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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

Reserving an empty region

7.7 Reserving an empty region

You can reserve an empty block of memory with a scatter file, such as the area used for the stack. Use the EMPTY attribute for the execution region in the scatter-loading description.

The block of memory does not form part of the load region, but is assigned for use at execution time. Because it is created as a dummy ZI region, the linker uses the following symbols to access it:
  • Image$$region_name$$ZI$$Base.
  • Image$$region_name$$ZI$$Limit.
  • Image$$region_name$$ZI$$Length.
If the length is given as a negative value, the address is taken to be the end address of the region. This must be an absolute address and not a relative one.
In the following example, the execution region definition STACK 0x800000 EMPTY –0x10000 defines a region called STACK that starts at address 0x7F0000 and ends at address 0x800000:
LR_1 0x80000                          ; load region starts at 0x80000   
{
    STACK 0x800000 EMPTY -0x10000     ; region ends at 0x800000 because of the
                                      ; negative length. The start of the region
                                      ; is calculated using the length.
    {
                                      ; Empty region for placing the stack
    }

    HEAP +0 EMPTY 0x10000             ; region starts at the end of previous
                                      ; region. End of region calculated using
                                      ; positive length
    {
                                      ; Empty region for placing the heap
    }
    …                                 ; rest of scatter-loading description
}

Note

The dummy ZI region that is created for an EMPTY execution region is not initialized to zero at runtime.
If the address is in relative (+offset) form and the length is negative, the linker generates an error.
The following figure shows a diagrammatic representation for this example.
Figure 7-5 Reserving a region for the stack
Reserving a region for the stack

In this example, the linker generates the symbols:
Image$$STACK$$ZI$$Base      = 0x7f0000
Image$$STACK$$ZI$$Limit     = 0x800000
Image$$STACK$$ZI$$Length    = 0x10000
Image$$HEAP$$ZI$$Base       = 0x800000
Image$$HEAP$$ZI$$Limit      = 0x810000
Image$$HEAP$$ZI$$Length     = 0x10000

Note

The EMPTY attribute applies only to an execution region. The linker generates a warning and ignores an EMPTY attribute used in a load region definition.
The linker checks that the address space used for the EMPTY region does not coincide with any other execution region.
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