Heap size

[Daniel Reisfeld]

Hi,

I'm using the latest everything for the nrf52840 with SES.

When I define a global variable that require substantial memory I get the linkage errors:

 .bss is too large to fit in RAM memory segment

   .heap is too large to fit in RAM memory segment

   section .heap overlaps absolute placed section .stack

   .bss is too large to fit in RAM memory segment

   .heap is too large to fit in RAM memory segment

   section .heap overlaps absolute placed section .stack

I don't use any malloc (all memory is preallocated or on the stack).

My questions:

1. How can I determine the various RAM memory allocation segments?

2. Same as (1) but for the Flash.

3. Does Softdevice use heap memory?

Thanks,

Daniel

[tlongeri]

I haven't used the latest SDK, but I think you should look at your linker script (.ld) file. At the beginning it should define the RAM and FLASH memory regions which determine the amount of memory available to the applications (they are offset from the flash/RAM start addresses to leave room for the SoftDevice). It should also have the line '#include "nrf_common.ld"' which refers to <sdk dir>/modules/nrfx/mdk/nrf_common.ld. That file determines how sections like .bss and .data are allocated, and may clear things up if you know or can learn how to read linker scripts.

Daniel Reisfeld said:

3. Does Softdevice use heap memory?

Nope (at least not the same heap that your application uses, although I nevertheless don't expect that it uses a heap), if you are not using malloc then you should be able to safely eliminate the heap by adding -D__HEAP_SIZE=0 to your compiler flags (I've done that without any trouble, albeit using SDK v12.3.0). This old question talks a bit about that.

Removing the heap won't solve your problem if .bss alone is too large, though. It seems like the variable you're defining really is too large to fit in memory (you're not defining global variables in header files, are you?).

[Daniel Reisfeld]

Tlongeri,

I'm using global variables (of course not in header files) in order to have full control of memory, and I would like to use the full capacity of the RAM and Flash (which I cannot do if using the out of the box definitions). It is vital to my application and I would hate adding external memory where there is sufficient memory in the soc.

It seems SES is utilizing "thumb_ct0.s" rather than the ld files. I would like avoiding messing assembly files and there must be a clean way to do so. 

Would you kindly check how to do so using SES and double check the dynamic memory usage of the Softdevice? Please also refer to the Flash side.

Thanks,

Daniel

[Daniel Reisfeld]

A simplified example:

static int x[100000];
int y = x[0];

[Sigurd]

Daniel Reisfeld said:

A simplified example:

static int x[100000];
int y = x[0];

That won't work. Each int is 4 bytes, so that would consume 400KB RAM!

[Daniel Reisfeld]

Thanks! you are right of course :) I did manage to use all the RAM memory.

Would you kindly explain how can I define variables in the flash?

Daniel 

[Sigurd]

If you want to store it in flash, use #define or const keyword.

#define MY_VAR 0x10
const uint8_t my_data[5] = {0x70, 0xDC, 0x9B, 0xDD ,0xBB };

[Daniel Reisfeld]

I managed to do doing something like 

struct T {
char t = 0;
};
static const T a[500000];
char* b = (char*)&a;

Sigurd, thanks so much for all your help,

Daniel