How to place variables in Flash

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Variables are usually placed in RAM and constant data is placed in Flash.

However, there are some special situations where data in Flash is modified (i.e. making it a variable) and the code needs to work on the possibly modified data.

This article describes how to place such a symbol using the SEGGER linker.

Requirements

  • Embedded Studio v6.34a or later

Example Situation

The following code will be executed by a bootloader.

const unsigned int ValidMarker = 0xA5A5A5A5;
const unsigned int FW_Valid;   values

int CheckFirmware() {
  if (FW_Valid == ValidMarker) {
    return 1;
  } else {
   return 0;
  }
}

However, this implementation brings a potential problem with it.

Variable FW_Valid could be removed by optimization, because we comparing two constant.

A solution for that is to tell the compiler that the variable is changed outside of the application (volatile).

volatile const unsigned int FW_Valid;

But volatile and const cannot be used together (const overrides volatile).

So we have to create a normal volatile variable:

volatile unsigned int FW_Valid;

The next Issue is that the created Variable is now placed in RAM. However we can place the variable explicitly in Flash with a linker script:

place in FLASH { symbol FW_Valid };

Now the variable also might be removed by optimization, because it is not directly written anywhere.

So we have to tell the linker to keep the variable:

keep { symbol FW_Valid };