Reducing the physical memory for the allocator

[Prabhu nath <gprabhunath@xxxxxxxxx>]

Dear All,

         I am trying to experiment the following. 
   
* I have a 1 GB of RAM and running Linux 2.6.34 on a Intel Machine.
* I want memory allocator to get only 768 MB of RAM.
* The rest 256 MB of RAM, Kernel should see it has IO memory

Is there any option that I can pass to the kernel so that it only takes 768 MB of RAM.

Request you to help me in this regard. 

Thanks,
Prabhu