/fd0 /dev/hda1

LILO bootable disk

First make the root diskette as described in the previous section. There are two ways to proceed:

• Mount the root diskette and install the kernel and LILO on it. This way we have a self-contained diskette that boots and has the root file system. It works for the kernel and the utilities as described in this article, but you would quickly run out of disk space when adding more features to the kernel or more utilities to the root file system.
• mke2fs a different diskette for the kernel and LILO. This way we still have two diskettes and more space on each.

The LILO diskette is either the root diskette to which we will add LILO or the separate diskette on which we will install LILO.

Mount the LILO diskette and add the kernel and boot loader to it:

mount /dev/fd0 /home/lennartb/myboot/mnt/
mkdir /home/lennartb/myboot/mnt/boot
cp /home/lennartb/myboot/linux/arch/i386/boot/zImage \
   /home/lennartb/myboot/mnt/boot
cp /boot/lilo/boot.b /home/lennartb/myboot/mnt/boot

Create the file /home/lennartb/myboot/lilo.conf as follows:

boot=/dev/fd0
install=/home/lennartb/myboot/mnt/boot/boot.b
map=/home/lennartb/myboot/mnt/boot/map
delay=100
compact
image=/home/lennartb/myboot/mnt/boot/zImage
  label=linux
  root=/dev/fd0

All relevant files mentioned in lilo.conf are on the mounted diskette. The delay option will wait 10 seconds, so we have the opportunity to type a command line in LILO. The compact option makes loading much faster. Try it without and compare.

Run LILO. This will add a map file to the /boot directory on the diskette and it will add a boot sector to the diskette. The boot sector will load the loader in boot.b, this will load the map file and the map file contains a list of sectors of the kernel, so this can be loaded.

lilo -C /home/lennartb/myboot/lilo.conf

Finally unmount the LILO diskette.

unount /dev/fd0

RAM disk

A RAM disk has the following advantages.

• Once the system is booted, programs will load faster.
• Once the system is booted, the diskette can be removed from the drive. Therefore the floppy drive can be used to load other programs or to restore a hard disk partition from backup diskettes.
• The RAM disk image can be stored compressed on the diskette. A full 1.44MB or even 2MB file system can be stored compressed on a diskette together with a kernel. A compressed 4MB or 8MB file system is also possible as long as it is not completely filled. After booting the system, you can copy programs from other diskettes into the RAM disk.

The RAM disk has the following disadvantages:

• It requires more RAM to run. The RAM disk configuration described in this system will boot on a system with 4MB of RAM, but this is pretty much the limit. More kernel features or more utilities would make this RAM disk unusable on a 4MB system.
• It requires more steps to create or to adapt the root file system.
• Changes made to the root file system are not permanent. This is either an advantage (security) or a disadvantage (configuration changes are not possible).

First we have to create an image file for the RAM disk. We limit its size to 1000K, so it still runs on a 4MB machine. Create a file system on the image file. The -N 200 option is necessary to create enough inodes on the file system, as there are a lot of symbolic links.

dd if=/dev/zero of=/home/lennartb/myboot/root.img \
   bs=1k count=1000
mke2fs -F -N 200 /home/lennartb/myboot/root.img

Next mount the image file using a loop device and copy all files of the root file system to it. The loop option to mount makes it possible to mount a regular file as if it were a block device.

mount -o loop /home/lennartb/myboot/root.img \
   /home/lennartb/myboot/mnt
cp -a /home/lennartb/myboot/rootfs/* \
   /home/lennartb/myboot/mnt
umount /home/lennartb/myboot/mnt

Compress the root file system image. This image file will be copied to a diskette.

gzip -9 /home/lennartb/myboot/root.img

Create a new LILO diskette, mount it and add the kernel, root file system image and boot loader to it:

mke2fs /dev/fd0
mount /dev/fd0 /home/lennartb/myboot/mnt/
mkdir /home/lennartb/myboot/mnt/boot
cp /home/lennartb/myboot/linux/arch/i386/boot/zImage \
   /home/lennartb/myboot/mnt/boot
cp /home/lennartb/myboot/root.img.gz \
   /home/lennartb/myboot/mnt/boot
cp /boot/lilo/boot.b /home/lennartb/myboot/mnt/boot

Create the file /home/lennartb/myboot/lilo-initrd.conf as follows:

boot=/dev/fd0
install=/home/lennartb/myboot/mnt/boot/boot.b
map=/home/lennartb/myboot/mnt/boot/map
delay=100
compact
image=/home/lennartb/myboot/mnt/boot/zImage
  label=linux
  root=/dev/ram
  initrd=/home/lennartb/myboot/mnt/boot/root.img.gz
image=/home/lennartb/myboot/mnt/boot/zImage
  label=noram
  root=/dev/fd0

The configuration file is almost the same, except for the initrd option. This causes the boot loader to load a RAM disk image into memory. The Linux kernel will decompress it to a RAM disk device and mount this as the root file system ³. Further we added a noram option, so it is also possible to use this LILO diskette without a RAM disk.

Run LILO:

lilo -C /home/lennartb/myboot/lilo-initrd.conf

Finally unmount the LILO diskette:

unount /dev/fd0