SPRUI03E June   2015  – January 2023

 

  1.   Read This First
    1.     About This Manual
    2.     How to Use This Manual
    3.     Notational Conventions
    4.     Related Documentation From Texas Instruments
    5.     Trademarks
  2. 1 Introduction to the Software Development Tools
    1. 1.1 Software Development Tools Overview
    2. 1.2 Tools Descriptions
  3. 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
    8. 2.8 Loading a Program
  4. 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
    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
  5. 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  The 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 C6X_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 Unit Specifier Field
      4. 4.6.4 Operand Field
      5. 4.6.5 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.       80
        3. 4.8.3.2 Local Labels of the Form name?
        4.       82
      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  Register Pairs
      9. 4.8.9  Register Quads (C6600 Only)
      10. 4.8.10 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
      5. 4.9.5 Expression Examples
    10. 4.10 Built-in Functions and Operators
      1. 4.10.1 Built-In Math and Trigonometric Functions
      2. 4.10.2 C6x Built-In ELF Relocation Generating Operators
        1. 4.10.2.1 $DPR_BYTE(sym) / $DPR_HWORD(sym) / $DPR_WORD(sym)
        2. 4.10.2.2 $LABEL_DIFF(x,y) Operator
    11. 4.11 Source Listings
    12. 4.12 Debugging Assembly Source
    13. 4.13 Cross-Reference Listings
  6. 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.      .common
      9.      .copy/.include
      10.      .cstruct/.cunion/.endstruct/.endunion/.tag
      11.      .data
      12.      .double
      13.      .drlist/.drnolist
      14.      .elfsym
      15.      .emsg/.mmsg/.wmsg
      16.      .end
      17.      .farcommon/.nearcommon
      18.      .fclist/.fcnolist
      19.      .field
      20.      .float
      21.      .global/.def/.ref
      22.      .group/.gmember/.endgroup
      23.      .half/.short/.uhalf/.ushort
      24.      .if/.elseif/.else/.endif
      25.      .int/.unint/.long/.ulong/.word/.uword
      26.      .label
      27.      .length/.width
      28.      .list/.nolist
      29.      .loop/.endloop/.break
      30.      .macro/.endm
      31.      .map/.clearmap
      32.      .mlib
      33.      .mlist/.mnolist
      34.      .newblock
      35.      .nocmp
      36.      .noremark/.remark
      37.      .option
      38.      .page
      39.      .retain / .retainrefs
      40.      .sect
      41.      .set/.equ
      42.      .space/.bes
      43.      .sslist/.ssnolist
      44.      .string/.cstring
      45.      .struct/.endstruct/.tag
      46.      .symdepend
      47.      .tab
      48.      .text
      49.      .title
      50.      .unasg/.undefine
      51.      .union/.endunion/.tag
      52.      .usect
      53.      .var
      54.      .weak
  7. 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
  8. 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
  9. 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 and --multithread Options)
      9. 8.4.9  Do Not Remove Unused Sections (--unused_section_elimination Option)
      10. 8.4.10 Linker Command File Preprocessing (--disable_pp, --define and --undefine Options)
      11. 8.4.11 Error Correcting Code Testing (--ecc Options)
      12. 8.4.12 Define an Entry Point (--entry_point Option)
      13. 8.4.13 Set Default Fill Value (--fill_value Option)
      14. 8.4.14 Define Heap Size (--heap_size Option)
      15. 8.4.15 Hiding Symbols
      16. 8.4.16 Alter the Library Search Algorithm (--library, --search_path, and C6X_C_DIR )
        1. 8.4.16.1 Name an Alternate Library Directory (--search_path Option)
        2. 8.4.16.2 Name an Alternate Library Directory (C6X_C_DIR Environment Variable)
