What is better - a Dimensioned variable or a local variable?

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

Hello all,

Question - what is better, a Dimensioned variable (Global) or a Local variable? There is probably a lot of opinions on this and it likely depends on the circumstances. So to better define the question allow me to restate.

When a sub or function is called a LOT what is the time difference between using a variable that is already dimensioned, and one whose memory space must have an allocation created? Is it a waste of time to do a Local since in my cases these subs/functions are used virtually continuously? Also in my specific case, there are a lot of events firing which may add to the overhead - not really sure about all this - not my strongest point!

Opinions and feedback are appreciated!

Thanks,
Tim

(BASCOM-AVR version : 2.0.7.8 )

snipsnip · Joined: 10 Feb 2014 Posts: 74 Location: Melbourne

I'm still learning all this stuff - but anyway here's my 2 cents...

It all depends Laughing

If you're low on memory locals will help.
globals are faster, but a nightmare to keep track of.

what I've just started doing is to use locals everywhere when developing the code. Make every function / sub stand alone with no side affects

once all the functions are done, work on the structure - once you get to this bit you'll know what needs to be converted to global, what subs / functions should be unpacked / put inline (gosub) etc (save the standalone functions so they can be used again elsewhere!)

But yeah, I don't think the time required to pass values to a sub is a big deal, shitty code / structure will be make the speed issue suck heaps more.

cheers

EDC · Joined: 26 Mar 2014 Posts: 1133

I was curious and I made a short test in the simulator. Create a local variable costing 15 cycles even if you do not use it. I forgot to remove the Local and the result was the same for the declared and local.

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

Thanks for the responses.

Doesn't that all change when you add in continuous, or seemingly so, interrupts that occur while performing some action. So the uC has to save its current data, pop over to the calling interupt, then go back and restore the previous data before resuming? It might do the same with a global but not really sure.

Tim

Duval JP · Joined: 22 Jun 2004 Posts: 1197 Location: France

good question Tim
but sometime another question must be asked: is that I have enough ram to use all these global variables?

compromise is the art of micro-controller

jp Wink