SPRU513Z August   2001  – October 2023 SM320F28335-EP

 

  1.   1
  2.   Read This First
    1.     About This Manual
    2.     How to Use This Manual
    3.     Notational Conventions
    4.     Related Documentation From Texas Instruments
    5.     Trademarks
  3. 1 Introduction to the Software Development Tools
    1. 1.1 Software Development Tools Overview
    2. 1.2 Tools Descriptions
  4. 2 Introduction to Object Modules
    1. 2.1 Object File Format Specifications
    2. 2.2 Executable Object Files
    3. 2.3 Introduction to Sections
      1. 2.3.1 Special Section Names
    4. 2.4 How the Assembler Handles Sections
      1. 2.4.1 Uninitialized Sections
      2. 2.4.2 Initialized Sections
      3. 2.4.3 User-Named Sections
      4. 2.4.4 Current Section
      5. 2.4.5 Section Program Counters
      6. 2.4.6 Subsections
      7. 2.4.7 Using Sections Directives
    5. 2.5 How the Linker Handles Sections
      1. 2.5.1 Combining Input Sections
      2. 2.5.2 Placing Sections
    6. 2.6 Symbols
      1. 2.6.1 Global (External) Symbols
      2. 2.6.2 Local Symbols
      3. 2.6.3 Weak Symbols
      4. 2.6.4 The Symbol Table
    7. 2.7 Symbolic Relocations
      1. 2.7.1 Expressions With Multiple Relocatable Symbols (COFF Only)
    8. 2.8 Loading a Program
  5. 3 Program Loading and Running
    1. 3.1 Loading
      1. 3.1.1 Load and Run Addresses
      2. 3.1.2 Bootstrap Loading
        1. 3.1.2.1 Boot, Load, and Run Addresses
        2. 3.1.2.2 Primary Bootloader
        3. 3.1.2.3 Secondary Bootloader
        4. 3.1.2.4 Boot Table
        5. 3.1.2.5 Bootloader Routine
          1. 3.1.2.5.1 Sample Secondary Bootloader Routine
    2. 3.2 Entry Point
    3. 3.3 Run-Time Initialization
      1. 3.3.1 The _c_int00 Function
      2. 3.3.2 RAM Model vs. ROM Model
        1. 3.3.2.1 Autoinitializing Variables at Run Time (--rom_model)
        2. 3.3.2.2 Initializing Variables at Load Time (--ram_model)
        3. 3.3.2.3 The --rom_model and --ram_model Linker Options
      3. 3.3.3 About Linker-Generated Copy Tables
        1. 3.3.3.1 BINIT
        2. 3.3.3.2 CINIT
    4. 3.4 Arguments to main
    5. 3.5 Run-Time Relocation
    6. 3.6 Additional Information
  6. 4 Assembler Description
    1. 4.1  Assembler Overview
    2. 4.2  The Assembler's Role in the Software Development Flow
    3. 4.3  Invoking the Assembler
    4. 4.4  Controlling Application Binary Interface
    5. 4.5  Naming Alternate Directories for Assembler Input
      1. 4.5.1 Using the --include_path Assembler Option
      2. 4.5.2 Using the C2000_A_DIR Environment Variable
    6. 4.6  Source Statement Format
      1. 4.6.1 Label Field
      2. 4.6.2 Mnemonic Field
      3. 4.6.3 Operand Field
      4. 4.6.4 Comment Field
    7. 4.7  Literal Constants
      1. 4.7.1 Integer Literals
        1. 4.7.1.1 Binary Integer Literals
        2. 4.7.1.2 Octal Integer Literals
        3. 4.7.1.3 Decimal Integer Literals
        4. 4.7.1.4 Hexadecimal Integer Literals
        5. 4.7.1.5 Character Literals
      2. 4.7.2 Character String Literals
      3. 4.7.3 Floating-Point Literals
    8. 4.8  Assembler Symbols
      1. 4.8.1 Identifiers
      2. 4.8.2 Labels
      3. 4.8.3 Local Labels
        1. 4.8.3.1 Local Labels of the Form $n
        2.       85
        3. 4.8.3.2 Local Labels of the Form name?
        4.       87
      4. 4.8.4 Symbolic Constants
      5. 4.8.5 Defining Symbolic Constants (--asm_define Option)
      6. 4.8.6 Predefined Symbolic Constants
      7. 4.8.7 Registers
      8. 4.8.8 Substitution Symbols
    9. 4.9  Expressions
      1. 4.9.1 Mathematical and Logical Operators
      2. 4.9.2 Relational Operators and Conditional Expressions
      3. 4.9.3 Well-Defined Expressions
      4. 4.9.4 Legal Expressions
    10. 4.10 Built-in Functions and Operators
      1. 4.10.1 Built-In Math and Trigonometric Functions
