How to optimize or reduce RAM size in embedded system software?

Unlike speed optimisation, RAM optimisation might be something that requires "a little bit here, a little bit there" all through the code. On the other hand, there may turn out to be some "low hanging fruit".

Arrays and Lookup Tables

Arrays and look-up tables can be good "low-hanging fruit". If you can get a memory map from the linker, check that for large items in RAM.

Check for look-up tables that haven't used the const declaration properly, which puts them in RAM instead of ROM. Especially look out for look-up tables of pointers, which need the const on the correct side of the *, or may need two const declarations. E.g.:

const my_struct_t * param_lookup[] = {...};  // Table is in RAM!
my_struct_t * const param_lookup[] = {...};  // In ROM
const char * const strings[] = {...};    // Two const may be needed; also in ROM

Stack and heap

Perhaps your linker config reserves large amounts of RAM for heap and stack, larger than necessary for your application.

If you don't use heap, you can possibly eliminate that.

If you measure your stack usage and it's well under the allocation, you may be able to reduce the allocation. For ARM processors, there can be several stacks, for several of the operating modes, and you may find that the stacks allocated for the exception or interrupt operating modes are larger than needed.

Other

If you've checked for the easy savings, and still need more, you might need to go through your code and save "here a little, there a little". You can check things like:

Global vs local variables

Check for unnecessary use of static or global variables, where a local variable (on the stack) can be used instead. I've seen code that needed a small temporary array in a function, which was declared static, evidently because "it would take too much st