Building Tiny Linux Systems with Busybox, Part 2: Building the Kernel

You'll need the genromfs program to generate the ROM file system image. If you are using Debian or Red Hat, it's already packaged for you as part of the standard system; you'll just need to run the command to install it. If your Linux distribution doesn't include a prepackaged version, at this writing the program can be found at ftp://ibiblio.org/pub/Linux/system/recovery/genromfs-0.3.tar.gz. Install the program, change directory to the directory that contains tiny-linux and run these commands:

genromfs -d tiny-linux -f fs

This creates the ROM file system image in the file fs. Now, compress the file system image using the gzip command:

gzip -9 fs

That will create the file fs.gz, which is about half the size of the uncompressed version.

We'll need one more program to build our floppy: syslinux. This is an i386 bootstrap program that will load a kernel and a compressed root file system image from a floppy, hard disk or CD. Again, it's prepackaged with Debian or Red Hat and can be found, at this writing, at ftp://ibiblio.org/pub/Linux/system/boot/loaders/syslinux-1.48.tar.gz.

Create an MS-DOS file system on a floppy by using the Linux mformat command (or by another means). Put the floppy in your drive but don't mount it, and install the syslinux bootstrap with this command:

syslinux /dev/fd0

That will copy a first-stage bootstrap onto the first block of the floppy and a second-stage bootstrap into the file LDLINUX.SYS in the MS-DOS file system of the floppy. I'm assuming you have a directory called /mnt; substitute whatever directory you usually use for mounting floppy disks in the shell commands below. Now, it's time to copy our kernel and root file system onto the floppy:

mount -t msdos /dev/fd0 /mnt
cp fs.gz /mnt
cp /usr/src/linux/arch/i386/boot/bzImage /mnt/linux

Create the configuration file /mnt/syslinux.cfg with an editor:

TIMEOUT 20
DEFAULT linux
LABEL linux
    KERNEL linux
    APPEND root=/dev/ram0 initrd=fs.gz

This tells syslinux to wait for two seconds and then boot the default system. You can interrupt the default boot during those two seconds by pressing the shift key. Typing linux at the prompt will do the same thing as the default.

The kernel is booted with the arguments root=/dev/ram0 and initrd=fs.gz. These arguments tell the kernel that the root is a RAM disk, and that the RAM disk is loaded from the compressed ROM file system image fs.gz. Although the ``ROM'' root file system is actually in RAM, it will not be writable because the Linux ROM file system driver used in this example is meant to work with real ROMs and thus doesn't support writing.

Configure an i386 PC to boot from the floppy. Note that this is a setting in the BIOS preferences of most new PCs, and they are often configured to boot from the hard disk without first looking for a bootstrap on the floppy. Place the floppy in the first floppy drive, and restart the system. You should see something like the following:

SYSLINUX 1.48 1999-09-26
Copyright (C) 1994-1999 H. Peter Anvin
loading fs.gz.........
loading linux...............
Uncompressing linux, OK, booting the kernel

Tons of cybercrud about every device driver and facility in the kernel race by too rapidly to read...

RAMDISK: Compressed image found at block 0.
VFS: Mounted root (romfs file system)
Please press Enter to activate this console.

You've done it! Press Enter and you should see something like this:

Busybox v0.46 (2000-09-12-22:16+0000) built-in shell
Enter 'help' for a list of built-in commands.
/ # _

Pop out the floppy disk; it's not being used any longer. The root file system is entirely in RAM. You should be able to look around the system and try out commands, but you won't have any writable storage. This is the simplest, bootable-system, running busybox that I could think of, and thus I've left out the files in /dev that you'd need to mount writable RAM disks, floppies and hard disks. You now should be able to figure out what those devices are, add them and build a tiny Linux system specialized to your application.