can someone please tell me what is the role or function of the supervisor engine available in some cisco switches?
I tried to find this answer, but all documents and books jump directly to redendency !!
In modular devices, it's the module that controls the device. It's usually replacable so you can replace a dead one, perform an upgrade, and/or install two for redundancy. The chassis itself is traditionally "dumb", so that it can last through one or more generations of supervisor technology.
in Catalyst 6500, does the Cisco IOS be installed only on Supervisor ?
Yes, the 6500 supervisor module houses the IOS (or CatOS). The line cards are dumb devices that don't run IOS.
You'll only copy the code to the supervisor. Most cards are completely dumb, but some do receive a microcopy of the code from the supervisor.