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Preface Arm Compiler Tools Overview armclang Reference armlink Reference armlink Command-line Options --any_contingency --any_placement=algorithm --any_sort_order=order --api, --no_api --autoat, --no_autoat --bare_metal_pie --base_platform --be8 --be32 --bestdebug, --no_bestdebug --blx_arm_thumb, --no_blx_arm_thumb --blx_thumb_arm, --no_blx_thumb_arm --bpabi --branchnop, --no_branchnop --callgraph, --no_callgraph --callgraph_file=filename --callgraph_output=fmt --callgraph_subset=symbol[,symbol,...] --cgfile=type --cgsymbol=type --cgundefined=type --comment_section, --no_comment_section --cppinit, --no_cppinit --cpu=list (armlink) --cpu=name (armlink) --crosser_veneershare, --no_crosser_veneershare --datacompressor=opt --debug, --no_debug --diag_error=tag[,tag,…] (armlink) --diag_remark=tag[,tag,…] (armlink) --diag_style={arm|ide|gnu} (armlink) --diag_suppress=tag[,tag,…] (armlink) --diag_warning=tag[,tag,…] (armlink) --dll --dynamic_linker=name --eager_load_debug, --no_eager_load_debug --eh_frame_hdr --edit=file_list --emit_debug_overlay_relocs --emit_debug_overlay_section --emit_non_debug_relocs --emit_relocs --entry=location --errors=filename --exceptions, --no_exceptions --export_all, --no_export_all --export_dynamic, --no_export_dynamic --filtercomment, --no_filtercomment --fini=symbol --first=section_id --force_explicit_attr --force_so_throw, --no_force_so_throw --fpic --fpu=list (armlink) --fpu=name (armlink) --got=type --gnu_linker_defined_syms --help (armlink) --import_cmse_lib_in=filename --import_cmse_lib_out=filename --import_unresolved, --no_import_unresolved --info=topic[,topic,…] (armlink) --info_lib_prefix=opt --init=symbol --inline, --no_inline --inline_type=type --inlineveneer, --no_inlineveneer input-file-list (armlink) --keep=section_id (armlink) --keep_intermediate --largeregions, --no_largeregions --last=section_id --legacyalign, --no_legacyalign --libpath=pathlist --library=name --library_security=protection --library_type=lib --list=filename --list_mapping_symbols, --no_list_mapping_symbols --load_addr_map_info, --no_load_addr_map_info --locals, --no_locals --lto, --no_lto --lto_keep_all_symbols, --no_lto_keep_all_symbols --lto_intermediate_filename --lto_level --lto_relocation_model --mangled, --unmangled --map, --no_map --max_er_extension=size --max_veneer_passes=value --max_visibility=type --merge, --no_merge --merge_litpools, --no_merge_litpools --muldefweak, --no_muldefweak -o filename, --output=filename (armlink) --output_float_abi=option --overlay_veneers --override_visibility -Omax (armlink) --pad=num --paged --pagesize=pagesize --partial --pie --piveneer, --no_piveneer --pixolib --pltgot=type --pltgot_opts=mode --predefine="string" --preinit, --no_preinit --privacy (armlink) --ref_cpp_init, --no_ref_cpp_init --ref_pre_init, --no_ref_pre_init --reloc --remarks --remove, --no_remove --ro_base=address --ropi --rosplit --rw_base=address --rwpi --scanlib, --no_scanlib --scatter=filename --section_index_display=type --shared --show_cmdline (armlink) --show_full_path --show_parent_lib --show_sec_idx --soname=name --sort=algorithm --split --startup=symbol, --no_startup --stdlib --strict --strict_flags, --no_strict_flags --strict_ph, --no_strict_ph --strict_preserve8_require8 --strict_relocations, --no_strict_relocations --strict_symbols, --no_strict_symbols --strict_visibility, --no_strict_visibility --symbols, --no_symbols --symdefs=filename --symver_script=filename --symver_soname --sysv --tailreorder, --no_tailreorder --tiebreaker=option --unaligned_access, --no_unaligned_access --undefined=symbol --undefined_and_export=symbol --unresolved=symbol --use_definition_visibility --userlibpath=pathlist --veneerinject, --no_veneerinject --veneer_inject_type=type --veneer_pool_size=size --veneershare, --no_veneershare --verbose --version_number (armlink) --via=filename (armlink) --vsn (armlink) --xo_base=address --xref, --no_xref --xrefdbg, --no_xrefdbg --xref{from|to}=object(section) --zi_base=address Linking Models Supported by armlink Overview of linking models Bare-metal linking model overview Partial linking model overview Base Platform Application Binary Interface (BPABI) Base Platform linking model overview SysV linking model overview Concepts common to both BPABI and SysV linking mod Image Structure and Generation The structure of an Arm ELF image Views of the image at each link stage Input sections, output sections, regions, and prog Load view and execution view of an image Methods of specifying an image memory map with the Image entry points Restrictions on image structure Simple images Types of simple image Type 1 image structure, one load region and contig Type 2 image structure, one load region and non-co Type 3 image structure, multiple load regions and Section placement with the linker Default section placement Section placement with the FIRST and LAST attribut Section alignment with the linker Linker support for creating demand-paged files Linker reordering of execution regions containing Linker-generated veneers What is a veneer? Veneer sharing Veneer types Generation of position independent to absolute ven Reuse of veneers when scatter-loading Generation of secure gateway veneers Command-line options used to control the generatio Weak references and definitions How the linker performs library searching, selecti How the linker searches for the Arm standard libra Specifying user libraries when linking How the linker resolves references The strict family of linker options Linker Optimization Features Elimination of common section groups Elimination of unused sections Optimization with RW data compression How the linker chooses a compressor Options available to override the compression algo How