Where is the OS stored on disk and how does the boot loader locate it?

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http://stackoverflow.com – This might be OS specific I am not sure (I am working on Linux). When the BIOS brings the boot loader in the memory and the CPU starts executing it, how does it locate the OS? Is it stored in a special disk partition/well-known sector etc? If it is stored on the file system, then the boot loader needs to interact with the file system code which is stored in the OS. How is this chicken-egg problem resolved? PS - I am just trying to fill the gaps in my knowledge here. (HowTos)