Odd Memory Usage

[object01]

I'm very new to embedded software, and I'm hoping someone can help me understand why I'm observing this:

I'm working with a PIC16F88, and up until recently I've been getting lots of "Not enough RAM for all variables" errors. As a pure shot-in-the-dark, I commented out a 'char buffer[59]' declaration at global scope, and commented out all references to it. All of a sudden, my code compiles and I have 59% RAM free! Put the buffer back, and I'm back to "not enough RAM." (PCM 3.189) How is this possible?

[rwyoung]

There is not an exact 1:1 relationship between the number of bytes you allocate in your code and the number of bytes the compiler uses. Your array of 59 characters is about 16% of total available RAM. If the functions where you were using the array require accessor variables or scratchpad then the size goes up. Also in some cases the compiler use the RETLW method of accessing RAM arrays and that can take more space too.

If your functions use argument lists there will be scratchpad RAM required for passing arguments.

If the compiler can't resolve the scope of a variable it may not reuse that RAM eventhough it could have.

Another thing that seems to eat up RAM is the way the compiler assigns things to banks. It never quite gets things fit perfectly so you loose a few bytes here and there. You can override the automatic assignment of variables by doing it yourself.

Lots of interactions cause the memory used or available to be off.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[PCM programmer]

[prwatCCS]

I have found it sometime helps to place large arrays at the beginning of the next bank of ram, rather than let the compiler try and fit it (and fail).
_________________
Peter Willis
Development Director
Howard Eaton Lighting Ltd UK