        3. 8.4.16.3 Exhaustively Read and Search Libraries (--reread_libs and --priority Options)
      17. 8.4.17 Change Symbol Localization
        1. 8.4.17.1 Make All Global Symbols Static (--make_static Option)
      18. 8.4.18 Create a Map File (--map_file Option)
      19. 8.4.19 Managing Map File Contents (--mapfile_contents Option)
      20. 8.4.20 Disable Name Demangling (--no_demangle)
      21. 8.4.21 Merging of Symbolic Debugging Information
      22. 8.4.22 Strip Symbolic Information (--no_symtable Option)
      23. 8.4.23 Name an Output Module (--output_file Option)
      24. 8.4.24 Prioritizing Function Placement (--preferred_order Option)
      25. 8.4.25 C Language Options (--ram_model and --rom_model Options)
      26. 8.4.26 Retain Discarded Sections (--retain Option)
      27. 8.4.27 Scan All Libraries for Duplicate Symbol Definitions (--scan_libraries)
      28. 8.4.28 Define Stack Size (--stack_size Option)
      29. 8.4.29 Mapping of Symbols (--symbol_map Option)
      30. 8.4.30 Generate Far Call Trampolines (--trampolines Option)
        1. 8.4.30.1 Advantages and Disadvantages of Using Trampolines
        2. 8.4.30.2 Minimizing the Number of Trampolines Required (--minimize_trampolines Option)
        3. 8.4.30.3 Making Trampoline Reservations Adjacent (--trampoline_min_spacing Option)
        4. 8.4.30.4 Carrying Trampolines From Load Space to Run Space
      31. 8.4.31 Introduce an Unresolved Symbol (--undef_sym Option)
      32. 8.4.32 Display a Message When an Undefined Output Section Is Created (--warn_sections)
      33. 8.4.33 Generate XML Link Information File (--xml_link_info Option)
      34. 8.4.34 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 Binding
          2. 8.5.5.2.2 Named Memory
          3. 8.5.5.2.3 Controlling Placement Using The HIGH Location Specifier
            1. 8.5.5.2.3.1 Linker Placement With the HIGH Specifier
            2.         262
            3. 8.5.5.2.3.2 Linker Placement Without HIGH Specifier
          4. 8.5.5.2.4 Alignment and Blocking
          5. 8.5.5.2.5 Alignment With Padding
        3. 8.5.5.3 Specifying Input Sections
          1. 8.5.5.3.1 The Most Common Method of Specifying Section Contents
          2.        268
        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.        272
        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.       277
        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  Special Section Types (DSECT, COPY, NOLOAD, and NOINIT)
      9. 8.5.9  Configuring Error Correcting Code (ECC) with the Linker
        1. 8.5.9.1 Using the ECC Specifier in the Memory Map
        2. 8.5.9.2 Using the ECC Directive
        3. 8.5.9.3 Using the VFILL Specifier in the Memory Map
      10. 8.5.10 Assigning Symbols at Link Time
        1. 8.5.10.1 Syntax of Assignment Statements
        2. 8.5.10.2 Assigning the SPC to a Symbol
        3. 8.5.10.3 Assignment Expressions
        4. 8.5.10.4 Symbols Automatically Defined by the Linker
        5. 8.5.10.5 Assigning Exact Start, End, and Size Values of a Section to a Symbol
        6. 8.5.10.6 Why the Dot Operator Does Not Always Work
        7. 8.5.10.7 Address and Dimension Operators
          1. 8.5.10.7.1 Input Items
          2. 8.5.10.7.2 Output Section
          3. 8.5.10.7.3 GROUPs
          4. 8.5.10.7.4 UNIONs
      11. 8.5.11 Creating and Filling Holes
        1. 8.5.11.1 Initialized and Uninitialized Sections
        2. 8.5.11.2 Creating Holes
        3. 8.5.11.3 Filling Holes
        4. 8.5.11.4 Explicit Initialization of Uninitialized Sections
    6. 8.6  Linker Symbols
      1. 8.6.1 Using Linker Symbols in C/C++ Applications
      2. 8.6.2 Declaring Weak Symbols
      3. 8.6.3 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.       331
      6. 8.8.6 Copy Table Contents
      7. 8.8.7 General Purpose Copy Routine
    9. 8.9  Partial (Incremental) Linking
