3.9.2 Use the Volatile Keyword for Necessary Memory Accesses

The compiler analyzes data flow to avoid memory accesses whenever possible. If you have code that depends on memory accesses exactly as written in the C/C++ code, you must use the volatile keyword to identify these accesses. The compiler does not optimize out any references to volatile variables.

In the following example, the loop waits for a location to be read as 0xFF:

unsigned int *ctrl; while (*ctrl !=0xFF);

In this example, *ctrl is a loop-invariant expression, so the loop is optimized down to a single memory read. To correct this, declare ctrl as:

volatile unsigned int *ctrl;