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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

Root execution regions and the FIXED attribute

7.2.3 Root execution regions and the FIXED attribute

You can use the FIXED attribute for an execution region in a scatter file to create root regions that load and execute at fixed addresses.

Use the FIXED execution region attribute to ensure that the load address and execution address of a specific region are the same.
You can use the FIXED attribute to place any execution region at a specific address in ROM.
For example, the following memory map shows fixed execution regions:
Figure 7-3 Memory map for fixed execution regions
Memory map for fixed execution regions

The following example shows the corresponding scatter-loading description:
LR_1 0x040000              ; load region starts at 0x40000   
{                          ; start of execution region descriptions      
    ER_RO 0x040000         ; load address = execution address
        * (+RO)            ; RO sections other than those in init.o
    ER_INIT 0x080000 FIXED ; load address and execution address of this
                           ; execution region are fixed at 0x80000
        init.o(+RO)        ; all RO sections from init.o
    …                    ; rest of scatter-loading description
You can use this to place a function or a block of data, such as a constant table or a checksum, at a fixed address in ROM so that it can be accessed easily through pointers.
If you specify, for example, that some initialization code is to be placed at start of ROM and a checksum at the end of ROM, some of the memory contents might be unused. Use the * or .ANY module selector to flood fill the region between the end of the initialization block and the start of the data block.
To make your code easier to maintain and debug, it is suggested that you use the minimum amount of placement specifications in scatter files and leave the detailed placement of functions and data to the linker.


There are some situations where using FIXED and a single load region are not appropriate. Other techniques for specifying fixed locations are:
  • If your loader can handle multiple load regions, place the RO code or data in its own load region.
  • If you do not require the function or data to be at a fixed location in ROM, use ABSOLUTE instead of FIXED. The loader then copies the data from the load region to the specified address in RAM. ABSOLUTE is the default attribute.
  • To place a data structure at the location of memory-mapped I/O, use two load regions and specify UNINIT. UNINIT ensures that the memory locations are not initialized to zero.

Example showing the misuse of the FIXED attribute

The following example shows common cases where the FIXED execution region attribute is misused:
LR1 0x8000
    ER_LOW +0 0x1000
; At this point the next available Load and Execution address is 0x8000 + size of
; contents of ER_LOW. The maximum size is limited to 0x1000 so the next available Load
; and Execution address is at most 0x9000
    ER_HIGH 0xF0000000 FIXED
; The required execution address and load address is 0xF0000000. The linker inserts
; 0xF0000000 - (0x8000 + size of(ER_LOW)) bytes of padding so that load address matches
; execution address
; The other common misuse of FIXED is to give a lower execution address than the next
; available load address.
LR_HIGH 0x100000000
    ER_LOW 0x1000 FIXED
; The next available load address in LR_HIGH is 0x10000000. The required Execution
; address is 0x1000. Because the next available load address in LR_HIGH must increase
; monotonically the linker cannot give ER_LOW a Load Address lower than 0x10000000
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