6.1.4 Sections

The compiler produces relocatable blocks of code and data called sections. The sections are allocated into memory in a variety of ways to conform to a variety of system configurations. For more information about sections and allocating them, see the introductory object file information in the MSP430 Assembly Language Tools User's Guide.

There are two basic types of sections:

• Initialized sections contain data or executable code. Initialized sections are usually, but not always, read-only. The C/C++ compiler creates the following initialized sections:
  • The .binit section contains boot time copy tables. This is a read-only section. For details on BINIT, see the MSP430 Assembly Language Tools User's Guide.
  • The .cinit section contains tables for initializing variables and constants. This is a read-only section. The compiler does not create this section; instead, the linker does.
  • The .init_array section contains the table of pointers to initialization routines for global C++ objects.
  • The .ovly section contains copy tables for unions in which different sections have the same run address. This is a read-only section.
  • The .data section contains initialized global and static variables.
  • For EABI only, the .mspabi.exidx section contains the index table for exception handling. The .mspabi.extab section contains unwinding instructions for exception handling. These sections are read-only. See the --exceptions option for details.
  • The .const section contains string constants, string literals, switch tables, and data defined with the C/C++ qualifier const (provided the constant is not also defined as volatile or one of the exceptions described in Section 5.8.1). This is a read-only section. String literals are placed in the .const:.string subsection to enable greater link-time placement control.
  • The .text section contains all the executable code and compiler-generated constants. This section is usually read-only.
  • The .TI.crctab section contains CRC checking tables. This is a read-only section.
• Uninitialized sections reserve space in memory (usually RAM). A program can use this space at run time to create and store variables. The compiler creates the following uninitialized sections:
  • The .bss section reserves space for global and static variables. At boot or load time, the C/C++ boot routine or the loader copies data out of the .cinit section (which can be in ROM) and stores it in the .bss section.
  • For EABI only, the .bss section reserves space for uninitialized global and static variables. Uninitialized variables that are also unused are usually created as common symbols (unless you specify --common=off) instead of being placed in .bss so that they can be excluded from the resulting application.
  • The .stack section reserves memory for the C/C++ software stack.
  • The .sysmem section reserves space for dynamic memory allocation. This space is used by dynamic memory allocation routines, such as malloc(), calloc(), realloc(), or new(). If a C/C++ program does not use these functions, the compiler does not create the .sysmem section.

The assembler creates the default sections .text, .bss, and .data. You can instruct the compiler to create additional sections by using the CODE_SECTION and DATA_SECTION pragmas (see Section 5.12.6 and Section 5.12.8).

The linker takes the individual sections from different object files and combines sections that have the same name. The resulting output sections and the appropriate placement in memory for each section are listed in Table 6-1. You can place these output sections anywhere in the address space as needed to meet system requirements.

You can use the SECTIONS directive in the linker command file to customize the section-allocation process. For more information about allocating sections into memory, see the linker description chapter in the MSP430 Assembly Language Tools User's Guide.