Книга: Real-Time Concepts for Embedded Systems

2.3 Executable and Linking Format

2.3 Executable and Linking Format

Typically an object file contains

· general information about the object file, such as file size, binary code and data size, and source file name from which it was created,

· machine-architecture-specific binary instructions and data

· symbol table and the symbol relocation table, and

· debug information, which the debugger uses.

The manner in which this information is organized in the object file is the object file format. The idea behind a standard object file format is to allow development tools which might be produced by different vendors-such as a compiler, assembler, linker, and debugger that conform to the well-defined standard-to interoperate with each other.

This interoperability means a developer can choose a compiler from vendor A to produce object code used to form a final executable image by a linker from vendor B. This concept gives the end developer great flexibility in choice for development tools because the developer can select a tool based on its functional strength rather than its vendor.

Two common object file formats are the common object file format (COFF) and the executable and linking format (ELF). These file formats are incompatible with each other; therefore, be sure to select the tools, including the debugger, that recognize the format chosen for development.

We focus our discussion on ELF because it supersedes COFF. Understanding the object file format allows the embedded developer to map an executable image into the target embedded system for static storage, as well as for runtime loading and execution. To do so, we need to discuss the specifics of ELF, as well as how it relates to the linker.

Using the ELF object file format, the compiler organizes the compiled program into various system-defined, as well as user-defined, content groupings called sections. The program's binary instructions, binary data, symbol table, relocation table, and debug information are organized and contained in various sections. Each section has a type. Content is placed into a section if the section type matches the type of the content being stored.

A section also contains important information such as the load address and the run address. The concept of load address versus run address is important because the run address and the load address can be different in embedded systems. This knowledge can also be helpful in understanding embedded system loader and link loader concepts introduced in Chapter 3.

Chapter 1 discusses the idea that embedded systems typically have some form of ROM for non-volatile storage and that the software for an embedded system can be stored in ROM. Modifiable data must reside in RAM. Programs that require fast execution speed also execute out of RAM. Commonly therefore, a small program in ROM, called a loader, copies the initialized variables into RAM, transfers the program code into RAM, and begins program execution out of RAM. This physical ROM storage address is referred to as the section's load address. The section's run address refers to the location where the section is at the time of execution. For example, if a section is copied into RAM for execution, the section's run address refers to an address in RAM, which is the destination address of the loader copy operation. The linker uses the program's run address for symbol resolutions.

The ELF file format has two different interpretations, as shown in Figure 2.4. The linker interprets the file as a linkable module described by the section header table, while the loader interprets the file as an executable module described by the program header table.

Figure 2.4: Executable and linking format.

Listing 2.1 shows both the section header and the program header, as represented in C programming structures. We describe the relevant fields during the course of this discussion.

Listing 2.1: Section header and program header.

typedef struct {
 Elf32_Word sh_name;
 Elf32_Word sh_type;
 Elf32_Word sh_flags;
 Elf32_Addr sh_addr;
 Elf32_Off sh_offset;
 Elf32_Word sh_size;
 Elf32_Word sh_link;
 Elf32_Word sh_info;
 Elf32_Word sh_addralign;
 Elf32_Word sh_entsize;
} Elf32_Shdr;

typedef struct {
 Elf32_Word p_type;
 Elf32_Off p_offset;
 Elf32_Addr p_vaddr;
 Elf32_Addr p_paddr;
 Elf32_Word p_filesz;
 Elf32_Word p_memsz;
 Elf32_Word p_flags;
 Elf32_Word p_align;
} Elf32_Phdr;

One of the fields in the section header structure is sh_type, which specifies the type of a section. Table 2.1 lists some section types.

Table 2.1: Section types.

The sh_flags field in the section header specifies the attribute of a section. Table 2.2 lists some of these attributes.

Table 2.2: Section attributes.

Some common system-created default sections with predefined names for the PROGBITS are.text,.sdata,.data,.sbss, and.bss. Program code and constant data are contained in the.text section. This section is read-only because code and constant data are not expected to change during the lifetime of the program execution. The.sbss and.bss sections contain uninitialized data. The.sbss section stores small data, which is the data such as variables with sizes that fit into a specific size. This size limit is architecture-dependent. The result is that the compiler and the assembler can generate smaller and more efficient code to access these data items. The.sdata and.data sections contain initialized data items. The small data concept described for.sbss applies to.sdata. A.text section with executable code has the EXECINSTR attribute. The.sdata and.data sections have the WRITE attribute. The.sbss and.bss sections have both the WRITE and the ALLOC attributes.

Other common system-defined sections are.symtab containing the symbol table,.strtab containing the string table for the program symbols,.shstrtab containing the string table for the section names, and.relaname containing the relocation information for the section named name. We have discussed the role of the symbol table (SYMTAB) previously. In Figure 2.3, the symbol name is shown as part of the symbol table. In practice, each entry in the symbol table contains a reference to the string table (STRTAB) where the character representation of the name is stored.

The developer can define custom sections by invoking the linker command.section. For example, where the source files states

.section my_section

the linker creates a new section called my_section. The reasons for creating custom named sections are explained shortly.

The sh_addr is the address where the program section should reside in the target memory. The p_paddr is the address where the program segment should reside in the target memory. The sh_addr and the p_paddr fields refer to the load addresses. The loader uses the load address field from the section header as the starting address for the image transfer from non-volatile memory to RAM.

For many embedded applications, the run address is the same as the load address. These embedded applications are directly downloaded into the target system memory for immediate execution without the need for any code or data transfer from one memory type or location to another. This practice is common during the development phase. We revisit this topic in Chapter 3, which covers the topic of image transfer from the host system to the target system.

Оглавление книги