where is volatile variables stored in memory layout ?

The storage of a variable is not decided based on if it is volatile or not. It can be anywhere as is the case with normal variable. So the variable may be on stack,heap or in the data section of executable, depending on how it gets defined. The volatile qualifier just tells the compiler that this variable may change in ways that are not apparent to you, it can be changed at any point of time, for example is hardware registers (variables mapped to h/w registers). So compiler disables any optimizations such as caching on this variable.

Volatile is type qualifier not a storage class specifier.