    11. 4.11 TMS320C28x Assembler Extensions
      1. 4.11.1 C28x Support
      2. 4.11.2 C28x FPU32 and FPU64 Extensions
      3. 4.11.3 C28x CLA Extensions
    12. 4.12 Source Listings
    13. 4.13 Debugging Assembly Source
    14. 4.14 Cross-Reference Listings
    15. 4.15 Smart Encoding
    16. 4.16 Pipeline Conflict Detection
      1. 4.16.1 Protected and Unprotected Pipeline Instructions
      2. 4.16.2 Pipeline Conflict Prevention and Detection
      3. 4.16.3 Pipeline Conflicts Detected
  7. 5 Assembler Directives
    1. 5.1  Directives Summary
    2. 5.2  Directives that Define Sections
    3. 5.3  Directives that Initialize Values
    4. 5.4  Directives that Perform Alignment and Reserve Space
    5. 5.5  Directives that Format the Output Listings
    6. 5.6  Directives that Reference Other Files
    7. 5.7  Directives that Enable Conditional Assembly
    8. 5.8  Directives that Define Union or Structure Types
    9. 5.9  Directives that Define Enumerated Types
    10. 5.10 Directives that Define Symbols at Assembly Time
    11. 5.11 Miscellaneous Directives
    12. 5.12 Directives Reference
      1.      .align
      2.      .asg/.define/.eval
      3.      .asmfunc/.endasmfunc
      4.      .bits
      5.      .bss
      6.      .byte/.ubyte/.char/.uchar
      7.      .cdecls
      8.      .clink
      9.      .common
      10.      .copy/.include
      11.      .cstruct/.cunion/.endstruct/.endunion/.tag
      12.      .data
      13.      .drlist/.drnolist
      14.      .elfsym
      15.      .emsg/.mmsg/.wmsg
      16.      .end
      17.      .fclist/.fcnolist
      18.      .field
      19.      .float/.xfloat/.xldouble
      20.      .global/.def/.ref
      21.      .group/.gmember/.endgroup
      22.      .if/.elseif/.else/.endif
      23.      .int/.unint/.word/.uword
      24.      .label
      25.      .length/.width
      26.      .list/.nolist
      27.      .long/.ulong/.xlong
      28.      .loop/.endloop/.break
      29.      .macro/.endm
      30.      .mlib
      31.      .mlist/.mnolist
      32.      .newblock
      33.      .option
      34.      .page
      35.      .retain / .retainrefs
      36.      .sblock
      37.      .sect
      38.      .set
      39.      .space/.bes
      40.      .sslist/.ssnolist
      41.      .string/.cstring/.pstring
      42.      .struct/.endstruct/.tag
      43.      .symdepend
      44.      .tab
      45.      .text
      46.      .title
      47.      .unasg/.undefine
      48.      .union/.endunion/.tag
      49.      .usect
      50.      .var
      51.      .weak
  8. 6 Macro Language Description
    1. 6.1  Using Macros
    2. 6.2  Defining Macros
    3. 6.3  Macro Parameters/Substitution Symbols
      1. 6.3.1 Directives That Define Substitution Symbols
      2. 6.3.2 Built-In Substitution Symbol Functions
      3. 6.3.3 Recursive Substitution Symbols
      4. 6.3.4 Forced Substitution
      5. 6.3.5 Accessing Individual Characters of Subscripted Substitution Symbols
      6. 6.3.6 Substitution Symbols as Local Variables in Macros
    4. 6.4  Macro Libraries
    5. 6.5  Using Conditional Assembly in Macros
    6. 6.6  Using Labels in Macros
    7. 6.7  Producing Messages in Macros
    8. 6.8  Using Directives to Format the Output Listing
    9. 6.9  Using Recursive and Nested Macros
    10. 6.10 Macro Directives Summary
  9. 7 Archiver Description
    1. 7.1 Archiver Overview
    2. 7.2 The Archiver's Role in the Software Development Flow
    3. 7.3 Invoking the Archiver
    4. 7.4 Archiver Examples
    5. 7.5 Library Information Archiver Description
      1. 7.5.1 Invoking the Library Information Archiver
      2. 7.5.2 Library Information Archiver Example
      3. 7.5.3 Listing the Contents of an Index Library
      4. 7.5.4 Requirements
  10. 8 Linker Description
    1. 8.1  Linker Overview
