Chapter 4. Configure the boot loader

When a PC boots the CPU it runs code from Read-Only Memory. This code is the Basic Input/Output System, or BIOS. The BIOS then loads a boot loader from the Master Boot Record of the first hard disk.[1] In turn, the boot loader reads the operating system into memory and then runs it.[2]

Neither the BIOS nor the boot loader are strictly necessary. For example, there are versions of Linux that run directly from the flash memory which usually contains the BIOS. Linux was originally designed to run without an interactive boot loader, by placing the kernel at particular sectors of the disk.

The benefits of using a boot loader are:

• Multiple operating systems can be booted. See the Linux + Windows HOWTO for more information.

• Parameters can be passed to the kernel interactively. This is useful for solving hardware problems; for example, some interrupt lines can be disabled, direct memory access to some drives can be disabled, and so on. See the Linux BootPrompt-HOWTO for a list of kernel parameters.

• Differing kernels can be interactively loaded. This is useful when deploying a new kernel, as it provides simple fallback to a proven kernel.

For these reasons systems administrators want to be able to interactively control the boot loader from the serial console.

LILO, GRUB and SYSLINUX are popular boot loaders for IBM PCs. Find which of these boot loaders your Linux installation uses and then follow the instructions for your boot loader in the following section.