F3036 OPEN SOURCE OPERATING SYSTEM CHAPTER 2: PART 2 UNDERSTAND BOOT LOADERS IN LINUX

1. F3036 OPEN SOURCE OPERATING SYSTEMCHAPTER 2: PART 2UNDERSTAND BOOT LOADERS IN LINUX

2. What is Boot Loaders ? • 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. • Most new computers are shipped with boot loaders for some version of Microsoft Windows or the Mac OS. If a computer is to be used with Linux, a special boot loader must be installed.

3. Linux Boot Loader • For Linux, the two most common boot loaders are known as : • LILO (Linux Loader) • GRUB (Grand Unified Boot loader)

4. LILO (Linux Loader) • LILO comes as standard on all distributions of Linux. • As one of the older/oldest Linux boot loaders, its continued strong Linux community support has enabled it to evolve over time and stay viable as a usable modern-day boot loader. • Some new functionality includes an enhanced user interface.

5. Making LILO your boot loader • What you will need to do to use LILO as your boot loader depends on whether you are installing the OS fresh or have already installed Linux and are planning on moving to LILO. • If you're starting fresh, you can jump straight to the Configuring LILO section. • If you already have a Linux distribution installed, you usually get an option to install and configure LILO (and can boot your machine into your new Linux install).

6. LILO: Initial boot process • When LILO initially loads, it brings up in order each of the letters -- L-I-L-O. If all the letters come up, the first stage boot was successful. Anything less indicates a problem: 1) L: The first stage boot loader has been loaded. If LILO stops here, there were problems loading the second stage boot loader. This is usually accompanied by an error code. The common problems at this stage are media problems or incorrect disk parameters specified in your lilo.conf file. 2) LI: The second stage boot loader has been loaded. LILO halting at this point indicates the second stage boot loader could not be executed. Again, this can be due to problems similar to just L: loading or if the boot.b file has been corrupted, moved, or deleted. 3) LIL: The second stage boot loader has now been executed. At this point, media problem could again be responsible or the map file (as specified in the lilo.conf file) could have had problems finding the descriptor tables. 4) LIL?: Loaded to the same point as above. This usually means the second stage boot loader loaded at an incorrect address, caused most likely by boot.b being in a different place than specified in the lilo.conf file. 5) LIL-: Loaded to the same point as above. Problem loading the descriptor table, most likely due to a corrupt descriptor table. 6) LILO: LILO has successfully loaded with no errors.

7. GRUB (Grand Unified Boot Loader) • More recently, the GRand Unified Boot loader (commonly known as GRUB) seems to have outshine LILO. • GNU GRUB is actively developed by the Free Software Foundation and based on the original GRUB program, originally created by Erich Stefan Boleyn.

8. GRUB: Features • GRUB is now being replaced by GRUB2. • The original GRUB is being renamed to GRUB Legacy; apart from fixing bugs, it will no longer be actively developed. • GRUB2 will be a complete rewrite of the original boot loader. • To date, the following features are the core of the changes: • Replacement of Stage 1.5 with the creation of a compact core image • Support for dynamic loading to the core image • Trend towards making the overall GRUB framework object oriented • Support for different hardware architectures and different platforms (other than Linux)

9. Making GRUB your boot loader • As with LILO, the steps you will need to perform to use GRUB as your active boot loader depend on whether you are installing the OS fresh or have already installed Linux and are planning on moving to GRUB. • If you're starting fresh, you can jump straight to the Configuring GRUB section. • If you already have a Linux distribution installed, you usually get an option to install and configure LILO (and can boot your machine into your new Linux install). • For existing Linux users wanting to migrate to GRUB, get the latest version of GRUB .

10. GRUB: Initial boot process • When GRUB initially loads, like LILO it loads its first stage from the MBR. • Once this has loaded, it then enters an intermediate stage between the common boot loader stages one and two (or for argument's sake, Stage 1.5). • Stage 1.5 is present to enable regular file system access to the GRUB configuration files in /boot/grub rather than accessing using disk blocks. • We then enter stage two of the boot loader where GRUB loads the grub.conf file. • You should now see the GRUB GUI. For any native Windows user, this seems a much more friendly experience than that of LILO. • User should now be looking at a screen giving them two options: to boot into Red Hat Linux or to boot into Windows XP. By default, it will load Linux.

11. LILO vs. GRUB • All boot loaders work in a similar way to fulfill a common purpose. But LILO and GRUB do have a number of differences: • LILO has no interactive command interface, whereas GRUB does. • LILO does not support booting from a network, whereas GRUB does. • LILO stores information regarding the location of the operating systems it can to load physically on the MBR. If you change your LILO config file, you have to rewrite the LILO stage one boot loader to the MBR. Compared with GRUB, this is a much more risky option since a misconfigured MBR could leave the system unbootable. With GRUB, if the configuration file is configured incorrectly, it will simply default to the GRUB command-line interface.