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

Question

i am working on embedded software projects in automotive domain. In one of my projects, the application software consumes almost 99% of RAM memory. Actual RAM size available is 12KB. we use TMS470R1B1 Titan F05 microcontroller. I have done some optimisation like finding unused messages in software and deleting them but its still not worth reducing RAM. could you please suggest some good ways to reduce the RAM by some software optimisation?

Answer 1

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 stack space". If this happens enough times in the code, it would actually save total memory usage overall to make such variables local again. It might require an increase in the stack size, but will save more memory on reduced global/static variables. (As a side benefit, the functions are more likely to be re-entrant, thread-safe.)

Smaller variables

Variables that can be smaller, e.g. int16_t (short) or int8_t (char) instead of int32_t (int).

Enum variable size

enum variable size may be bigger than necessary. I can't remember what ARM compilers typically do, but some compilers I've used in the past by default made enum variables 2 bytes even though the enum definition really only required 1 byte to store its range. Check compiler settings.

Algorithm implementation

Rework your algorithms. Some algorithms have have a range of possible implementations with a speed/memory trade-off. E.g. AES encryption can use an on-the-fly key calculation which means you don't have to have the entire expanded key in memory. That saves memory, but it's slower.

Answer 2

Deleting unused string literals won't have any effect on RAM usage because they aren't stored in RAM but in ROM. The same goes for code.

What you need to do is cut back on actual variables and possibly the size of your stack/stacks. I'd look for arrays that can be resized and unused varaibles. Also, it's best to avoid dynamic allocation because of the danger of memory fragmentation.

Aside from that, you'll want to make sure that constant data such as lookup tables are stored in ROM. This can usually be achieved with the const keyword.