Boot Loader

BOOT LOADER

A boot loader, also called a boot manager, is a small program that places the operating system (OS) of a computer into memory. When a computer is powered-up or restarted, the basic input/output system (BIOS) performs some initial tests, and then transfers control to the master boot record (MBR) where the boot loader resides. 

How does GRUB work?

 When a computer boots, the BIOS transfers control to the first boot device, which can be a hard disk, a floppy disk, a CD-ROM, or any other BIOS-recognized device. We'll concentrate on hard disks, for the sake of simplicity.

 The first sector on a hard is called the Master Boot Record (MBR). This sector is only 512 bytes long and contains a small piece of code (446 bytes) called the primary boot loader and the partition table (64 bytes) describing the primary and extended partitions.

 By default, MBR code looks for the partition marked as active and once such a partition is found, it loads its boot sector into memory and passes control to it.

GRUB replaces the default MBR with its own code.

 Furthermore, GRUB works in stages.

Stage 1 is located in the MBR and mainly points to Stage 2, since the MBR is too small to contain all of the needed data.

Stage 2 points to its configuration file, which contains all of the complex user interface and options we are normally familiar with when talking about GRUB. Stage 2 can be located anywhere on the disk. If Stage 2 cannot find its configuration table, GRUB will cease the boot sequence and present the user with a command line for manual configuration.

Stage 1.5 also exists and might be used if the boot information is small enough to fit in the area immediately after MBR.

 The Stage architecture allows GRUB to be large (~20-30K) and therefore fairly complex and highly configurable, compared to most bootloaders, which are sparse and simple to fit within the limitations of the Partition Table.

For Linux, the two most common boot loaders are known as LILO (LInux LOader) and LOADLIN (LOAD LINux). An alternative boot loader, called GRUB (GRand Unified Bootloader), is used with Red HatLinux. LILO is the most popular boot loader among computer users that employ Linux as the main, or only, operating system. The primary advantage of LILO is the fact that it allows for fast boot-up. LOADLIN is preferred by some users whose computers have multiple operating systems, and who spend relatively little time in Linux. LOADLIN is sometimes used as a backup boot loader for Linux in case LILO fails. GRUB is preferred by many users of Red Hat Linux, because it is the default boot loader for that distribution.

GRUB (GRand Unified Bootloader) is a boot loader package developed to support multiple operating systems and allow the user to select among them during boot-up.

 GRUB is a bootloader (can also be spelled boot loader) capable of loading a variety of free and proprietary operating systems. GRUB will work well with Linux, DOS, Windows, or BSD. GRUB stands for GRand Unified Bootloader.

GRUB can be run from or be installed to any device (floppy disk, hard disk, CD-ROM, USB drive, network drive) and can load operating systems from just as many locations, including network drives. It can also decompress operating system images before booting them.

What about LILO?

You may have heard about another Linux bootloader called LILO (stands for LInux LOader). While a sensible option for many Linux users, I believe that GRUB is a better choice, for several reasons:

1.      LILO supports only up to 16 different boot selections; GRUB supports an unlimited number of boot entries.

2.      LILO cannot boot from network; GRUB can.

3.      LILO must be written again every time you change the configuration file; GRUB does not.

4.      LILO does not have an interactive command interface.