    10. 8.10 Linking C/C++ Code
      1. 8.10.1 Run-Time Initialization
      2. 8.10.2 Object Libraries and Run-Time Support
      3. 8.10.3 Setting the Size of the Stack and Heap Sections
      4. 8.10.4 Initializing and AutoInitialzing Variables at Run Time
    11. 8.11 Linker Example
  10. 9 Object File Utilities
    1. 9.1 Invoking the Object File Display Utility
    2. 9.2 Invoking the Disassembler
    3. 9.3 Invoking the Name Utility
    4. 9.4 Invoking the Strip Utility
  11. 10Hex Conversion Utility Description
    1. 10.1  The Hex Conversion Utility's Role in the Software Development Flow
    2. 10.2  Invoking the Hex Conversion Utility
      1. 10.2.1 Invoking the Hex Conversion Utility From the Command Line
      2. 10.2.2 Invoking the Hex Conversion Utility With a Command File
    3. 10.3  Understanding Memory Widths
      1. 10.3.1 Target Width
      2. 10.3.2 Specifying the Memory Width
      3. 10.3.3 Partitioning Data Into Output Files
      4. 10.3.4 Specifying Word Order for Output Words
    4. 10.4  The ROMS Directive
      1. 10.4.1 When to Use the ROMS Directive
      2. 10.4.2 An Example of the ROMS Directive
    5. 10.5  The SECTIONS Directive
    6. 10.6  The Load Image Format (--load_image Option)
      1. 10.6.1 Load Image Section Formation
      2. 10.6.2 Load Image Characteristics
    7. 10.7  Excluding a Specified Section
    8. 10.8  Assigning Output Filenames
    9. 10.9  Image Mode and the --fill Option
      1. 10.9.1 Generating a Memory Image
      2. 10.9.2 Specifying a Fill Value
      3. 10.9.3 Steps to Follow in Using Image Mode
    10. 10.10 Array Output Format
    11. 10.11 Controlling the ROM Device Address
    12. 10.12 Control Hex Conversion Utility Diagnostics
    13. 10.13 Description of the Object Formats
      1. 10.13.1 ASCII-Hex Object Format (--ascii Option)
      2. 10.13.2 Intel MCS-86 Object Format (--intel Option)
      3. 10.13.3 Motorola Exorciser Object Format (--motorola Option)
      4. 10.13.4 Extended Tektronix Object Format (--tektronix Option)
      5. 10.13.5 Texas Instruments SDSMAC (TI-Tagged) Object Format (--ti_tagged Option)
      6. 10.13.6 TI-TXT Hex Format (--ti_txt Option)
        1. 10.13.6.1 TI-TXT Object Format
  12. 11Sharing C/C++ Header Files With Assembly Source
    1. 11.1 Overview of the .cdecls Directive
    2. 11.2 Notes on C/C++ Conversions
      1. 11.2.1  Comments
      2. 11.2.2  Conditional Compilation (#if/#else/#ifdef/etc.)
      3. 11.2.3  Pragmas
      4. 11.2.4  The #error and #warning Directives
      5. 11.2.5  Predefined symbol __ASM_HEADER__
      6. 11.2.6  Usage Within C/C++ asm( ) Statements
      7. 11.2.7  The #include Directive
      8. 11.2.8  Conversion of #define Macros
      9. 11.2.9  The #undef Directive
      10. 11.2.10 Enumerations
      11. 11.2.11 C Strings
      12. 11.2.12 C/C++ Built-In Functions
      13. 11.2.13 Structures and Unions
      14. 11.2.14 Function/Variable Prototypes
      15. 11.2.15 C Constant Suffixes
      16. 11.2.16 Basic C/C++ Types
    3. 11.3 Notes on C++ Specific Conversions
      1. 11.3.1 Name Mangling
      2. 11.3.2 Derived Classes
      3. 11.3.3 Templates
      4. 11.3.4 Virtual Functions
    4. 11.4 Special Assembler Support
      1. 11.4.1 Enumerations (.enum/.emember/.endenum)
      2. 11.4.2 The .define Directive
      3. 11.4.3 The .undefine/.unasg Directives
      4. 11.4.4 The $defined( ) Built-In Function
      5. 11.4.5 The $sizeof Built-In Function
      6. 11.4.6 Structure/Union Alignment and $alignof( )
      7. 11.4.7 The .cstring Directive
  13.   A Symbolic Debugging Directives
    1.     A.1 DWARF Debugging Format
    2.     A.2 Debug Directive Syntax
  14.   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 Far Call Trampoline List
      7.      B.2.7 Symbol Table
  15.   C Glossary
    1.     C.1 Terminology
  16.   D Revision History
  17.   428
  18.   D Earlier Revisions

