如何在Arduino中获取全局变量的闪存地址

Question

I have a static global variable in my Arduino (ATmega328P for example) which is, as any other static non-PROGMEM storage, automatically loaded to SRAM after startup. I set it to a default value which is replaced by the stored value from EEPROM in setup() . I would like to offer the ability to reset the variable to its original default value without defining the value again in my code.
Is it possible to get the flash address of a static global variable in Arduino to load its original data from flash to SRAM via pgm_read_byte() for example? If so, how?

PS The static global variable in question is a rather large structure. Re-defining it is out of question.

Answer 1

Constant data can be made addressable by declaring it as PROGMEM . Since AVR program memory is organized as 16-bit words in a separate address space, special functions have to be used for reading.

#include <assert.h>
#include <string.h>                        // memcpy
#include <avr/pgmspace.h>                  // AVR PROGMEM, memcpy_P

#define SIZE 1000
struct large {                             // arbitrary large data structure
  unsigned char data[SIZE];
};

struct large l;                            // global variable (in SRAM)

const struct large l_default PROGMEM = {   // default value (in PROGMEM)
  { 0xaa, 0xab, 0x0ac, 0xad, 0xae, 0xaf }
};

void setup(void)                           // load values from EEPROM
{
  memset(&l, 1, SIZE);                     // simulate reading from real EEPROM
}

int main(void)
{
  setup();                                 // initialize from EEPROM
  assert(l.data[0] == 1);

  memcpy_P(l.data, &l_default, SIZE);      // reset to defaults (from PROGMEM)
  assert(l.data[0] == 0xaa);

  return 0;
}

Confirming that sizes from the above example are as expected: 1000 additional bytes in Program for the default data, 1000 bytes total in Data for the global variable.

$ avr-size -C -x main.bin
AVR Memory Usage
----------------
Device: Unknown

Program:    1252 bytes
(.text + .data + .bootloader)

Data:       1000 bytes
(.data + .bss + .noinit)

The default data (starting with aaabacadaeaf and continuing with 994 zeros) has been placed in program memory right after the interrupt vectors. It can be accessed at runtime through the pgm_read_ family of functions.

$ avr-objdump -s main.bin

main.bin:     file format elf32-avr

Contents of section .text:
0000 0c942802 0c943a02 0c943a02 0c943a02  ..(...:...:...:.
0010 0c943a02 0c943a02 0c943a02 0c943a02  ..:...:...:...:.
0020 0c943a02 0c943a02 0c943a02 0c943a02  ..:...:...:...:.
0030 0c943a02 0c943a02 0c943a02 0c943a02  ..:...:...:...:.
0040 0c943a02 0c943a02 0c943a02 0c943a02  ..:...:...:...:.
0050 0c943a02 0c943a02 0c943a02 0c943a02  ..:...:...:...:.
0060 0c943a02 0c943a02 aaabacad aeaf0000  ..:...:.........
0070 00000000 00000000 00000000 00000000  ................
0080 00000000 00000000 00000000 00000000  ................
0090 00000000 00000000 00000000 00000000  ................
[...]