In c how to declare the variable of 89v51rd2 on chip RAM from 000 to 2FF ?
Standard 'C' provides no way to do this, so it relies upon a specific language extension provided by Keil's C51 implementation.
All of the language extensions provided by Keil's C51 implementation are described in the Manual...
Your Keil installation also includes a whole folder full of Examples - also available here: http://www.keil.com/download/list/c51.htm
In particular, look at the one titled, "8051 Memory Write Example Program"...
As with any new product, it pays to invest time in familiarising yourself with the support available:
http://www.keil.com/support/
do not forget to set the appropiate SFR in (a local copy of) startup.a51. doing it in main will not do
Erik