The master boot record itself holds two things -- either some 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.
The BIOS is the startup device of the PC that starts to move to 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 the primary boot loader. The primary boot that resides in the MBR is a 512-byte image containing both program code and a small partition table . The first 446 bytes are the primary boot loader, which contains both executable code and error message text. The next sixty-four bytes are the partition table, which contains a record for each of four partitions (sixteen bytes each). The MBR ends with two bytes that are defined as the magic number (0xAA55). The magic number serves as a validation check of the MBR..
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.
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.
The first- and second-stage boot loaders combined are called Linux Loader (LILO) or GRand Unified Bootloader (GRUB) in the x86 PC environment. Because LILO has some disadvantages that were corrected in GRUB, let's look into GRUB.
On an embedded platform, a bootstrap environment is used when the system is powered on, or reset. Examples include U-Boot, RedBoot, and MicroMonitor from Lucent. Embedded platforms are commonly shipped with a boot monitor. These programs reside in special region of flash memory on the target hardware and provide the means to download a Linux kernel image into flash memory and subsequently execute it. In addition to having the ability to store and boot a Linux image, these boot monitors perform some level of system test and hardware initialization. In an embedded target, these boot monitors commonly cover both the first- and second-stage boot loaders.
Reference:
1-Boot loader showdown: Getting to know LILO and GRUB
http://www.ibm.com/developerworks/linux/library/l-bootload.html
2-Inside the Linux boot process
http://www.ibm.com/developerworks/linux/library/l-linuxboot/
