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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

Example of next_fit algorithm showing behavior of full regions, selectors, and priority

7.4.6 Example of next_fit algorithm showing behavior of full regions, selectors, and priority

This example shows the operation of the next_fit placement algorithm for RO-CODE sections in sections.o.

The input section properties and ordering are shown in the following table:

Table 7-3 Input section properties for placement of sections with next_fit

Name Size
sec1 0x14
sec2 0x14
sec3 0x10
sec4 0x4
sec5 0x4
sec6 0x4
The scatter file used for the examples is:
LR 0x100
{
  ER_1 0x100 0x20
  {
     .ANY1(+RO-CODE)
  }
  ER_2 0x200 0x20
  {
     .ANY2(+RO)
  }
  ER_3 0x300 0x20
  {
     .ANY3(+RO)
  }
}

Note

This example has --any_contingency disabled.
The next_fit algorithm is different to the others in that it never revisits a region that is considered to be full. This example also shows the interaction between priority and specificity of selectors - this is the same for all the algorithms.
    Execution Region ER_1 (Base: 0x00000100, Size: 0x00000014, Max: 0x00000020, ABSOLUTE)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00000100   0x00000014   Code   RO            1    sec1                sections.o


    Execution Region ER_2 (Base: 0x00000200, Size: 0x0000001c, Max: 0x00000020, ABSOLUTE)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00000200   0x00000010   Code   RO            3    sec3                sections.o
    0x00000210   0x00000004   Code   RO            4    sec4                sections.o
    0x00000214   0x00000004   Code   RO            5    sec5                sections.o
    0x00000218   0x00000004   Code   RO            6    sec6                sections.o


    Execution Region ER_3 (Base: 0x00000300, Size: 0x00000014, Max: 0x00000020, ABSOLUTE)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00000300   0x00000014   Code   RO            2    sec2                sections.o
In this example:
  • The linker places sec1 in ER_1 because ER_1 has the most specific selector. ER_1 now has 0x6 bytes remaining.
  • The linker then tries to place sec2 in ER_1, because it has the most specific selector, but there is not enough space. Therefore, ER_1 is marked as full and is not considered in subsequent placement steps. The linker chooses ER_3 for sec2 because it has higher priority than ER_2.
  • The linker then tries to place sec3 in ER_3. It does not fit, so ER_3 is marked as full and the linker places sec3 in ER_2.
  • The linker now processes sec4. This is 0x4 bytes so it can fit in either ER_1 or ER_3. Because both of these sections have previously been marked as full, they are not considered. The linker places all remaining sections in ER_2.
  • If another section sec7 of size 0x8 exists, and is processed after sec6 the example fails to link. The algorithm does not attempt to place the section in ER_1 or ER_3 because they have previously been marked as full.
Related reference
Non-ConfidentialPDF file icon PDF versionARM DUI0377H
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