CODEWITHSUNDEEP
cstackembeddedmemory-address
A Shaved Wookie
A Shaved Wookie

Reputation: 3

Standard C: Storing arrays in off-chip RAM

I would like to know if I can choose the storage location of arrays in c. There are a couple of questions already on here with some helpful info, but I'm looking for some extra info.

I have an embedded system with a soft-core ARM cortex implemented on an FPGA.

Upon start-up code is loaded from memory and executed by the processor. My code is in assembley and contains some c functions. One particular function is a uART interrupt which I have included below

void UART_ISR()
{
int count, n=1000, t1=0, t2=1, display=0, y, z;

int x[1000];    //storage array for first 1000 terms of Fibonacci series

x[1] = t1;
x[2] = t2;

printf("\n\nFibonacci Series: \n\n %d \n %d \n ", t1, t2); 

count=2;    /* count=2 because first two terms are already displayed. */

while (count<n)  
{
  display=t1+t2;
  t1=t2;
  t2=display;

  x[count] = t2;
  ++count;

  printf(" %d \n",display);
}

printf("\n\n Finished. Sequence written to memory. Reading sequence from memory.....:\n\n");

for (z=0; z<10000; z++){} // Delay 


for (y=0; y<1000; y++) {    //Read variables from memory
    printf("%d \n",x[y]);
}
}

So basically the first 1000 values of the Fibonacci series are printed and stored in array X and then values from the array are printed to the screen again after a short delay.

Please correct me if I'm wrong but the values in the array X are stored on the stack as they are computed in the for loop and retrieved from the stack when the array is read from memory.

Here is he memory map of the system

Is there a function in c that allows me to store the array X in the off-chip ram for later retrieval?

Please let me know if you need any more info

Thanks very much for helping

--W

Upvotes: 0

Views: 143

Answers (1)

unwind
unwind

Reputation: 400029

No, not "in C" as in "specified by the language".

The C language doesn't care about where things are stored, it specifies nothing about the existance of RAM at particular addresses.

But, actual implementations in the form of compilers, assemblers and linkers, often care a great deal about this.

With gcc for instance, you can use the section variable attribute to force a variable into a particular section.

You can then control the linker to map that section to a particular memory area.

UPDATE:

The other way to do this is manually, by not letting the compiler in on the secret and doing it yourself.

Something like:

int *external_array = (int *) 0x00140be0;
memcpy(external_array, x, sizeof x);

will copy the required number of bytes to the external memory. You can then read it back by swapping the two first arguments in the memcpy() call.

Note that this is way more manual, low-level and fragile, compared to letting the compiler/linker dynamic duo Just Make it Work for you.

Also, it seems very unlikely that you want to do all of that work from an ISR.

Upvotes: 1

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