8.2. The boot process in closer look

You can boot Linux either from a floppy or from the hard disk. The installation section in the Installation and Getting Started guide (XXX citation) tells you how to install Linux so you can boot it the way you want to.

When a PC is booted, the BIOS will do various tests to check that everything looks all right, [1] and will then start the actual booting. It will choose a disk drive (typically the first floppy drive, if there is a floppy inserted, otherwise the first hard disk, if one is installed in the computer; the order might be configurable, however) and will then read its very first sector. This is called the boot sector; for a hard disk, it is also called the master boot record, since a hard disk can contain several partitions, each with their own boot sectors.

The boot sector contains a small program (small enough to fit into one sector) whose responsibility is to read the actual operating system from the disk and start it. When booting Linux from a floppy disk, the boot sector contains code that just reads the first few hundred blocks (depending on the actual kernel size, of course) to a predetermined place in memory. On a Linux boot floppy, there is no filesystem, the kernel is just stored in consecutive sectors, since this simplifies the boot process. It is possible, however, to boot from a floppy with a filesystem, by using LILO, the LInux LOader.

When booting from the hard disk, the code in the master boot record will examine the partition table (also in the master boot record), identify the active partition (the partition that is marked to be bootable), read the boot sector from that partition, and then start the code in that boot sector. The code in the partition's boot sector does what a floppy disk's boot sector does: it will read in the kernel from the partition and start it. The details vary, however, since it is generally not useful to have a separate partition for just the kernel image, so the code in the partition's boot sector can't just read the disk in sequential order, it has to find the sectors wherever the filesystem has put them. There are several ways around this problem, but the most common way is to use LILO. (The details about how to do this are irrelevant for this discussion, however; see the LILO documentation for more information; it is most thorough.)

When booting with LILO, it will normally go right ahead and read in and boot the default kernel. It is also possible to configure LILO to be able to boot one of several kernels, or even other operating systems than Linux, and it is possible for the user to choose which kernel or operating system is to be booted at boot time. LILO can be configured so that if one holds down the alt, shift, or ctrl key at boot time (when LILO is loaded), LILO will ask what is to be booted and not boot the default right away. Alternatively, LILO can be configured so that it will always ask, with an optional timeout that will cause the default kernel to be booted.

With LILO, it is also possible to give a kernel command line argument, after the name of the kernel or operating system.

Booting from floppy and from hard disk have both their advantages, but generally booting from the hard disk is nicer, since it avoids the hassle of playing around with floppies. It is also faster. However, it can be more troublesome to install the system to boot from the hard disk, so many people will first boot from floppy, then, when the system is otherwise installed and working well, will install LILO and start booting from the hard disk.

After the Linux kernel has been read into the memory, by whatever means, and is started for real, roughly the following things happen:



This is called the power on self test, or POST for short.