.cstruct/.cunion/.endstruct/.endunion/.tag

Declare C Structure Type

Syntax

[stag]       .cstruct|.cunion    [expr]

[mem0]     element                 [expr0]
[mem1]     element                 [expr1] 
      .                 .                 . 
      .                 .                 . 

[memn]     .tag stag                [exprn]

[memN]     element                 [exprN]

[size]         .endstruct|.endunion

label       .tag                     stag

Description

The .cstruct and .cunion directives have been added to support ease of sharing of common data structures between assembly and C code. The .cstruct and .cunion directives can be used exactly like the existing .struct and .union directives except that they are guaranteed to perform data layout matching the layout used by the C compiler for C struct and union data types.

In particular, the .cstruct and .cunion directives force the same alignment and padding as used by the C compiler when such types are nested within compound data structures.

The .endstruct directive terminates the structure definition. The .endunion directive terminates the union definition.

The .tag directive gives structure characteristics to a label, simplifying the symbolic representation and providing the ability to define structures that contain other structures. The .tag directive does not allocate memory. The structure tag (stag) of a .tag directive must have been previously defined.

Following are descriptions of the parameters used with the .struct, .endstruct, and .tag directives:

  • The stag is the structure's tag. Its value is associated with the beginning of the structure. If no stag is present, the assembler puts the structure members in the global symbol table with the value of their absolute offset from the top of the structure. The stag is optional for .struct, but is required for .tag.
  • The element is one of the following descriptors: .byte, .char, .int, .long, .word, .double, .half, .short, .string, .float, and .field. All of these except .tag are typical directives that initialize memory. Following a .struct directive, these directives describe the structure element's size. They do not allocate memory. A .tag directive is a special case because stag must be used (as in the definition of stag).
  • The expr is an optional expression indicating the beginning offset of the structure. The default starting point for a structure is 0.
  • The exprn/N is an optional expression for the number of elements described. This value defaults to 1. A .string element is considered to be one byte in size, and a .field element is one bit.
  • The memn/N is an optional label for a member of the structure. This label is absolute and equates to the present offset from the beginning of the structure. A label for a structure member cannot be declared global.
  • The size is an optional label for the total size of the structure.

Example

This example illustrates a structure in C that will be accessed in assembly code.

typedef struct STRUCT1 
; {     int i0;      /* offset 0 */ 
;       short s0;    /* offset 4 */ 
; } struct1;         /* size 8, alignment 4 */ 
;
; typedef struct STRUCT2 
; {     struct1 st1; /* offset 0 */ 
;       short s1;    /* offset 8 */ 
; } struct2;         /* size 12, alignment 4 */ 
; 
; The structure will get the following offsets once the C compiler lays out the structure
; elements according to the C standard rules:
; 
; offsetof(struct1, i0) = 0 
; offsetof(struct1, s0) = 4 
; sizeof(struct1)       = 8  
; offsetof(struct2, s1) = 0 
; offsetof(struct2, i1) = 8 
; sizeof(struct2)       = 12 
; 
; Attempts to replicate this structure in assembly using the .struct/.union directives will not
; create the correct offsets because the assembler tries to use the most compact arrangement:
struct1     .struct 
i0          .int                ; bytes 0-3 
s0          .short; bytes 4-5 
struct1len  .endstruct          ; size 6, alignment 4 
struct2     .struct 
st1         .tag struct1        ; bytes 0-5 
s1          .short              ; bytes 6-7 
endstruct2  .endstruct          ; size 8, alignment 4 
        .sect "data1" 
        .word struct1.i0        ; 0 
        .word struct1.s0        ; 4 
        .word struct1len        ; 6 
        .sect "data2" 
        .word struct2.st1       ; 0 
        .word struct2.s1        ; 6 
        .word endstruct2        ; 8 
;
; The .cstruct/.cunion directives calculate offsets in the same manner as the C compiler. The resulting
; assembly structure can be used to access the elements of the C structure. Compare the difference 
; in the offsets of those structures defined via .struct above and the offsets for the C code.
cstruct1    .cstruct 
i0          .int                ; bytes 0-3 
s0          .short; bytes 4-5 
cstruct1len .endstruct          ; size 8, alignment 4 
cstruct2    .cstruct 
st1         .tag cstruct1       ; bytes 0-7 
s1          .short; bytes 8-9 
cendstruct2 .endstruct          ; size 12, alignment 4 
        .sect "data3" 
        .word cstruct1.i0, struct1.i0   ; 0 
        .word cstruct1.s0, struct1.s0   ; 4
        .word cstruct1len, struct1len   ; 8 
        .sect "data4" 
        .word cstruct2.st1, struct2.st1 ; 0 
        .word cstruct2.s1, struct2.s1   ; 8 
        .word cendstruct2, endstruct2   ; 12