bootloader in linux

Boot loader in linux

The boot loader phase varies by platform. Since the earlier phases are not specific to the OS, the boot process is considered to start:

* For x86 or x86-64: when the partition boot sector code is executed in real mode and loads the first stage boot loader (typically a part of LILO or GRUB).

From that point, the boot process continues as follows:

The first stage boot loader loads the remainder of the boot loader, which typically gives a prompt asking which operating system (or type of session) the user wishes to initialize. Under LILO, this is done via the map installer which reads the configuration file /etc/lilo.conf to identify the available systems. It includes data such as boot partition and kernel location for each, as well as customized options if any. Upon selection, the appropriate kernel is loaded into RAM memory as an image file ("initrd"), and along with the appropriate parameters, control is passed to it.

LILO and GRUB differ in some ways:

* LILO does not understand file systems, so it uses raw disk offsets and the BIOS for data load. It loads the menu code, and then depending on the response loads either the 512 byte disk sectors for an MBR system such as Microsoft Windows, or the kernel image for Linux.
* GRUB by contrast does have understanding of the common ext2 and ext3 file systems. Because GRUB stores its data in a configuration file rather than the MBR and contains a command line interface, it is often easier to rectify or modify GRUB if misconfigured or corrupt.

GRUB

Source: Red Hat GRUB description.

1. The first stage loader is read by the BIOS from the MBR (master boot record).
2. The first stage loads the rest of the boot loader (second stage). If the second stage is on a large drive, sometimes an intermediate 1.5 stage is loaded, which contains extra code to allow cylinders above 1024, or LBA type drives, to be read. The 1.5 boot loader is stored (if needed) in the MBR or the boot partition.
3. The second stage boot loader executes, and displays the GRUB startup menu. It also allows choice of operating environment, and examination of system parameters.
4. When an operating system is chosen, it is loaded and control is passed.

GRUB supports both direct and chain-loading boot methods, LBA, ext2, and "a true command-based, pre-OS environment on x86 machines". It contains three interfaces: a selection menu, a configuration editor, and a command line console.

LILO

LILO, the older of the two boot loaders, is almost identical to GRUB in process, except that it does not contain a command line interface. Thus all changes must be made to its configuration and written to the MBR, and then the system restarted. An error in configuration can therefore leave a disk unable to be booted without use of a separate boot device (floppy disk etc) containing a program capable of fixing this. Additionally it does not understand file systems, instead locations of image files are stored within the MBR directly and the BIOS is used to access them directly.

Loadlin


Yet another way to boot Linux is from DOS or Windows 9x, where the Linux kernel completely replaces the running copy of this operating system. This can be useful in the case of hardware which needs to be switched on via software and for which such configuration programs are only available for DOS, whereas not for Linux, those being proprietary to the manufacturer and kept an industry secret. This tedious booting method is less necessary nowadays, as Linux has drivers for a multitude of hardware devices, but it used to be helpful in the past.
Another case is when the Linux is located on a storage device which is not available to the BIOS for booting: DOS or Windows can load the appropriate drivers to make up for the BIOS limitation, and boot Linux from there.