Volatile Keyword

Hi sir,
what is volatile keyword and how it's works?
volatile with const qualifier
i was read some read some articles but i didn't fully understand that concept.can you explain with some examples

 

A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change:

1. Memory-mapped peripheral registers
2. Global variables modified by an interrupt service routine
3. Global variables accessed by multiple tasks within a multi-threaded application

We'll talk about each of these cases in the sections that follow.

 

it is keyword which does not work if it having any moderations..

 

Volatile is a qualifier that is applied to a variable when it is declared. It tells the compiler that the value of the variable may change at any time-without any action being taken by the code the compiler finds nearby. Syntax:
To declare a variable volatile, include the keyword volatile before or after the data type in the variable definition.
volatile int foo;
int volatile foo;