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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

Type 2 image, one load region and non-contiguous execution regions

7.13.3 Type 2 image, one load region and non-contiguous execution regions

A Type 2 image consists of a single load region in the load view and three execution regions in the execution view. It is similar to images of Type 1 except that the RW execution region is not contiguous with the RO execution region.

--ro_base=address specifies the load and execution address of the region containing the RO output section. --rw_base=address specifies the execution address for the RW execution region.
For images that contain execute-only (XO) sections, the XO execution region is placed at the address specified by --ro_base. The RO execution region is placed contiguously and immediately after the XO execution region.
If you use --xo_base address, then the XO execution region is placed in a separate load region at the specified address.

Example for single load region and multiple execution regions

The following example shows the scatter-loading description equivalent to using --ro_base=0x010000 --rw_base=0x040000:
LR_1 0x010000        ; Defines the load region name as LR_1
{
    ER_RO +0         ; The first execution region is called ER_RO and starts at end
                     ; of previous region. Because there is no previous region, the
                     ; address is 0x010000.
    {
        * (+RO)      ; All RO sections are placed consecutively into this region.
    }
    ER_RW 0x040000   ; Second execution region is called ER_RW and starts at 0x040000.
    {
        * (+RW)      ; All RW sections are placed consecutively into this region.
    }
    ER_ZI +0         ; The last execution region is called ER_ZI.
                     ; The address is 0x040000 + size of ER_RW region.
    {
        * (+ZI)      ; All ZI sections are placed consecutively here.
    }
}
In this example:
  • This description creates an image with one load region, named LR_1, with a load address of 0x010000.
  • The image has three execution regions, named ER_RO, ER_RW, and ER_ZI, that contain the RO, RW, and ZI output sections respectively. The RO region is a root region. The execution address of ER_RO is 0x010000.
  • The ER_RW execution region is not contiguous with ER_RO. Its execution address is 0x040000.
  • The ER_ZI execution region is placed immediately following the end of the preceding execution region, ER_RW.

RWPI example variant

This is similar to images of Type 2 with --rw_base where the RW execution region is separate from the RO execution region. However, --rwpi marks the execution regions containing the RW output section as position-independent.
The following example shows the scatter-loading description equivalent to using --ro_base=0x010000 --rw_base=0x018000 --rwpi:
LR_1 0x010000           ; The first load region is at 0x010000.
{
    ER_RO +0            ; Default ABSOLUTE attribute is inherited from parent.
                        ; The execution address is 0x010000. The code and RO data
                        ; cannot be moved.
    {
        * (+RO)         ; All the RO sections go here.
    }
    ER_RW 0x018000 PI   ; PI attribute overrides ABSOLUTE
    {
        * (+RW)         ; The RW sections are placed at 0x018000 and they can be
                        ; moved.
    }
    ER_ZI +0            ; ER_ZI region placed after ER_RW region.
    {
        * (+ZI)         ; All the ZI sections are placed consecutively here.
    }
}
ER_RO, the RO execution region, inherits the ABSOLUTE attribute from the load region LR_1. The next execution region, ER_RW, is marked as PI. Also, because the ER_ZI region has an offset of +0, it inherits the PI attribute from the ER_RW region.
Similar scatter-loading descriptions can also be written to correspond to the usage of other combinations of --ropi and --rwpi with Type 2 and Type 3 images.

Note

Be aware that if an image contains execute-only memory, RWPI is not supported. armlink gives an error if you use --rwpi to link such an image.
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