Understanding the Linux Kernel, 3rd 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^[*] 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;^[†] following this approach, only the operating system whose kernel image is stored in the active partition can be booted. As we will see later, Linux is more flexible because it replaces the rudimentary program included in the MBR with a sophisticated program—the “boot loader”—that allows users to select the operating system to be booted.

Kernel images of earlier Linux versions—up to the 2.4 series—included a minimal “boot loader” program in the first 512 bytes; thus, copying a kernel image starting from the first sector made the floppy bootable. On the other hand, kernel images of Linux 2.6 no longer include such boot loader; thus, in order to boot from floppy disk, a suitable boot loader has ...