Global memory initialization

[Pret]

Hi there,

As we all know, global variables are being set to 0 right before main() occurs. But how can I disable this?

The problem that I have is inside a bootloader, where, if I want to use 1k bytes of RAM, the compiler adds 2k bytes of ROM just to clear it with "CLRF" for each byte. That's rather ridiculous for a bootloader. Any idea how can I disable this and use my own clear loop?

Thanks,
Cezane.

[Pret]

I have found in manual that "static" keyword forces the value of specified global variable to 0. I wasn't expecting that...

[temtronic]

curious...I just did a small program for the 18F46K22 and it's 397 words long.When I insert the #zero_ram directive, the program is only 13 words longer, no where near the 2K you've seen. I'd like to see your code(small program) that shows this 2K.

btw I've always been under the impression that unless _you_ , the programmer, specifically place values in RAM variables, that they can be anything, random in fact, based upon powerup time, static charge, whatever. I know there's nothing inside a PIC to force RAM to a known state upon powerup.


hth
jay

[Pret]

Hi temtronic, try using this:

[Ttelmah]

Static for a global, clearing to zero, is in the CCS manual:
"Variable is globally active and initialized to 0".
This is actually always the case in C. Look at:
<http://stackoverflow.com/questions/3373108/why-are-static-variables-auto-initialized-to-zero>
It is now part of the ANSI standard, which is why CCS does this...

In CCS, global variables, are not zeroed. Global variables are inherently 'static', without being declared as such (since the code always runs). Variables declared in main are also inherently 'static' without being declared as such. However they are only initialised, if they are declared as static, or if there is a value allocated to them.

ZERO_RAM, forces all of RAM to be zeroed (fairly obvious...).
Adds a tiny block of code to zero them. Size varies with the processor (PIC16's with bank switching, may take up to perhaps 30 bytes, while on processors with simpler memory maps, this is much smaller). Certainly not 2K. Declaring the variable as static, requires initialisation of a single defined area of memory. Not quite so efficient to do, but still not 2K.

So one solution if you want the variable to not be initialised, is to declare it externally (so global), not static. It is then not initialised. Use zero_ram, so it is initialised to zero.

Demo:

[RF_Developer]

[Pret]

Thanks guys.

As a personal programming tactic, I'm always initializing variables before using them, regardless of their location (global or local). In this specific case, in main() I'm calling mem_set(&se_data, 0, sizeof(se_data)), which has the obvious effect. Additionally, I wanted to limit the scope of my se_data to my .c file. And here we are...

Ttelmah, Here is a sample code:

[Ttelmah]

It's a 'late V4 compiler bug'.....

I don't see why you want to declare the variable as static.
No point at all. A global variable is always static in CCS, with the one difference that it is not initialised.
You gain nothing by declaring it as static.

[Pret]

What I was hoping with static is to limit the visibility of that variable to that particular module. I wanted to give access to the module (driver) only through its routines, and deny access to any internal variables. Some kind of encapsulation.

[jeremiah]

To be honest, I've never tried, but I'm not sure if CCS supports that feature of static or not. Have you tested to see if it actually provides that?

[Pret]

That's exactly my point. It doesn't. The main purpose of static keyword is not working.

[jeremiah]

My apologies then. I only saw the discussion on the initialization part. I missed the scoping discussion.
#module might be something to play with. I haven't used it, but it might help with what you want.

[Ttelmah]

CCS supports what you want, with 'EXCEPT' in the EXPORT options.

Best Wishes

[Pret]

Well yeah, but it's far from being elegant...