Comment by deng

"volatile", according to the standard, simply is: "An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine."

Or simpler: don't assume anything what you think you might know about this object, just do as you're told.

And yes, that for instance prohibits putting a value from a memory address into a register for further use, which would be a simple case of data optimization. Instead, a fresh retrieval from memory must be done on each access.

However, if your system has caching or an MMU is outside of the spec. The compiler does not care. If you tell the compiler to give you the byte at address 0x1000, it will do so. 'volatile' just forbids the compiler to deduce the value from already available knowledge. If a hardware cache or MMU messes with that, that's your problem, not the compiler's.