compression is applied Considerations when working with RW data compressi Function inlining with the linker Factors that influence function inlining About branches that optimize to a NOP Linker reordering of tail calling sections Restrictions on reordering of tail calling section Linker merging of comment sections Merging identical constants Accessing and Managing Symbols with armlink About mapping symbols Linker-defined symbols Region-related symbols Types of region-related symbols Image$$ execution region symbols Load$$ execution region symbols Load$$LR$$ load region symbols Region name values when not scatter-loading Linker defined symbols and scatter files Methods of importing linker-defined symbols in C a Methods of importing linker-defined symbols in Arm Section-related symbols Types of section-related symbols Image symbols Input section symbols Access symbols in another image Creating a symdefs file Outputting a subset of the global symbols Reading a symdefs file Symdefs file format Edit the symbol tables with a steering file Specifying steering files on the linker command-li Steering file command summary Steering file format Hide and rename global symbols with a steering fil Use of $Super$$ and $Sub$$ to patch symbol definit 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 Placing the stack and heap with a scatter file Scatter-loading command-line options Scatter-loading images with a simple memory map Scatter-loading images with a complex memory map Root region and the initial entry point Effect of the ABSOLUTE attribute on a root region Effect of the FIXED attribute on a root region Methods of placing functions and data at specific Placing functions and data in a named section Placing __at sections at a specific address Restrictions on placing __at sections Automatically placing __at sections Manually placing __at sections Placing a key in flash memory with an __at section Example of how to explicitly place a named section Placement of unassigned sections Default rules for placing unassigned sections Command-line options for controlling the placement Prioritizing the placement of unassigned 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 Placing veneers with a scatter file Placement of CMSE veneer sections for a Secure ima Reserving an empty block of memory Characteristics of a reserved empty block of memor Example of reserving an empty block of memory Placement of Arm C and C++ library code Placing code in a root region Placing Arm C library code Placing Arm C++ library code Aligning regions to page boundaries Aligning execution regions and input sections Preprocessing a scatter file Default behavior for armclang -E in a scatter file Using other preprocessors in 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 BNF notation used in scatter-loading description s Syntax of a scatter file Load region descriptions Components of a load region description Syntax of a load region description Load region attributes Inheritance rules for load region address attribut Inheritance rules for the RELOC address attribute Considerations when using a relative address +offs Execution region descriptions Components of an execution region description Syntax of an execution region description Execution region attributes Inheritance rules for execution region address att Considerations when using a relative address +offs Input section descriptions Components of an input section description Syntax of an input section description Examples of module and input section specification Expression evaluation in scatter files Expression usage in scatter files Expression rules in scatter files Execution address built-in functions for use in sc ScatterAssert function and load address related fu Symbol related function in a scatter file AlignExpr(expr, align) function GetPageSize() function SizeOfHeaders() function Example of aligning a base address in execution sp Scatter files containing relative base address loa BPABI and SysV Shared Libraries and Executables About the Base Platform Application Binary Interfa Platforms supported by the BPABI Features common to all BPABI models About importing and exporting symbols for BPABI mo Symbol visibility for BPABI models Automatic import and export for BPABI models Manual import and export for BPABI models Symbol versioning for BPABI models RW compression for BPABI models SysV linking model SysV standard memory model Using the C and C++ libraries Using a dynamic Linker Automatic dynamic symbol table rules in the SysV l Symbol definitions defined for SysV compatibility Addressing modes in the SysV linking model Thread local storage in the SysV linking model Linker command-line options for the SysV linking m Bare metal and DLL-like memory models BPABI standard memory model Customization of the BPABI standard memory model Linker command-line options for bare metal and DLL Mandatory symbol versioning in the BPABI DLL-like Automatic dynamic symbol table rules in the BPABI Addressing modes in the BPABI DLL-like model C++ initialization in the BPABI DLL-like model Symbol versioning Overview of symbol versioning Embedded symbols The symbol versioning script file Example of creating versioned symbols Linker options for enabling implicit symbol versio Features of the Base Platform Linking Model Restrictions on the use of scatter files with the Scatter files for the Base Platform linking model Placement of PLT sequences with the Base Platform Linker Steering File Command Reference EXPORT steering file command HIDE steering file command IMPORT steering file command RENAME steering file command REQUIRE steering file command RESOLVE steering file command SHOW steering file command fromelf Reference armar Reference armasm Legacy Assembler Reference Appendixes

