I work on ATmega8 and I need to optimize my code as well as I can, because I am over limit and I saw something surprising. I have a few 8bit integers declared as global variables and a few 8bit integer declered as local variables. And in some case when I change global variable to local I get extra bytes of program and in same cases when I change local variable to global I also get extra bytes. Can anyone explain me why in same cases global variables generate smaller size of program and sometimes local variables do that?
Electronic – AVR local and global variable program size
avrmemory
Related Solutions
Your moving average filter is rather inefficient; there's no need to shift the data, nor to calculate the total each time again. Instead of a FIFO implement it as a round robin:
*i points to the oldest sample in the list*
total -= samples[i]
total += new_sample
samples[i] = new_sample
i = (i+1) mod 8
average = total >> 3
So you replace the oldest sample by the new one in the list, after subtracting the oldest from the total and adding the new one. Then let the index point to the next position, which now is the oldest, to be replaced by the next sample.
FIR vs IIR
The moving average filter is a FIR (Finite Impulse Response) filter with all coefficients equal. A FIR filter is inherently stable, and its main disadvantage is that it needs more storage than the typical IIR filter. In the case of the moving average it's only the samples array, but for a generic FIR filter you need a second array with the coefficients.
Your other solution is an IIR (Infinite Impulse Response) filter; it uses a combination of the previous output and the new input to create the new output value. It's not necessarily stable (though yours is), but often needs less storage: yours only needs the previous out value. A bi-quad filter only needs a 4 sample storage (Direct Form 1, shown below) or just 2 samples in Direct Form 2.
Turns out this is a known bug with avr-gcc (PR50652), and was fixed in version 4.6.2. Unfortunately, Ubuntu repositories (which I was using) utilize version 4.5.3.
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Best Answer
Global variables are placed in the
.datasection of your program (if explicitly initialized) or in the.bsssection (if not initialized). The initial values for global variables are copied over from flash upon start up by the__do_copy_data()function (added by the linker scripts if needed) and the initialization of the variables in.bssto zero (this is mandated by the C standard) is done by__do_clear_bss(). Your program includes symbols which tell it the size of these two segments. Code must be included to handle these events. Note that sometimes these variables can be bound to a register.Local variables on the other hand reside either in registers or the stack. Being a modified Harvard Machine, AVR has a large number of registers, thus, if local variables can be put there, they will, as to avoid memory access. This means that for local variables, if residing in the stack, there must be code to
As a result, depending on the amount of code that needs to be added to the binary will change. For further details, you can see what is actually happening by disassembling your binary (
avr-objdumpis a good starting place) and by listing the symbols in the file (avr-nm).Cheers.