William Chops Westfield wrote:

> On Jun 26, 2004, at 5:11 PM, Gerhard Fiedler wrote:
>
> >> Before these changes C18/PIC18F encouraged the use of poor programming
> >> practices because using global variables in access ram was so much
> >> more
> >> efficient than passing arguments and using local variables.
> >
> > Isn't that the case with pretty much all 8bit micros?
>
> Well, no.  At the recent AVR seminar, they specifically reccomended
> against global variables.   Seems that the compiler tries to put global
> variables in RAM, while local variables get allocated in registers.
> It also seems that the avr has enough pointer registers and addressing
> mores that it is more efficient to access fields of a structure than
> the equivilent global variables, since accessing global ram can take a
> 4 byte instruction (to hold the whole 16+ bit ram address), while
> offsets from a pointer register are two-byte instructions.
>
> While interesting, these suggestions made me somewhat unimpressed with
> the optimization capabilities of whatever compiler they were using.
> Any compiler ought to be smart enough to put all the global variables
> into registers (if they fit.)  (to be fair, it wasn't clear whether
> they meant this mainly for the cases where not all the variables fit in
> registers; using local variables makes it possibles (or at least a lot
> easier) to determine when you can reuse a location because it's not
> used anymore...)  Any good compiler ought  to be able to reference
> global memory as offsets to a global heap pointer if the improvement in
> memory utilization is that significant...  (although I don't know how
> hard that would be, really.)
>
> BillW

Exactly! And with regard to how difficult it would be, answer: not very hard.

Regards
Sergio Masci

http://www.xcprod.com

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