6. Putting them together: Making the diskette(s)

6.1. Transferring the kernel with LILO

First, make sure you have a recent version of LILO.

You must 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

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 -s KERNEL'') 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 -N 24 -m 0 /dev/fd0 KERNEL_BLOCKS

        mount -o dev /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

        cp /boot/boot.b /mnt/boot

        cp bdlilo.conf KERNEL /mnt

        lilo -v -C bdlilo.conf -r /mnt

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 diskette in the drive and go to Section 6.3.

6.2. Transferring the kernel without LILO

If you are not using LILO, transfer the kernel to the bootdisk with dd:

        % dd if=KERNEL of=/dev/fd0 bs=1k
        353+1 records in
        353+1 records out

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

6.3. Setting the ramdisk word

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:

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.

1. If you are building a single disk, the compressed root filesystem will be placed right after the kernel, so the offset will be the first free block (which should be the same as 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,

2. If you are building a two-disk set, the root filesystem will begin at block zero of the second disk, so the offset will be zero. Bit 14 will be set to 1 and bit 15 will be 1. The decimal value will be 2^14 + 2^15 = 49152 in this case.

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.

Do not believe what the rdev/ramsize manpage says about ramdisk size. The manpage is obsolete. As of kernel 2.0 or so, the ramdisk word no longer determines the ramdisk size; the word is instead interpreted according to the table at the beginning of section Section 6.3. For a detailed explanation, see the documentation file ramdisk.txt or http://www.linuxhq.com/kernel/v2.4/doc/ramdisk.txt.html.

6.4. Transferring the root filesystem

The last step is to transfer the root filesystem.

• If the root filesystem will be placed on the same disk as the kernel, transfer it using dd with the seek option, which specifies how many blocks to skip:

  dd if=rootfs.gz of=/dev/fd0 bs=1k seek=KERNEL_BLOCKS

• If the root filesystem will be placed on a second disk, remove the first diskette, put the second diskette in the drive, then transfer the root filesystem to it:

  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.