Understanding the Linux Kernel, Second Edition

Ancient Age: The Boot Loader

The boot loader is the program invoked by the BIOS to load the image of an operating system kernel into RAM. Let’s briefly sketch how boot loaders work in IBM’s PC architecture.

To boot from a floppy disk, the instructions stored in its first sector are loaded in RAM and executed; these instructions copy all the remaining sectors containing the kernel image into RAM.

Booting from a hard disk is done differently. The first sector of the hard disk, named the Master Boot Record (MBR), includes the partition table^[138] and a small program, which loads the first sector of the partition containing the operating system to be started. Some operating systems, such as Microsoft Windows 98, identify this partition by means of an active flag included in the partition table;^[139] following this approach, only the operating system whose kernel image is stored in the active partition can be booted. As we shall see later, Linux is more flexible because it replaces the rudimentary program included in the MBR with a sophisticated program such as LILO or GRand Unified Bootloader (GRUB) that allows users to select the operating system to be booted.

Booting Linux from Floppy Disk

The only way to store a Linux kernel on a single floppy disk is to compress the kernel image. As we shall see, compression is done at compile time and decompression is done by the loader.

If the Linux kernel is loaded from a floppy disk, the boot loader is quite simple. It is coded in the arch/i386/boot/bootsect.S ...