AVR-Chat

Don Kinzer wrote:

>... if there are any calls to other functions in
> the code region of interest the compiler must
> assume that any particular global variable
> has been modified ...

Good point.

> ... unless it can determine conclusively otherwise.

Which, I think, it generally can't for the C language.

What I was getting at, as I'm sure you realized, is the common case of busy-waiting on a change in a global variable.  Without the volatile modifier, the compiler will see no reason to repeatedly read the value as, while executing the busy-wait loop, it is impossible for the code that it is compiling to modify the value, either by a direct assignment or by an external function call.  The volatile qualifier "tells" the compiler that "something else" might modify it while the loop is running.

In other news, I am getting so fed up with this issue cropping up several times a year on various discussion groups that I am trying to prepare a definitive statement on when you should use the volatile qualifier and exactly what effect it has.  I am hoping that, if I can get the appropriate peer reviews, people will reference this when the subject first arises.  This might avoid the posting of yet more miss-information (which, as you point out, I am contributing to myself).

Graham Davies
ECROS Technology
www.ecrostech.com