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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

Placement of sections with overlays

7.6 Placement of sections with overlays

You can place multiple execution regions at the same address with overlays.

The OVERLAY attribute allows you to place multiple execution regions at the same address. An overlay manager is required to make sure that only one execution region is instantiated at a time. ARM® Compiler does not provide an overlay manager.
The following example shows the definition of a static section in RAM followed by a series of overlays. Here, only one of these sections is instantiated at a time.
EMB_APP 0x8000 
{
    …
    STATIC_RAM 0x0                  ; contains most of the RW and ZI code/data
    {
            * (+RW,+ZI)
    }
    OVERLAY_A_RAM 0x1000 OVERLAY    ; start address of overlay…
    {
            module1.o (+RW,+ZI)
    }
    OVERLAY_B_RAM 0x1000 OVERLAY
    {
            module2.o (+RW,+ZI)
    }
    …                        ; rest of scatter-loading description
}
A region marked as OVERLAY is not initialized by the C library at startup. The contents of the memory used by the overlay region are the responsibility of an overlay manager. If the region contains initialized data, use the NOCOMPRESS attribute to prevent RW data compression.
You can use the linker defined symbols to obtain the addresses required to copy the code and data.
You can use the OVERLAY attribute on a single region that is not at the same address as a different region. Therefore, you can use an overlay region as a method to prevent the initialization of particular regions by the C library startup code. As with any overlay region you must manual initialize them in your code.
An overlay region can have a relative base. The behavior of an overlay region with a +offset base address depends on the regions that precede it and the value of +offset. The linker places consecutive +offset regions at the same base address if they have the same +offset value.
When a +offset execution region ER follows a contiguous overlapping block of overlay execution regions the base address of ER is:
limit address of the overlapping block of overlay execution regions + offset
The following table shows the effect of +offset when used with the OVERLAY attribute. REGION1 appears immediately before REGION2 in the scatter file:

Table 7-8 Using relative offset in overlays

REGION1 is set with OVERLAY +offset REGION2 Base Address
NO <offset> REGION1 Limit + <offset>
YES +0 REGION1 Base Address
YES <non-zero offset> REGION1 Limit + <non-zero offset>
The following example shows the use of relative offsets with overlays and the effect on execution region addresses:
EMB_APP 0x8000
{
    CODE 0x8000
    {
        *(+RO)
    }
    # REGION1 Base = CODE limit
    REGION1 +0 OVERLAY
    {
        module1.o(*)
    }
    # REGION2 Base = REGION1 Base
    REGION2 +0 OVERLAY
    {
        module2.o(*)
    }
    # REGION3 Base = REGION2 Base = REGION1 Base
    REGION3 +0 OVERLAY
    {
        module3.o(*)
    }
    # REGION4 Base = REGION3 Limit + 4
    Region4 +4 OVERLAY
    {
        module4.o(*)
    }
}
If the length of the non-overlay area is unknown, you can use a zero relative offset to specify the start address of an overlay so that it is placed immediately after the end of the static section.
You can use the following command-line options to add extra debug information to the image:
  • --emit_debug_overlay_relocs.
  • --emit_debug_overlay_section.
These permit an overlay-aware debugger to track which overlay is currently active.
Non-ConfidentialPDF file icon PDF versionARM DUI0377H
Copyright © 2007, 2008, 2011, 2012, 2014-2016 ARM. All rights reserved. 

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