Volatile variable

volatile is a type qualifier not a storage class specifier, so it does not determine storage location at all; it affects the definition of a variable's type, not its storage.

It simply forces the compiler to explicitly read a variable whose type is volatile from the variable's storage location (wherever that may be) rather than assuming that some previously read value in a register for example remains valid.

In C volatile just tells the compiler - "You don't have enough knowledge to assume the value of this variable hasn't changed". There is no "section" eg BSS, CSS for it.

Consider it a flag to the compiler to prevent certain types of optimisations. Its very handy in embedded programming, where memory at a certain address may "change" due to a hardware device input.

Here's a good explanation: http://www.embedded.com/columns/programmingpointers/174300478?_requestid=137658

The volatility of a variable does not change the place in which a variable is stored. What it changes is the semantics around how it is accessed with respect to reads and writes.

I do not believe the C standard says anything about the implementation of volatile. But typically, volatile guarantees release semantics for write operations on a variable and aquire semantics on read operations of a variable. This will not be true for every implementation though and you should read up on what your specific compiler guarantees