Methods of placing functions and data at specific addresses

C6.2.3 Methods of placing functions and data at specific addresses

There are various methods available to place functions and data at specific addresses.

Placing functions and data at specific addresses

To place a single function or data item at a fixed address, you must enable the linker to process the function or data separately from the rest of the input files.

Where they are required, the compiler normally produces RO, RW, and ZI sections from a single source file. These sections contain all the code and data from the source file.

Note:

For images targeted at Arm®v7‑M or Armv8‑M, the compiler might generate execute-only (XO) sections.

Typically, you create a scatter file that defines an execution region at the required address with a section description that selects only one section.

To place a function or variable at a specific address, it must be placed in its own section. There are several ways to do this:

  • By default, the compiler places each function and variable in individual ELF sections. This can be overridden using the -fno-function-sections or -fno-data-sections compiler options.
  • Place the function or data item in its own source file.
  • Use __attribute__((section("name"))) to place functions and variables in a specially named section, .ARM.__at_address, where address is the address to place the function or variable. For example, __attribute__((section(".ARM.__at_0x4000"))).

    To place ZI data at a specific address, use the variable attribute __attribute__((section("name"))) with the special name .bss.ARM.__at_address

    These specially named sections are called __at sections.

  • Use the .section directive from assembly language. In assembly code, the smallest locatable unit is a .section.

Placing a variable at a specific address without scatter-loading

This example shows how to modify your source code to place code and data at specific addresses, and does not require a scatter file.

To place code and data at specific addresses without a scatter file:

  1. Create the source file main.c containing the following code:

    #include <stdio.h>
    
    extern int sqr(int n1);
    const int gValue __attribute__((section(".ARM.__at_0x5000"))) = 3; // Place at 0x5000
    int main(void)
    {
        int squared;
        squared=sqr(gValue);
        printf("Value squared is: %d\n", squared);
        return 0;
    }
  2. Create the source file function.c containing the following code:

    int sqr(int n1)
    {
        return n1*n1;
    }
  3. Compile and link the sources:

    armclang --target=arm-arm-none-eabi -march=armv8-a -c function.c
    armclang --target=arm-arm-none-eabi -march=armv8-a -c main.c
    armlink --map function.o main.o -o squared.axf

    The --map option displays the memory map of the image. Also, --autoat is the default.

In this example, __attribute__((section(".ARM.__AT_0x5000"))) specifies that the global variable gValue is to be placed at the absolute address 0x5000. gValue is placed in the execution region ER$$.ARM.__AT_0x5000 and load region LR$$.ARM.__AT_0x5000.

The memory map shows:

…
  Load Region LR$$.ARM.__AT_0x5000 (Base: 0x00005000, Size: 0x00000004, Max: 0x00000004, ABSOLUTE)

    Execution Region ER$$.ARM.__AT_0x5000 (Base: 0x00005000, Size: 0x00000004, Max: 0x00000004, ABSOLUTE, UNINIT)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00005000   0x00000004   Data   RO           18    .ARM.__AT_0x5000  main.o

Example of how to place a variable in a named section with scatter-loading

This example shows how to modify your source code to place code and data in a specific section using a scatter file.

