我的BSS部分的尺寸是否重要？

Question

I have written a small application for my heating system in my apartment, at the end of compilation I am becoming the following output :

 'Invoking: Cross ARM GNU Print Size'
arm-none-eabi-size --format=berkeley "Phaethon_F446.elf"
text       data     bss     dec     hex filename
142935      924   77140  220999   35f47 Phaethon_F446.elf

I am using the STM32F446RE with 128 KB RAM, if I am correct, the bss section needs 75KB of RAM; that means more than 50% of the RAM is gone.

• Am I wrong?

• Kills bss my RAM or this value is represented wrong from the compiler?

• What should I change in my software?

Answer 1

Yes, you are right, according to the report, the ~50% of the RAM is being taken by .bss. To be sure, you could generate the map file to find out how (or where) the variables are being allocated (-Wl,-Map=output.map).

Regarding a solution, you have to optimise the software (are all the variables needed? Are you using compiler optimisations as -O3/... options ? Can you re-use variables?) I have no idea about your code/algorithms/etc, but it looks like a lot of bytes just for a heating system.

The linker script will show you the borders (that is, the current size) of the .bss section (and how your memory is splitted), just in case you want to modify it (you can choose how the memory is distributed).