On 18/04/2012 03:36, William Swanson wrote: > On Tue, Apr 17, 2012 at 7:03 AM, David Brown<da...@westcontrol.com> wrote: >> You have to be particularly careful about read-modify-write codings. >> The msp430 can do some of these actions atomically, but it is possible >> that the compiler will split the actions up. For example, if you write >> "a += 1; b = a" then then compiler might read "a" into a register, do >> the addition, then save the value to "a" as three separate actions, so >> that it can re-use the saved value for "b". > > If you declare "a" as "volatile," that will tie the compiler's hands > and force the reads and writes to occur exactly as specified.
The details of whether something like "a += 1" must be a read, modify, write, or a single RMW instruction, are not specified even if "a" is volatile. Of course, in this example, if "a" is volatile then "b = a" would force a re-read of "a", negating any benefits of keeping it in a register. > Any > variable shared between ISR-land and normal code should be "volatile" > to avoid surprises like this. > That's okay as a beginner's rule, as you err on the side of caution, but there are lots of situations when non-volatile variables are more efficient. You just have to be careful about where and when you force volatile accesses on the non-volatile variables, or use memory barriers to ensure everything is done in the required orders. ------------------------------------------------------------------------------ Better than sec? Nothing is better than sec when it comes to monitoring Big Data applications. Try Boundary one-second resolution app monitoring today. Free. http://p.sf.net/sfu/Boundary-dev2dev _______________________________________________ Mspgcc-users mailing list Mspgcc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mspgcc-users