    2. 8.2  The Linker's Role in the Software Development Flow
    3. 8.3  Invoking the Linker
    4. 8.4  Linker Options
      1. 8.4.1  Wildcards in File, Section, and Symbol Patterns
      2. 8.4.2  Specifying C/C++ Symbols with Linker Options
      3. 8.4.3  Relocation Capabilities (--absolute_exe and --relocatable Options)
        1. 8.4.3.1 Producing an Absolute Output Module (--absolute_exe option)
        2. 8.4.3.2 Producing a Relocatable Output Module (--relocatable option)
      4. 8.4.4  Allocate Memory for Use by the Loader to Pass Arguments (--arg_size Option)
      5. 8.4.5  Compression (--cinit_compression and --copy_compression Option)
      6. 8.4.6  Compress DWARF Information (--compress_dwarf Option)
      7. 8.4.7  Control Linker Diagnostics
      8. 8.4.8  Automatic Library Selection (--disable_auto_rts Option)
      9. 8.4.9  Disable Conditional Linking (--disable_clink Option)
      10. 8.4.10 Do Not Remove Unused Sections (--unused_section_elimination Option)
      11. 8.4.11 Linker Command File Preprocessing (--disable_pp, --define and --undefine Options)
      12. 8.4.12 Error Correcting Code Testing (--ecc Options)
      13. 8.4.13 Define an Entry Point (--entry_point Option)
      14. 8.4.14 Set Default Fill Value (--fill_value Option)
      15. 8.4.15 Define Heap Size (--heap_size Option)
      16. 8.4.16 Hiding Symbols
      17. 8.4.17 Alter the Library Search Algorithm (--library, --search_path, and C2000_C_DIR )
        1. 8.4.17.1 Name an Alternate Library Directory (--search_path Option)
        2. 8.4.17.2 Name an Alternate Library Directory ( C2000_C_DIR Environment Variable)
        3. 8.4.17.3 Exhaustively Read and Search Libraries (--reread_libs and --priority Options)
      18. 8.4.18 Change Symbol Localization
        1. 8.4.18.1 Make All Global Symbols Static (--make_static Option)
      19. 8.4.19 Create a Map File (--map_file Option)
      20. 8.4.20 Manage Map File Contents (--mapfile_contents Option)
      21. 8.4.21 Disable Name Demangling (--no_demangle)
      22. 8.4.22 Disable Merging of Symbolic Debugging Information (--no_sym_merge Option)
      23. 8.4.23 Strip Symbolic Information (--no_symtable Option)
      24. 8.4.24 Name an Output Module (--output_file Option)
      25. 8.4.25 Prioritizing Function Placement (--preferred_order Option)
      26. 8.4.26 C Language Options (--ram_model and --rom_model Options)
      27. 8.4.27 Retain Discarded Sections (--retain Option)
      28. 8.4.28 Create an Absolute Listing File (--run_abs Option)
      29. 8.4.29 Scan All Libraries for Duplicate Symbol Definitions (--scan_libraries)
      30. 8.4.30 Define Stack Size (--stack_size Option)
      31. 8.4.31 Mapping of Symbols (--symbol_map Option)
      32. 8.4.32 Introduce an Unresolved Symbol (--undef_sym Option)
      33. 8.4.33 Display a Message When an Undefined Output Section Is Created (--warn_sections)
      34. 8.4.34 Generate XML Link Information File (--xml_link_info Option)
      35. 8.4.35 Zero Initialization (--zero_init Option)
    5. 8.5  Linker Command Files
      1. 8.5.1  Reserved Names in Linker Command Files
      2. 8.5.2  Constants in Linker Command Files
      3. 8.5.3  Accessing Files and Libraries from a Linker Command File
      4. 8.5.4  The MEMORY Directive
        1. 8.5.4.1 Default Memory Model
        2. 8.5.4.2 MEMORY Directive Syntax
        3. 8.5.4.3 Expressions and Address Operators
      5. 8.5.5  The SECTIONS Directive
        1. 8.5.5.1 SECTIONS Directive Syntax
        2. 8.5.5.2 Section Allocation and Placement
          1. 8.5.5.2.1 Example: Placing Functions in RAM
          2. 8.5.5.2.2 Binding
          3. 8.5.5.2.3 Named Memory
          4. 8.5.5.2.4 Controlling Placement Using The HIGH Location Specifier
            1. 8.5.5.2.4.1 Linker Placement With the HIGH Specifier
            2.         265
