variable memory allocation

The mydata declarations would be placed in the .bss section of RAM.
The data2 declarations would be allocated on the stack, which is not checked at compile time. It's purely on the you make sure that you don't allocate too much on the stack. Once you overflow the stack unpredictable things may happen.

this sound fair to me. we can keep track of the stack size.

but if i have stack size of 160 then i can not create array size of 161? will stack expand if i create one?

i tried this a little although i don't know if i tested it correctly but i was able to create a 256 byte local array when having stack size of only 160 byte .
does anyone have some theoretical details about this ? some compiler manual or something like that? 

i will really very kind of help me little. 

Luis, the stack can grow beyond ram size. Sure, nothing can be stored there then, but the CPU doesn't are as long as it doesn't try to read something back (e.g. a return address).
It may even happen that you define a large local array and the stack pointer now points into peripheral register area, where a return address can be stored and fetched from (of course with disastrous effects on the hardware configuration of the affected module).
And if you have a function that calls itself (recursion), the memory (ram or whatever) will be quickly written with the return address form top of ram down until finally the return address is written to a register that is password protected and triggers a reset.

"The stack grows as the program flows." :)

To clarify what's going on: the compiler assumes a limitless stack when generating code. It doesn't know how much of the available ram is already required on the stack by other modules that are later linked, or by recursions. It doesn't even know how much ram the target may have. Only the linker knows this, but the linker doesn't know how much stack the code might need during program execution. It can only ensure that at least as much as configured will be available.