Boney:
Thanks for your replays guys.
So it is impossible to control the SRAM memory =(. Guys could you please help me finding good resources for AVR programming. I really want to edit the arduino bootloader (Optiboot) to read initial (Power On) SRAM random values for finding any pattern they are following. I know that it is completely random but i need to know how the values depends on temperature and voltage.
I would suggest that the best method to achieve your goal, is not to write protect the memory (which is impossible), but rather carefully construct your code to avoid using those areas. That means that you shouldn't use any libraries until you understand how they use memory. Personally, I would skip the Arduino IDE environment for this and deal with the AVR chip yourself directly. That way you have complete control over how and what areas of memory you use. While you can do this in C/C++ you need to write this code with an understanding of the assembly language produced by your code, since that will dictate how memory is used.
That and a thorough understanding of the data sheet for the particular AVR your intending to use is all you need to accomplish your purpose.