            3. 8.5.5.2.4.2 Linker Placement Without HIGH Specifier
          5. 8.5.5.2.5 Alignment and Blocking
          6. 8.5.5.2.6 Alignment With Padding
          7. 8.5.5.2.7 Using the Page Method
        3. 8.5.5.3 Specifying Input Sections
          1. 8.5.5.3.1 The Most Common Method of Specifying Section Contents
          2.        272
        4. 8.5.5.4 Using Multi-Level Subsections
        5. 8.5.5.5 Specifying Library or Archive Members as Input to Output Sections
          1. 8.5.5.5.1 Archive Members to Output Sections
          2.        276
        6. 8.5.5.6 Allocation Using Multiple Memory Ranges
        7. 8.5.5.7 Automatic Splitting of Output Sections Among Non-Contiguous Memory Ranges
      6. 8.5.6  Placing a Section at Different Load and Run Addresses
        1. 8.5.6.1 Specifying Load and Run Addresses
        2.       281
        3. 8.5.6.2 Referring to the Load Address by Using the .label Directive
      7. 8.5.7  Using GROUP and UNION Statements
        1. 8.5.7.1 Grouping Output Sections Together
        2. 8.5.7.2 Overlaying Sections With the UNION Statement
        3. 8.5.7.3 Using Memory for Multiple Purposes
        4. 8.5.7.4 Nesting UNIONs and GROUPs
        5. 8.5.7.5 Checking the Consistency of Allocators
        6. 8.5.7.6 Naming UNIONs and GROUPs
      8. 8.5.8  Overlaying Pages
        1. 8.5.8.1 Using the MEMORY Directive to Define Overlay Pages
        2. 8.5.8.2 Example of Overlay Pages
          1. 8.5.8.2.1 MEMORY Directive With Overlay Pages
          2.        294
        3. 8.5.8.3 Using Overlay Pages With the SECTIONS Directive
          1. 8.5.8.3.1 SECTIONS Directive Definition for Overlays
          2.        297
        4. 8.5.8.4 Memory Allocation for Overlaid Pages
      9. 8.5.9  Special Section Types (DSECT, COPY, NOLOAD, and NOINIT)
      10. 8.5.10 Configuring Error Correcting Code (ECC) with the Linker
        1. 8.5.10.1 Using the ECC Specifier in the Memory Map
        2. 8.5.10.2 Using the ECC Directive
        3. 8.5.10.3 Using the VFILL Specifier in the Memory Map
      11. 8.5.11 Assigning Symbols at Link Time
        1. 8.5.11.1 Syntax of Assignment Statements
        2. 8.5.11.2 Assigning the SPC to a Symbol
        3. 8.5.11.3 Assignment Expressions
        4. 8.5.11.4 Symbols Automatically Defined by the Linker
        5. 8.5.11.5 Assigning Exact Start, End, and Size Values of a Section to a Symbol
        6. 8.5.11.6 Why the Dot Operator Does Not Always Work
        7. 8.5.11.7 Address and Dimension Operators
          1. 8.5.11.7.1 Input Items
          2. 8.5.11.7.2 Output Section
          3. 8.5.11.7.3 GROUPs
          4. 8.5.11.7.4 UNIONs
        8. 8.5.11.8 LAST Operator
      12. 8.5.12 Creating and Filling Holes
        1. 8.5.12.1 Initialized and Uninitialized Sections
        2. 8.5.12.2 Creating Holes
        3. 8.5.12.3 Filling Holes
        4. 8.5.12.4 Explicit Initialization of Uninitialized Sections
    6. 8.6  Linker Symbols
      1. 8.6.1 Linker-Defined Functions and Arrays
      2. 8.6.2 Linker-Defined Integer Values
      3. 8.6.3 Linker-Defined Addresses
      4. 8.6.4 More About the _symval Operator
      5. 8.6.5 Weak Symbols
        1. 8.6.5.1 Weak Symbol References
        2. 8.6.5.2 Weak Symbol Definitions
      6. 8.6.6 Resolving Symbols with Object Libraries
    7. 8.7  Default Placement Algorithm
      1. 8.7.1 How the Allocation Algorithm Creates Output Sections
      2. 8.7.2 Reducing Memory Fragmentation
    8. 8.8  Using Linker-Generated Copy Tables
      1. 8.8.1 Using Copy Tables for Boot Loading
      2. 8.8.2 Using Built-in Link Operators in Copy Tables
      3. 8.8.3 Overlay Management Example
      4. 8.8.4 Generating Copy Tables With the table() Operator
        1. 8.8.4.1 The table() Operator
        2. 8.8.4.2 Boot-Time Copy Tables
        3. 8.8.4.3 Using the table() Operator to Manage Object Components
