Technical Note 	: SLE0010

Author	: Scott Evans

Created/Modified	: 29/9/99

Description	: The volatile keyword

This technical note describes the volatile keyword used in C and C++. If
the volatile keyword is used when declaring a variable it warns the
compiler that an external process can change the variable. These types
of variables are not subject to compiler optimisations. The compiler
will always produce code to write a volatile variable straight away and
read it each time it is referenced.

For example say we had some hardware that set a register while it was
busy and cleared the register when it had finished its processing. Now
you code some software that needs to wait for the hardware to finish
processing. It might look like this.

// Address of hardware busy register

unsigned long *const busy=(unsigned long *)0x8001000;

while(*busy)

{

	// Wait for hardware to clear register

}

Now chances are this will work, but in certain circumstances when the
compiler has been optimising the value of *busy might not be updated.
This could cause the software to get stuck in an infinite loop.

The way to be sure that this never happens is to declare the busy
variable as volatile.

// Address of hardware busy register

volatile unsigned long *const busy=(unsigned long *)0x8001000;

while(*busy)

{

	// Wait for hardware to clear register

}

The above bit of code will always produce the desired result since the
compiler is forced to always read the value of *busy when it is
referenced.