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Linker User Guide

Preface Overview of the Linker Linking Models Supported by armlink Image Structure and Generation Linker Optimization Features Getting Image Details Accessing and Managing Symbols with armlink Scatter-loading Features The scatter-loading mechanism Overview of scatter-loading When to use scatter-loading Linker-defined symbols that are not defined when s Specifying stack and heap using the scatter file Scatter-loading command-line options Scatter-loading images with a simple memory map Scatter-loading images with a complex memory map Scatter file with link to bit-band objects Root execution regions Root execution region and the initial entry point Root execution regions and the ABSOLUTE attribute Root execution regions and the FIXED attribute Methods of placing functions and data at specific Placement of code and data with __attribute__((sec Placement of __at sections at a specific address Restrictions on placing __at sections Automatic placement of __at sections Manual placement of __at sections Placement of a key in flash memory with an __at se Mapping a structure over a peripheral register wit Example of how to explicitly place a named section Placement of unassigned sections with the .ANY mod Placement rules when using multiple .ANY selectors Command-line options for controlling the placement Prioritization of .ANY sections Specify the maximum region size permitted for plac Examples of using placement algorithms for .ANY se Example of next_fit algorithm showing behavior of Examples of using sorting algorithms for .ANY sect Behavior when .ANY sections overflow because of li Placement of veneer input sections in a scatter fi Placement of sections with overlays Reserving an empty region Placement of ARM C and C++ library code Specifying ARM standard C and C++ libraries in a s Example of placing code in a root region Example of placing ARM C library code Example of placing ARM C++ library code Example of placing ARM library helper functions Creation of regions on page boundaries Overalignment of execution regions and input secti Preprocessing of a scatter file Example of using expression evaluation in a scatte Equivalent scatter-loading descriptions for simple Command-line options for creating simple images Type 1 image, one load region and contiguous execu Type 2 image, one load region and non-contiguous e Type 3 image, multiple load regions and non-contig How the linker resolves multiple matches when proc How the linker resolves path names when processing Scatter file to ELF mapping Scatter File Syntax Linker Command-line Options Linker Steering File Command Reference Via File Syntax

Scatter file to ELF mapping

7.16 Scatter file to ELF mapping

Shows how scatter file components map onto ELF.

For simple images, ELF executable files contain segments:
  • A load region is represented by an ELF program segment with type PT_LOAD.
  • An execution region is represented by one or more of the following ELF sections:
    • XO.
    • RO.
    • RW.
    • ZI.

Note

If XO and RO are mixed within an execution region, that execution region is treated as RO.
For example, you might have a scatter file similar to the following:
LOAD 0x8000
{
    EXEC_ROM +0
    {
        *(+RO)
    }
    RAM +0
    {
        *(+RW,+ZI)
    }
    HEAP +0x100 EMPTY 0x100
    {
    }
    STACK +0 EMPTY 0x400
    {
    }
}
This scatter file creates a single program segment with type PT_LOAD for the load region with address 0x8000.
A single output section with type SHT_PROGBITS is created to represent the contents of EXEC_ROM. Two output sections are created to represent RAM. The first has a type SHT_PROGBITS and contains the initialized read/write data. The second has a type of SHT_NOBITS and describes the zero-initialized data.
The heap and stack are described in the ELF file by SHT_NOBITS sections.
Enter the following fromelf command to see the scatter-loaded sections in the image:
fromelf --text -v my_image.axf
To display the symbol table, enter the command:
fromelf --text -s -v my_image.axf
The following is an example of the fromelf output showing the LOAD, EXEC_ROM, RAM, HEAP, and STACK sections:
…
========================================================================
** Program header #0
    Type          : PT_LOAD (1)
    File Offset   : 52 (0x34)
    Virtual Addr  : 0x00008000
    Physical Addr : 0x00008000
    Size in file  : 764 bytes (0x2fc)
    Size in memory: 2140 bytes (0x85c)
    Flags         : PF_X + PF_W + PF_R + PF_ARM_ENTRY (0x80000007)
    Alignment     : 4
========================================================================
** Section #1
    Name        : EXEC_ROM
…
    Addr        : 0x00008000
    File Offset : 52 (0x34)
    Size        : 740 bytes (0x2e4)
…
====================================
** Section #2
    Name        : RAM
…
    Addr        : 0x000082e4
    File Offset : 792 (0x318)
    Size        : 20 bytes (0x14)
…
====================================
** Section #3
    Name        : RAM
…
    Addr        : 0x000082f8
    File Offset : 812 (0x32c)
    Size        : 96 bytes (0x60)
…
====================================
** Section #4
    Name        : HEAP
…
    Addr        : 0x00008458
    File Offset : 812 (0x32c)
    Size        : 256 bytes (0x100)
…
====================================
** Section #5
    Name        : STACK
…
    Addr        : 0x00008558
    File Offset : 812 (0x32c)
    Size        : 1024 bytes (0x400)
…
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