        4. 8.8.4.4 Linker-Generated Copy Table Sections and Symbols
        5. 8.8.4.5 Splitting Object Components and Overlay Management
      5. 8.8.5 Compression
        1. 8.8.5.1 Compressed Copy Table Format
        2. 8.8.5.2 Compressed Section Representation in the Object File
        3. 8.8.5.3 Compressed Data Layout
        4. 8.8.5.4 Run-Time Decompression
        5. 8.8.5.5 Compression Algorithms
        6.       350
      6. 8.8.6 Copy Table Contents
      7. 8.8.7 General Purpose Copy Routine
    9. 8.9  Linker-Generated CRC Tables and CRC Over Memory Ranges
      1. 8.9.1 Using the crc_table() Operator in the SECTIONS Directive
        1. 8.9.1.1 Restrictions when using the crc_table() Operator
        2. 8.9.1.2 Examples
          1. 8.9.1.2.1 Using crc_table() Operator to Compute the CRC Value for .text Data
          2.        358
          3. 8.9.1.2.2 Specifying an Algorithm in the crc_table() Operator
          4.        360
          5. 8.9.1.2.3 Using a Single Table for Multiple Sections
          6.        362
          7. 8.9.1.2.4 Applying the crc_table() Operator to a GROUP or UNION
          8.        364
        3. 8.9.1.3 Interface When Using the crc_table() Operator
          1. 8.9.1.3.1 The CRC Table Header, crc_tbl.h
      2. 8.9.2 Using the crc() Operator in the MEMORY Directive
        1. 8.9.2.1 Restrictions when Using the crc() Operator
        2. 8.9.2.2 Using the VFILL Specifier within a GROUP
        3. 8.9.2.3 Generate CRC for Most or All of Flash Memory
        4. 8.9.2.4 Computing CRCs for Both Memory Ranges and Sections
        5. 8.9.2.5 Example Specifying Memory Range CRCs
        6. 8.9.2.6 Interface When Using the crc() Operator
      3. 8.9.3 A Special Note Regarding 16-Bit char
    10. 8.10 Partial (Incremental) Linking
    11. 8.11 Linking C/C++ Code
      1. 8.11.1 Run-Time Initialization
      2. 8.11.2 Object Libraries and Run-Time Support
      3. 8.11.3 Setting the Size of the Stack and Heap Sections
      4. 8.11.4 Initializing and AutoInitialzing Variables at Run Time
    12. 8.12 Linker Example
  11. 9 Absolute Lister Description
    1. 9.1 Producing an Absolute Listing
    2. 9.2 Invoking the Absolute Lister
    3. 9.3 Absolute Lister Example
  12. 10Cross-Reference Lister Description
    1. 10.1 Producing a Cross-Reference Listing
    2. 10.2 Invoking the Cross-Reference Lister
    3. 10.3 Cross-Reference Listing Example
  13. 11Object File Utilities
    1. 11.1 Invoking the Object File Display Utility
    2. 11.2 Invoking the Disassembler
    3. 11.3 Invoking the Name Utility
    4. 11.4 Invoking the Strip Utility
  14. 12Hex Conversion Utility Description
    1. 12.1  The Hex Conversion Utility's Role in the Software Development Flow
    2. 12.2  Invoking the Hex Conversion Utility
      1. 12.2.1 Invoking the Hex Conversion Utility From the Command Line
      2. 12.2.2 Invoking the Hex Conversion Utility With a Command File
    3. 12.3  Understanding Memory Widths
      1. 12.3.1 Target Width
      2. 12.3.2 Specifying the Memory Width
      3. 12.3.3 Partitioning Data Into Output Files
      4. 12.3.4 Specifying Word Order for Output Words
    4. 12.4  The ROMS Directive
      1. 12.4.1 When to Use the ROMS Directive
      2. 12.4.2 An Example of the ROMS Directive
    5. 12.5  The SECTIONS Directive
    6. 12.6  The Load Image Format (--load_image Option)
      1. 12.6.1 Load Image Section Formation
      2. 12.6.2 Load Image Characteristics
    7. 12.7  Excluding a Specified Section
    8. 12.8  Assigning Output Filenames
    9. 12.9  Image Mode and the --fill Option
      1. 12.9.1 Generating a Memory Image
      2. 12.9.2 Specifying a Fill Value
      3. 12.9.3 Steps to Follow in Using Image Mode
    10. 12.10 Array Output Format
    11. 12.11 Building a Table for an On-Chip Boot Loader
