What is Volatile variable ,can a variable be CONST VOLATILE

The reason to use volatile is to ensure that the compiler generates code to re-load a data item each time it is referenced in your program. Without volatile, the compiler may generate code that merely re-uses the value it already loaded into a register.
Volatile advises the compiler that the data may be modified in a manner that may not be determinable by the compiler. This could be, for example, when a pointer is mapped to a device's hardware registers. The device may independently change the values unbeknownst to the compiler.
A variable should be declared volatile whenever its value can be changed by something beyond the control of the program in which it appears, such as a concurrently executing thread.

Use of volatile
- An object that is a memory-mapped I/O port - An object variable that is shared between multiple concurrent processes - An object that is modified by an interrupt service routine - An automatic object declared in a function that calls setjmp and whose value is-changed between the call to setjmp and a corresponding call to longjmp

Can a variable be both const and volatile?
Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code