[Linker Script Tutorial] $8 - Linker and C code example

Alias names can be added to existing memory regions created with the MEMORY command. Each name corresponds to at most one memory region.

REGION_ALIAS(alias, region)

The REGION_ALIAS function creates an alias name alias for the memory region region. This allows a flexible mapping of output sections to memory regions. An example follows.
Suppose we have an application for embedded systems which come with various memory storage devices. All have a general purpose, volatile memory RAM that allows code execution or data storage. Some may have a read-only, non-volatile memory ROM that allows code execution and read-only data access. The last variant is a read-only, non-volatile memory ROM2 with read-only data access and no code execution capability. We have four output sections:

• .text program code;
• .rodata read-only data;
• .data read-write initialized data;
• .bss read-write zero initialized data.

The goal is to provide a linker command file that contains a system independent part defining the output sections and a system dependent part mapping the output sections to the memory regions available on the system. Our embedded systems come with three different memory setups A, B and C:

Section	Variant A	Variant B	Variant C
.text	RAM	ROM	ROM
.rodata	RAM	ROM	ROM2
.data	RAM	RAM/ROM	RAM/ROM2
.bss	RAM	RAM	RAM

The notation RAM/ROM or RAM/ROM2 means that this section is loaded into region ROM or ROM2 respectively. Please note that the load address of the .data section starts in all three variants at the end of the .rodata section.
The base linker script that deals with the output sections follows. It includes the system dependent linkcmds.memory file that describes the memory layout:

INCLUDE linkcmds.memory

SECTIONS
  {
    .text :
      {
        *(.text)
      } > REGION_TEXT
    .rodata :
      {
        *(.rodata)
        rodata_end = .;
      } > REGION_RODATA
    .data : AT (rodata_end)
      {
        data_start = .;
        *(.data)
      } > REGION_DATA
    data_size = SIZEOF(.data);
    data_load_start = LOADADDR(.data);
    .bss :
      {
        *(.bss)
      } > REGION_BSS
  }

Now we need three different linkcmds.memory files to define memory regions and alias names. The content of linkcmds.memory for the three variants A, B and C:

A

Here everything goes into the RAM.

MEMORY
  {
    RAM : ORIGIN = 0, LENGTH = 4M
  }

REGION_ALIAS("REGION_TEXT", RAM);
REGION_ALIAS("REGION_RODATA", RAM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);

B

Program code and read-only data go into the ROM. Read-write data goes into the RAM. An image of the initialized data is loaded into the ROM and will be copied during system start into the RAM.

MEMORY
  {
    ROM : ORIGIN = 0, LENGTH = 3M
    RAM : ORIGIN = 0x10000000, LENGTH = 1M
  }

REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);

C

Program code goes into the ROM. Read-only data goes into the ROM2. Read-write data goes into the RAM. An image of the initialized data is loaded into the ROM2 and will be copied during system start into the RAM.

MEMORY
  {
    ROM : ORIGIN = 0, LENGTH = 2M
    ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
    RAM : ORIGIN = 0x20000000, LENGTH = 1M
  }

REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM2);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);

It is possible to write a common system initialization routine to copy the .data section from ROM or ROM2 into the RAM if necessary:

#include <string.h>

extern char data_start [];
extern char data_size [];
extern char data_load_start [];

void copy_data(void)
{
  if (data_start != data_load_start)
    {
      memcpy(data_start, data_load_start, (size_t) data_size);
    }
}