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how to develop a mobile phone os?
I'm afraid that the people who can answer your question have probably already developed iOS, Android, Windows 7 Mobile, WebOS, etc, and may not be inclined to give away their proprietory secrets. You're going to have to find some way to setup and access all the various registers and memory loctations within a mobile device. You need to learn what specific chips, ASICS, Processors, etc are used for in a phone and be able to program them in machine code. Would you not be better getting the SDK for either iOS or Android and start by developing applications?
It's a little question with a very
complicated answer! Good luck in your quest!