      1. 12.11.1 Description of the Boot Table
      2. 12.11.2 The Boot Table Format
      3. 12.11.3 How to Build the Boot Table
        1. 12.11.3.1 Building the Boot Table
        2. 12.11.3.2 Leaving Room for the Boot Table
      4. 12.11.4 Booting From a Device Peripheral
      5. 12.11.5 Setting the Entry Point for the Boot Table
      6. 12.11.6 Using the C28x Boot Loader
        1. 12.11.6.1 Sample Command File for Booting From 8-Bit SPI Boot
        2.       429
        3. 12.11.6.2 Sample Command File for C28x 16-Bit Parallel Boot GP I/O
        4.       431
        5. 12.11.6.3 Sample Command File for Booting From 8-Bit SCI Boot
        6.       433
    12. 12.12 Using Secure Flash Boot on TMS320F2838x Devices
    13. 12.13 Controlling the ROM Device Address
    14. 12.14 Control Hex Conversion Utility Diagnostics
    15. 12.15 Description of the Object Formats
      1. 12.15.1 ASCII-Hex Object Format (--ascii Option)
      2. 12.15.2 Intel MCS-86 Object Format (--intel Option)
      3. 12.15.3 Motorola Exorciser Object Format (--motorola Option)
      4. 12.15.4 Extended Tektronix Object Format (--tektronix Option)
      5. 12.15.5 Texas Instruments SDSMAC (TI-Tagged) Object Format (--ti_tagged Option)
      6. 12.15.6 TI-TXT Hex Format (--ti_txt Option)
        1. 12.15.6.1 TI-TXT Object Format
    16. 12.16 Hex Conversion Utility Error Messages
  15. 13Sharing C/C++ Header Files With Assembly Source
    1. 13.1 Overview of the .cdecls Directive
    2. 13.2 Notes on C/C++ Conversions
      1. 13.2.1  Comments
      2. 13.2.2  Conditional Compilation (#if/#else/#ifdef/etc.)
      3. 13.2.3  Pragmas
      4. 13.2.4  The #error and #warning Directives
      5. 13.2.5  Predefined symbol __ASM_HEADER__
      6. 13.2.6  Usage Within C/C++ asm( ) Statements
      7. 13.2.7  The #include Directive
      8. 13.2.8  Conversion of #define Macros
      9. 13.2.9  The #undef Directive
      10. 13.2.10 Enumerations
      11. 13.2.11 C Strings
      12. 13.2.12 C/C++ Built-In Functions
      13. 13.2.13 Structures and Unions
      14. 13.2.14 Function/Variable Prototypes
      15. 13.2.15 C Constant Suffixes
      16. 13.2.16 Basic C/C++ Types
    3. 13.3 Notes on C++ Specific Conversions
      1. 13.3.1 Name Mangling
      2. 13.3.2 Derived Classes
      3. 13.3.3 Templates
      4. 13.3.4 Virtual Functions
    4. 13.4 Special Assembler Support
      1. 13.4.1 Enumerations (.enum/.emember/.endenum)
      2. 13.4.2 The .define Directive
      3. 13.4.3 The .undefine/.unasg Directives
      4. 13.4.4 The $defined( ) Built-In Function
      5. 13.4.5 The $sizeof Built-In Function
      6. 13.4.6 Structure/Union Alignment and $alignof( )
      7. 13.4.7 The .cstring Directive
  16.   A Symbolic Debugging Directives
    1.     A.1 DWARF Debugging Format
    2.     A.2 Debug Directive Syntax
  17.   B XML Link Information File Description
    1.     B.1 XML Information File Element Types
    2.     B.2 Document Elements
      1.      B.2.1 Header Elements
      2.      B.2.2 Input File List
      3.      B.2.3 Object Component List
      4.      B.2.4 Logical Group List
      5.      B.2.5 Placement Map
      6.      B.2.6 Symbol Table
  18.   C CRC Reference Implementation
    1.     C.1 Compilation Instructions
    2.     C.2 Reference CRC Calculation Routine
      1.      C.2.1 Reference Implementation of a CRC Calculation Function: ref_crc.c
    3.     C.3 Linker-Generated Copy Tables and CRC Tables
      1.      C.3.1 Main Routine for Example Application: main.c
      2.      C.3.2 Checking CRC Values: check_crc.c
      3.      C.3.3 Task1 Routine: task1.c
      4.      C.3.4 Task2 Routine: task2.c
      5.      C.3.5 Task3 Routine: task3.c
      6.      C.3.6 Example 1 Command File: ex1.cmd (for COFF)
  19.   D Glossary
    1.     D.1 Terminology
  20.   E Revision History

