How can I provision a persistent array of int in flash memory and access it as an array of int from mbed C++?

Mbed OS
2

Constant’s value cannot be changed so it’s rational to store it in the FLASH memory rather than in the SRAM. The modifier const tells the compiler that a name represents a constant and it shall be stored in the FLASH memory (in contrary to for example Arduino). As far as I know we cannot change that behaviour of the ARM compiler. Then we can access constants in the code without any additional provisions. We don’t have to specify that they should be read from FLASH (in contrary to for example Arduino).

If your question was how to create a variable (especially an array) in the FLASH memory and access it from the code then one way is to utilize Mbed’s Flash IAP API. But be aware of increased FLASH memory wear out and quite slow write speed (in comparison to using SRAM)!

An example is below:

#include "mbed.h"

FlashIAP flash;

int main() {
    printf("Starting...\r\n");

    flash.init();

    const uint32_t  flash_start = flash.get_flash_start();
    const uint32_t  flash_size = flash.get_flash_size();
    const uint32_t  flash_end = flash_start + flash_size;
    const uint32_t  page_size = flash.get_page_size();                  // in bytes
    uint32_t        sector_size = flash.get_sector_size(flash_end - 1); // in bytes
    uint32_t        addr = flash_end - sector_size;                     // address of first byte in the last sector

    printf("flash_start = 0x%.8x\r\n", flash_start);
    printf("flash_size = 0x%.8x\r\n", flash_size);
    printf("flash_end = 0x%.8x\r\n", flash_end);
    printf("page_size = 0x%.8x\r\n", page_size);
    printf("sector_size = 0x%.8x\r\n", sector_size);
    printf("addr = 0x%.8x\r\n", addr);
    printf("----------------------\r\n");

    // Create an array in SRAM (later on to be stored in the first page of last sector in FLASH)
    uint32_t        data_len = page_size/sizeof(uint32_t);      // calculate available space
    uint32_t*       data = new uint32_t[data_len] {1, 2, 3};    // create an array in SRAM and initialize the first three elements

    data[11] = 572; // set element 11 to 572

    // Print the first 15 elements
    for (int i = 0; i < 15; i++) {
        printf("data[%d] = %d\r\n", i, data[i]);
    }

    // Store the array 'data' in FLASH
    flash.erase(addr, sector_size);         // erase needs sector_size (not page size)
    flash.program(data, addr, page_size);   // write needs page_size


    printf("----------------------\r\n");

    // Clear the first 15 elements in SRAM (to see whether they are going to be updated with values stored in FLASH)
    for (int i = 0; i < 15; i++) {
        data[i] = 0;
        printf("data[%d] = %d\r\n", i, data[i]);
    }

    printf("----------------------\r\n");

    // Update the SRAM array with values stored in FLASH
    flash.read(data, addr, page_size);      // read needs page_size

    // Print the first 15 elements
    for (int i = 0; i < 15; i++) {
        printf("data[%d] = %d\r\n", i, data[i]);
    }

    delete[] data;
    flash.deinit();

    printf("----------------------\r\n");
    printf("Done.\r\n");
}