About volatile in C language

The modifier volatile tells the compiler that a variable's value may be changed in ways not explicitly
specified by the program. For example, a global variable's address may be passed to the operating
system's clock routine and used to hold the system time. In this situation, the contents of the variable
are altered without any explicit assignment statements in the program. This is important because
most C compilers automatically optimize certain expressions by assuming that a variable's content is
unchanging if it does not occur on the left side of an assignment statement; thus, it might not be
reexamined each time it is referenced. Also, some compilers change the order of evaluation of an
expression during the compilation process. The volatile modifier prevents these changes.
You can use const and volatile together. For example, if 0x30 is assumed to be the value of a port
that is changed by external conditions only, the following declaration would prevent any possibility
of accidental side effects:
const volatile char *port = (const volatile char *) 0x30;