HI all
can anyone let me know how does the boot loader will come to know whether the disk already contains one more operating system installed?

Regards
Arun

The short answer is it doesn't.

The long answer is Grub can find it out.

The major difference betwee a MS system boot loader (no name ) and the Linux boot loaders (Grub or Lilo) is the former has no target to boot .

MS system MBR is like a blind person searching a OS for booting. It does so by examining each of the 4 primary partitions in the bootable hard disk to see if any one has the boot flag switched on and proceed to boot it when the first one is found. In all partitioning programs of MS and Linux systems when a primary partition boot flag is switched on the other one will be automatically switched off and so there can never be more than one active primary partition.

Linux's Grub and Lilo do not use the boot flag at all. The partition to be booted in "always" declared up front. In Grub it is by
in Lilo it is by
As Grub counts from 0 and so hdb3, being the 3rd partition of the 2nd hard disk will be known as (hd1,2) in Grub. This "declare the boot partition first" feature is the key to why Linux can boot from either primary or logical partition while a "standard" MS system can only cope with the former.

Ulike Grub which refers to all bootable partitions by the same root statement Lilo uses
for non-Linux partitions or any partition that it has to boot by chainloading (A Lilo can chainload a fellow Linux too)

Grub is slightly more powerful than Lilo in that it can operate without an OS attached to and acts as a mini OS in its own right. Grub has a rich set of commands, available at any Grub prompt (when booted without a OS) or at a Grub shell (after booted to a Linux) that can interrogate the hard disks, display any of the text files (like /boot/grub/menu.lst or /etc/lilo.conf) and restore the boot loader.

In a Grub prompt if any of the partitions, say (hd2,10) has a boot loader inside it can be booted up by Grub manually by these lines
Therefore Linux boot loader Grub can find out any bootable partition just by trial and error.

Think I may have given away a Howto to hack any PC.

HI
Thanks for the reply..
But my question was , while installing the Linux , once we start configuring/Installng boot loder , how will it recognises that already one OS is installed on the disk??

Arun

The bootloader will not detect a different operating system (that's not its job). Depending on what distro you intend to install, it might recognize other operating systems and automatically configure the bootloader for them. It depends on how user-friendly your distro strives to be.

Again, the job of the bootloader is only to do what it's told. Grub's `root' command is able to validate linux (and BSD?) kernels on some types of partitions. For other things (Windows, Plan9, etc.) Grub should be told to `chainload' that operating system (sometimes, it should be told first to lie to the operating system it's chainloading).

It is the "duty" of every Linux installer to check every partition and include it for booting if a boot loader is found inside!!!. MS system always has a boot loader at the boot sector of the begining of the partition and so you need to point a gun at the installer's head to stop from booting it.

The partition id known to the installer/boot loader and that is why the installer can labelled it as a Windows but could not determine if it is a Win2k or XP even its partition id is 7 indicating a NTFS partition.