Red Hat ENTERPRISE LINUX 4 - USING AS Using Manual

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32 Chapter 5. Sections and RelocationFurther, 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 ldmixes 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 twopartial programs’ data sections with overlapping addresses after linking, by definition their absolutesections must overlap. Address {absolute 239} in one part of a program is always the sameaddress 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 atassembly time is by definition rendered {undefined U}--where U is filled in later. Since numbers arealways defined, the only way to generate an undefined address is to mention an undefined symbol. Areference to a named common block would be such a symbol: its value is unknown at assembly timeso it has section undefined.By analogy the word section is used to describe groups of sections in the linked program. ld putsall partial programs’ text sections in contiguous addresses in the linked program. It is customary torefer 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 exceptduring assembly.5.2. Linker Sectionsld deals with just four kinds of sections, summarized below.named sectionstext sectiondata sectionThese sections hold your program. as and ld treat them as separate but equal sections. Anythingyou can say of one section is true of another. When the program is running, however, it is cus-tomary for the text section to be unalterable. The text section is often shared among processes:it contains instructions, constants and the like. The data section of a running program is usuallyalterable: for example, C variables would be stored in the data section.bss sectionThis section contains zeroed bytes when your program begins running. It is used to hold uninitial-ized variables or common storage. The length of each partial program’s bss section is important,but because it starts out containing zeroed bytes there is no need to store explicit zero bytes inthe object file. The bss section was invented to eliminate those explicit zeros from object files.absolute sectionAddress 0 of this section is always "relocated" to runtime address 0. This is useful if you want torefer to an address that ld must not change when relocating. In this sense we speak of absoluteaddresses being "unrelocatable": they do not change during relocation.