(K&R, §A4.4) When you declare a variable to be volatile the C compiler takes the following precautions:

• It does not store the variable in a register.
• It computes the variable's address every time a piece of code references the variable.

void main(void)
{
    int i[100];
    volatile int a, b; /* a and b are not cached in registers. */

    a = 5;
    b = 20;

    i[a + b] = 15;
    i[a + b] = 30;
}

The compiler does not place the value of a, b, or a+b in registers. Also, the compiler recalculates a+b in both assignment statements.