Boot Loader

In simple term, boot loader is application which loads the operating system.

Before we see boot loader definition, we will see why we require boot loader.

When your machine is switched on, the BIOS reads the first 512 bytes of your bootable media (which is known as the master boot record, or MBR). You can store the boot record of only one operating system in a single MBR. This opens up the Operating system for you. So a problem becomes apparent when you require multiple operating systems in your machine.. Hence the need for more flexible boot loaders.

The master boot record itself holds two things -- either some of or all of the boot loader program and the partition table (which holds information regarding how the rest of the media is split up into partitions). When the BIOS loads, it looks for data stored in the first sector of the hard drive, the MBR; using the data stored in the MBR, the BIOS activates the boot loader.

Due to the very small amount of data the BIOS can access, most boot loaders load in two stages. In the first stage of the boot, the BIOS loads a part of the boot loader known as the initial program loader, or IPL. The IPL interrogates the partition table and subsequently is able to load data wherever it may exist on the various media. This action is used initially to locate the second stage boot loader, which holds the remainder of the loader.

The second stage boot loader is the real meat of the boot loader; many consider it the only real part of the boot loader. This contains the more disk-intensive parts of the loader, such as user interfaces and kernel loaders. These user interfaces can range from a simple command line to the all-singing, all-dancing GUIs.

Boot loaders are usually configured in one of two ways: either as a primary boot loader or as a secondary boot loader. Primary boot loaders are where the first stage of the boot loader is installed on the MBR (per the previous description). Secondary boot loaders are where the first stage of the boot loader is installed onto a bootable partition. A separate boot loader must then be installed into the MBR and configured to pass control to the secondary boot loader.

Many newer Linux boot loaders can be especially useful since they give you varying degrees of interaction, such as advanced GUI and encrypted passwords, along with the ability to load from a selection of operating systems. Thus you can have many operating systems coexisting on the same machine spanning multiple physical disks. This setup has become common because it helps many users reconcile the treasures of valuable datafiles originally generated by their Windows® machines with their shiny new Linux installs. Personally, I think this setup is great; I use a single box for both Linux and Windows.

Over time, boot loaders have been enhanced to include a greater range of functionality for the user. Such functionality and configuration vary from loader to loader, although the basic purpose remains the same.

Now let's look at two of the more popular boot loaders: LILO and GRUB.