What is the unique Feature of ARM, why it is so popular when compared to other 32-Bit Microcontrollers
Because people buy so many.
Homework again? First of all, your question implies that ARM is a microcontroller. This is wrong. ARM is a company that designs and licenses CPU cores. Why ARM-based microcontrollers are so popular? I suspect that this is because ARM have a very efficient marketing department. I'm serious. There could be other reasons too.
because it's British - obviously!!
;-)
"your question implies that ARM is a microcontroller. This is wrong."
Very true.
ARM cores are used in many things that are not microcontrollers!
In fact, ARM-based microcontrollers are a relatively recent application of ARM cores.
I don't know how microcontrollers relate to total ARM sales - maybe you could find out on the ARM site...?
What makes me like it is that so many companies makes ARM based micros. If one discontinues you are not totally screwed.
I have no doubt that many that have been screwed by a one supplier micro being discontinued shares this sentiment.
This, in a way, is what makes the ARM the big brother of the '51.
Erik