topic Re: How RAM and Flash actually work? in LPC Microcontrollers

How RAM and Flash actually work?

dimitrissideris — Wed, 22 Mar 2017 11:09:10 GMT

I am trying to understand the procedure memory handling in LPC mcus.

I am working with an LPCxpresso 4367.

I wanted to store a buffer in flash I tested this code:

#include "board.h"
#define FLASHA_BASE_ADDR 0x1A000000
static int *fb = (int *)FLASHA_BASE_ADDR;
int main(void)
{
   SystemCoreClockUpdate();
   Board_Init();
   int arsize = 10;
   for (int i = 0; i < arsize; i++)
       {
        fb[i] = 0xA;
      }
   while (1) {
       __WFI();
   }
}

And after adding -print-memory-usage linker option i get this result:

MFlashA512: 7984 B 512 KB 1.52%
RamLoc32: 352 B 32 KB 1.07%

I went through normally without declaring specific address to fb buffer and i tested this code:

#include "board.h"

#define FLASHA_BASE_ADDR 0x1A000000

int main(void)
{
   SystemCoreClockUpdate();
   Board_Init();
   int arsize = 10;
    int fb[arsize];

   for (int i = 0; i < arsize; i++)
       {
        fb[i] = 0xA;
      }

   while (1) {
       __WFI();
   }
}

MFlashA512: 8044 B 512 KB 1.53%
RamLoc32: 348 B 32 KB 1.06%

Can some explain what is going on?

When i put fb in flash, flash occupied space is less.

When i put fb in ram, ram occupied space is less.

Re: How RAM and Flash actually work?

converse — Wed, 22 Mar 2017 12:31:58 GMT

Your 2nd example place fb on the stack, so it is 'allocated ' at run time .

in fact your 1st example doesn't place fb in flash either... suggest you look up the meaning of static in C

Re: How RAM and Flash actually work?

dimitrissideris — Wed, 22 Mar 2017 12:45:51 GMT

Did run again the examples without static. Same results

example1:

MFlashA512: 7984 B 512 KB 1.52%
RamLoc32: 352 B 32 KB 1.07%

example2:

MFlashA512: 8044 B 512 KB 1.53%
RamLoc32: 348 B 32 KB 1.06%

As far as example2 is concerned, ok it is allocated on run time. But why flashA has more space occupied?

For example1 the question is if i can store this way a variable or a buffer in Flash memory

Re: How RAM and Flash actually work?

converse — Wed, 22 Mar 2017 15:53:20 GMT

In C "static" defines the scope of a variable - it says nothing about where it is stored. When applied to a variable defined in a module, the variable is only visible within that module. Look up "C storage classes" for more information.

Flash is like ROM - you cannot write to it. So you cannot place a buffer in flash and expect it to work. Flash is for storing stuff that doesn't (or very rarely) changes. Flash can be *programmed* in blocks (sectors, or pages) by using a special algorithm, depending on whose flash is used - in NXP this is achieved by using the IAP calls as described in the User Manual. For more info, see Flash memory - Wikipedia.

Re: How RAM and Flash actually work?

Carlos_Mendoza — Thu, 23 Mar 2017 22:06:51 GMT

Hi Dimitris,

In addition to Con Verse information, I think that the following post could also help you understand how data is stored in Flash and RAM:

text, data and bss: Code and Data Size Explained | MCU on Eclipse

Hope it helps!

Best Regards,
Carlos Mendoza
Technical Support Engineer