To modify your source code to place code and data in a specific section using a scatter file:

  1. Create the source file main.c containing the following code:

    #include <stdio.h>
    extern int sqr(int n1);
    int gSquared __attribute__((section("foo")));  // Place in section foo
    int main(void)
    {
        gSquared=sqr(3);
        printf("Value squared is: %d\n", gSquared);
        return 0;
    }
  2. Create the source file function.c containing the following code:

    int sqr(int n1)
    {
        return n1*n1;
    }
  3. Create the scatter file scatter.scat containing the following load region:

    LR1 0x0000 0x20000
    {
        ER1 0x0 0x2000
        {
            *(+RO)                ; rest of code and read-only data
        }
        ER2 0x8000 0x2000
        {
            main.o
        }
        ER3 0x10000 0x2000
        {
            function.o
            *(foo)                ; Place gSquared in ER3
        }
        ; RW and ZI data to be placed at 0x200000
        RAM 0x200000 (0x1FF00-0x2000)
        {
            *(+RW, +ZI)
        }
        ARM_LIB_STACK 0x800000 EMPTY -0x10000
        {
        }
        ARM_LIB_HEAP  +0 EMPTY 0x10000
        {
        }
    }

    The ARM_LIB_STACK and ARM_LIB_HEAP regions are required because the program is being linked with the semihosting libraries.

  4. Compile and link the sources:

    armclang --target=arm-arm-none-eabi -march=armv8-a -c function.c
    armclang --target=arm-arm-none-eabi -march=armv8-a -c main.c
    armlink --map --scatter=scatter.scat function.o main.o -o squared.axf

    The --map option displays the memory map of the image. Also, --autoat is the default.

In this example, __attribute__((section("foo"))) specifies that the global variable gSquared is to be placed in a section called foo. The scatter file specifies that the section foo is to be placed in the ER3 execution region.

The memory map shows:

  Load Region LR1 (Base: 0x00000000, Size: 0x00001570, Max: 0x00020000, ABSOLUTE)
…
    Execution Region ER3 (Base: 0x00010000, Size: 0x00000010, Max: 0x00002000, ABSOLUTE)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00010000   0x0000000c   Code   RO            3    .text               function.o
    0x0001000c   0x00000004   Data   RW           15    foo                 main.o
…

Note:

If you omit *(foo) from the scatter file, the section is placed in the region of the same type. That is RAM in this example.

Placing a variable at a specific address with scatter-loading

This example shows how to modify your source code to place code and data at a specific address using a scatter file.

To modify your source code to place code and data at a specific address using a scatter file:

  1. Create the source file main.c containing the following code:

    #include <stdio.h>
    extern int sqr(int n1);
    // Place at address 0x10000
    const int gValue __attribute__((section(".ARM.__at_0x10000"))) = 3;
    int main(void)
    {
        int squared;
        squared=sqr(gValue);
        printf("Value squared is: %d\n", squared);
        return 0;
    }
  2. Create the source file function.c containing the following code:

    int sqr(int n1)
    {
        return n1*n1;
    }
  3. Create the scatter file scatter.scat containing the following load region:

    LR1 0x0
    {
        ER1 0x0
        {
            *(+RO)                      ; rest of code and read-only data
        }
        ER2 +0
        {
            function.o
            *(.ARM.__at_0x10000)        ; Place gValue at 0x10000
        }
        ; RW and ZI data to be placed at 0x200000
        RAM 0x200000 (0x1FF00-0x2000)
        {
            *(+RW, +ZI)
        }
        ARM_LIB_STACK 0x800000 EMPTY -0x10000
        {
        }
        ARM_LIB_HEAP  +0 EMPTY 0x10000
        {
        }
    }

    The ARM_LIB_STACK and ARM_LIB_HEAP regions are required because the program is being linked with the semihosting libraries.

  4. Compile and link the sources:

    armclang --target=arm-arm-none-eabi -march=armv8-a -c function.c
    armclang --target=arm-arm-none-eabi -march=armv8-a -c main.c
    armlink --no_autoat --scatter=scatter.scat --map function.o main.o -o squared.axf

    The --map option displays the memory map of the image.

The memory map shows that the variable is placed in the ER2 execution region at address 0x10000:

…
    Execution Region ER2 (Base: 0x00002a54, Size: 0x0000d5b0, Max: 0xffffffff, ABSOLUTE)

    Base Addr    Size         Type   Attr      Idx    E Section Name        Object

    0x00002a54   0x0000001c   Code   RO            4    .text.sqr           function.o
    0x00002a70   0x0000d590   PAD
    0x00010000   0x00000004   Data   RO            9    .ARM.__at_0x10000   main.o

In this example, the size of ER1 is unknown. Therefore, gValue might be placed in ER1 or ER2. To make sure that gValue is placed in ER2, you must include the corresponding selector in ER2 and link with the --no_autoat command-line option. If you omit --no_autoat, gValue is placed in a separate load region LR$$.ARM.__at_0x10000 that contains the execution region ER$$.ARM.__at_0x10000.

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