If you don't want to rely on LILO (which modifies disk sectors and may cause problems when you use several operating systems or change operating systems frequently), you can use a boot floppy disk to start up Linux. A boot floppy disk is a single floppy disk that contains a complete copy of the Linux kernel and instructions for accessing the root partition on your hard drive. The boot floppy disk must be of the proper format to run on the first disk drive on your system (drive A in DOS terms). Linux cannot boot from a second (drive B) floppy disk drive. In many ways, a boot floppy disk is the easiest and most versatile method of starting Linux. If, for example, you have your hard disk partitioned to contain both DOS and Linux, with DOS the normal boot partition, simply turning on your PC boots DOS without a hitch. If you want Linux to boot, you insert the boot floppy disk and start the machine. Linux boots from the floppy disk, and then accesses the hard drives as if it had booted from them.
If you are using a boot floppy disk to start Linux, be sure to update the kernel image on the floppy disk every time you make a change to the system that involves rebuilding the kernel. Keep in mind that you must rebuild the kernel almost every time you add devices or device drivers. Use the procedure outlined in the following paragraphs to update your existing floppy disk, or, even better, create a new floppy disk and save your current floppy disk for emergencies. You are not prompted to create a new boot floppy disk when you make changes to the kernel, so you must remember to perform this step. To create a boot floppy disk, you need a blank, formatted, high-density floppy disk (1.44M or 1.2M, depending on the A drive on your system). Format the floppy disk under DOS to lay down the sector and track information properly for Linux to read. Some small kernels can fit on low-density floppy disks, but high-density drives are most likely to be used on your system because they are the standard. The high-density floppy disk can have information on it as well as the Linux kernel, but make sure you have enough disk space for the kernel image. Some versions of Linux (such as Slackware) can create the boot floppy disk as part of the normal setup routine. If you are using a distribution that has a setup routine, try choosing the Configure option on the menu, follow through the prompts, and see whether you are prompted to create a boot floppy disk at the end. Alternatively, some distributions have a separate menu option for creating the boot floppy disk. If you want to create the boot floppy disk manually, locate the Linux kernel on your system. Usually it is in the root directory. The kernel name changes depending on the version of Linux, but it is often called Image or vmlinux. Some versions of Linux store the kernel image in the /etc directory. Some versions of Linux also store the kernel in a compressed format. The names of compressed kernels end in z, as in vmlinuz or vmlinux.z. A compressed kernel takes up less space on the hard disk or floppy disk, and it is decompressed when the Linux kernel boots. A compressed kernel takes a little longer to load than a kernel that isn't compressed, but because it is decompressed only when the system boots, the trade-off is usually beneficial (unless you have tons of empty disk space). You should be able to find the kernel quite easily by watching the startup messages when you boot the system and noting the kernel name, and then using find or whereis to locate it. Much easier is to change to the root directory and look for a large file called Image or vmlinux. The file is owned by root and has only read permission in many distributions of Linux. For example, when you do a listing command (such as ls-l), you see an entry like the following: -r-------- 1 root root 457700 Aug 10 13:52 vmlinuz This entry shows a kernel image file of almost half a megabyte that is compressed. The date and time of the kernel match the last time you rebuilt the kernel or the time and date you installed Linux. Once you have identified the kernel file, instruct Linux that the file is the root device and indicate which partition it is on by using the rdev command. For example, to set the root device to the kernel vmlinuz in the root directory of the partition /dev/sda3, you would issue the command rdev /vmlinuz /dev/sda3 Because you must specify the path to the kernel completely, the leading slash is included to show the root directory. If you issue the rdev command by itself, it displays the current partition of the root filesystem: $rdev /dev/sda3 / You can use this command to check the current settings if you are not sure which partition is your root filesystem. (This chapter looks at the rdev command in a little more detail in a later section.) After you set the root device, you can copy the kernel to your formatted floppy disk. Use the cp command and the device name of the floppy disk: cp /vmlinuz /dev/fd0 Once the image file has been transferred, the floppy disk should be able to boot Linux. If it doesn't, either the image didn't transfer properly due to a lack of disk space or a corrupted disk sector, or there is a problem with the kernel image.
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