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how can add sim900(sim900.h) header to keil. it is possible to add
How to download it has nothing to do with Keil!
And, since nobody knows what particular "sim900 header file" you're talking about, nobody can tell you anything specific about it!
As to using it a Keil project, you've already asked that - and had the answer!
Perhaps what you really need to do is to go back to basics and start to get a proper understanding of the tools - by studying the documentation and working through the examples they provide...?
In your Keil installation, there is a Getting Started Guide - search for "GS51"
uVision IDE manual: http://www.keil.com/support/man/docs/uv4/
C51 Compiler manual: http://www.keil.com/support/man/docs/c51/
Worked examples here: http://www.keil.com/support/man/docs/uv4/uv4_examples.htm
There are books here: http://www.keil.com/books/8051books.asp