Posted: Sun Apr 23, 2006 9:04 pm Post subject: Boot Loader
Parts of a boot loader
A boot loader typically consists of three programs:
The boot sector program is directly loaded by the BIOS at boot time and is only 512 bytes in size. The second stage program is loaded by the boot sector program and it does everything you expect the boot loader to do. The boot loader installer is not run when the system is booted, but it is used to install the boot loader and the second stage program onto the boot disk. These have to be stored in special locations, so they cannot be copied with cp.
Boot Sector Program
The boot sector program can only be 512 bytes in size and not all 512 bytes are even available in all cases. The last two bytes must be 0x55 and 0xAA for the BIOS. The Master Boot Record on a hard disk contains the partition table of 64 bytes, so only the first 446 bytes can be used. If the boot sector program must exist on a DOS partition or on a DOS diskette, there must be a parameter block at the start of the boot sector.
Because of these size restrictions, boot sector programs are just about the only remaining examples of programs on the PC platform that are truly optimized for size and have to be written in assembly for this reason. Further, a boot sector program cannot do everything you want a boot loader to do. Usually a boot sector program does one of the following things (not all three in one program):
Load another boot sector. This is typical for a boot sector program that lives in the master boot record of a hard disk. It can find the first sector of the selected active partition and chain load that. The MBR program that came traditionally with MS-DOS has no ability to change the active partition at boot time. There are other boot sector programs that let you select a partition by pressing a key, such as the MBR program of LILO. Load a second stage boot loader. It is generally not possible for a boot sector program to look into the directory for a file with a specific name and load that into memory, but exceptions exist, at least for DOS file systems2. Most boot sector programs find the second stage by sector number rather than by name. The sector numbers have to be put into the boot sector by the boot loader installer. Load the kernel directly. A kernel is typically much larger than a second stage boot loader. The boot sector program in the Linux kernel loads the kernel directly into memory without the need for a second stage boot loader. As the kernel is located in contiguous sectors on a diskette, there is no need to traverse file system data structures. However, for bZimage kernels the boot sector program cheats; it invokes a subroutine in the setup part of the kernel for loading the rest of the kernel into high memory.
The boot loader e2boot fits into the first 1kB block of an ext2 partition (it is twice as big as a boot sector program), but with some tricks it finds both the kernel and the RAM disk by name on an ext2 partition and loads them into memory.
Also the boot sector on a DOS disk does not utilize a second stage boot loader to load the MS-DOS kernel files IO.SYS and MSDOS.SYS. The structure of an MSDOS file system is simple enough to find a file with a specific name in the root directory and load it into memory, at least part of it..
Second Stage of Boot Loader
This is the real boot program. It contains the user interface and the kernel loader. It can be anywhere from 6.5 kilobytes (LILO) to over 100 kilobytes (GRUB) in size. It contains the following functions:
User interface. It is either a simple command line (old versions of LILO), a menu or both. It allows you to select any number of operating systems and to specify additional parameters to the operating system. The available options are specified by a configuration file. Modern versions of boot loaders can show their menu in a bitmap picture. Operating system loader. loads the operating system into memory and runs it. Alternatively we can load another boot loader specific to another operating system and let it run. This is called chain loading. LOADLIN is not a complete boot loader, but it has only the second stage (without the user interface). It is run from DOS, and it can make use of DOS system calls to read files from disk. What makes its task harder than that of a normal boot loader, it that it must be able to work its way out of some types of memory managers.
Boot Loader Installer
The third part of the boot loader is only run when the boot loader is installed on a disk. As opposed to the boot sector program and second stage, this is a normal Linux program. In the case of LILO the installer must be rerun each time the configuration is changed or any file has been updated. It performs the following tasks:
Install the boot sector. If the boot sector will be installed in the MBR of a hard disk or on a DOS file system, not all 512 bytes may be overwritten, but the partition table or the DOS parameter block must be preserved. Tell the boot sector where the second stage boot loader is. Usually it writes one or more sector addresses into the boot loader. Tell the second stage boot loader where all relevant information is (configuration, kernels). This is the case with LILO. LILO creates a map file that contains all relevant sector addresses and puts pointers to the map file in the boot sector and/or second stage boot loader.
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