2.6.3 Weak Symbols

Weak symbols are symbols that may or may not be defined.

Note:

Weak symbols are supported only in EABI mode.

The linker processes symbols that are defined with a "weak" binding differently from symbols that are defined with global binding. Instead of including a weak symbol in the object file's symbol table (as it would for a global symbol), the linker only includes a weak symbol in the output of a "final" link if the symbol is required to resolve an otherwise unresolved reference.

This allows the linker to minimize the number of symbols it includes in the output file's symbol table by omitting those that are not needed to resolve references. Reducing the size of the output file's symbol table reduces the time required to link, especially if there are a large number of pre-loaded symbols to link against.

You can define a weak symbol using either the .weak assembly directive or the weak operator in the linker command file.

  • Using Assembly: To define a weak symbol in an input object file, the source file can be written in assembly. Use the .weak and .set directives in combination as shown in the following example, which defines a weak symbol "ext_addr_sym": 
                    .weak   ext_addr_sym
ext_addr_sym    .set    0x12345678

    Assemble the source file that defines weak symbols, and include the resulting object file in the link. The "ext_addr_sym" in this example is available as a weak symbol in a final link. It is a candidate for removal if the symbol is not referenced elsewhere in the application. See .weak directive.

  • Using the Linker Command File: To define a weak symbol in a linker command file, use the "weak" operator in an assignment expression to designate that the symbol as eligible for removal from the output file's symbol table if it is not referenced. In a linker command file, an assignment expression outside a MEMORY or SECTIONS directive can be used to define a weak linker-defined symbol. For example, you can define "ext_addr_sym" as follows: 
    weak(ext_addr_sym) = 0x12345678;

    If the linker command file performs the final link, then "ext_addr_sym" is presented to the linker as a weak symbol; it will not be included in the resulting output file if the symbol is not referenced. See Section 8.6.5.

  • Using C/C++ code: See information about the WEAK pragma and weak GCC-style variable attribute in the TMS320C28x Optimizing C/C++ Compiler User's Guide.

If there are multiple definitions of the same symbol, the linker uses certain rules to determine which definition takes precedence. Some definitions may have weak binding and others may have strong binding. "Strong" in this context means that the symbol has not been given a weak binding by either of the two methods described above. Some definitions may come from an input object file (that is, using assembly directives) and others may come from an assignment statement in a linker command file.

The linker uses the following guidelines to determine which definition is used when resolving references to a symbol:

  • A strongly bound symbol always takes precedence over a weakly bound symbol.
  • If two symbols are both strongly bound or both weakly bound, a symbol defined in a linker command file takes precedence over a symbol defined in an input object file.
  • If two symbols are both strongly bound and both are defined in an input object file, the linker provides a symbol redefinition error and halts the link process.