Linker Script: Change the Variable Memory Location

In this guide, we shall see how to add to linker script command to move the variable location from memory location to another (From RAM_D1 to RAM_D2 in this case on STM32H735 and can be applied to any ARM_GCC).

In this guide, we shall cover the following:

• Memory location before modification.
• Memory location after modification.

1. Memory Location Before the Modification:

We start from main.c file.

Within the main.c file, include the following:

#include "stdint.h"
#include "stdlib.h"

Declare an array of size 1000 of uint32_t which will hold 4000 Bytes in the RAM:

uint32_t data[1000]

Get the size of the unsigned it as following:

#define dataSize	sizeof(unsigned int)

in main code:

fill the array with random numbers to force the compiler to not optimize the array:

    for(int i=0;i<1000;i++)
    {
    	data[i]=rand()/dataSize;
    }

Add the while loop:

	while(1)
	{

	}

Compile the code now.

After the compilation is done, check the memory region D1 which is the default for STM32H735:

There is 5.85KB of data in the RAM, 3.91 for the array and 1.5 for the stack.

Now, open Memory details, select RAM_D1 then open .bss section:

As you can see, the compiler with the default setup of the linkerscript allocate 3.91KB of data in RAM_D1.
Let say, we need to store this buffer into external RAM or another RAM region.

2. Memory Location After The Modification:

In order to move the variable to another location, we need to create a section, we can simply do this using attribute as following for the buffer:

uint32_t data[1000]__attribute__ ((section (".RAMD2_data")));

We named the section RAMD2_data, by adding this to any other variable, the linkerscript will put the data into RAM_D2 rather than the default D1

Now, open the linkerscript (STM32H735IGKX_FLASH.ld in this case) and add the following line:

.RAMD2_data :
{
    KEEP(*(.RAMD2_data))
} >RAM_D2

Save the linkerscript and compile the code and check the memory region, you should see 3.91KB of RAM_D2 has been used as following:

By checking memory details, we can see the section we created:

We have successfully moved the data from RAM_D1 to RAM_D2. The same can be applied to external RAM to hold the variables.

Happy coding 🙂