Chapter 5. Sections and Relocation

5.1. Background

Roughly, a section is a range of addresses, with no gaps; all data "in" those addresses is treated the same for some particular purpose. For example there may be a "read only" section.

The linker ld reads many object files (partial programs) and combines their contents to form a runnable program. When as emits an object file, the partial program is assumed to start at address 0. ld assigns the final addresses for the partial program, so that different partial programs do not overlap. This is actually an oversimplification, but it suffices to explain how as uses sections.

ld moves blocks of bytes of your program to their run-time addresses. These blocks slide to their run-time addresses as rigid units; their length does not change and neither does the order of bytes within them. Such a rigid unit is called a section. Assigning run-time addresses to sections is called relocation. It includes the task of adjusting mentions of object-file addresses so they refer to the proper run-time addresses. For the H8/300 and H8/500, and for the Renesas / SuperH SH, as pads sections if needed to ensure they end on a word (sixteen bit) boundary.

An object file written by as has at least three sections, any of which may be empty. These are named text, data and bss sections.

When it generates COFF or ELF output, as can also generate whatever other named sections you specify using the .section directive (refer to Section 8.74 .section name). If you do not use any directives that place output in the .text or .data sections, these sections still exist, but are empty.

When as generates SOM or ELF output for the HPPA, as can also generate whatever other named sections you specify using the .space and .subspace directives. See [HP9000 Series 800 Assembly Language Reference Manual] (HP 92432-90001) for details on the .space and .subspace assembler directives.

Additionally, as uses different names for the standard text, data, and bss sections when generating SOM output. Program text is placed into the $CODE$ section, data into $DATA$, and BSS into $BSS$.

Within the object file, the text section starts at address 0, the data section follows, and the bss section follows the data section.

When generating either SOM or ELF output files on the HPPA, the text section starts at address 0, the data section at address 0x4000000, and the bss section follows the data section.

To let ld know which data changes when the sections are relocated, and how to change that data, as also writes to the object file details of the relocation needed. To perform relocation ld must know, each time an address in the object file is mentioned:

In fact, every address as ever uses is expressed as

(section) + (offset into section)

Further, most expressions as computes have this section-relative nature. (For some object formats, such as SOM for the HPPA, some expressions are symbol-relative instead.)

In this manual we use the notation {secname N} to mean "offset N into section secname."

Apart from text, data and bss sections you need to know about the absolute section. When ld mixes partial programs, addresses in the absolute section remain unchanged. For example, address {absolute 0} is "relocated" to run-time address 0 by ld. Although the linker never arranges two partial programs' data sections with overlapping addresses after linking, by definition their absolute sections must overlap. Address {absolute 239} in one part of a program is always the same address when the program is running as address {absolute 239} in any other part of the program.

The idea of sections is extended to the undefined section. Any address whose section is unknown at assembly time is by definition rendered {undefined U}--where U is filled in later. Since numbers are always defined, the only way to generate an undefined address is to mention an undefined symbol. A reference to a named common block would be such a symbol: its value is unknown at assembly time so it has section undefined.

By analogy the word section is used to describe groups of sections in the linked program. ld puts all partial programs' text sections in contiguous addresses in the linked program. It is customary to refer to the text section of a program, meaning all the addresses of all partial programs' text sections. Likewise for data and bss sections.

Some sections are manipulated by ld; others are invented for use of as and have no meaning except during assembly.