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Chapter 5. Sections and Relocation

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.

5.2. Linker Sections

ld

deals with just four kinds of sections, summarized below.

named sections

text section

data section

These sections hold your program.

as

and

ld

treat them as separate but equal sections. Anything

you 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 usually

alterable: for example, C variables would be stored in the data section.

bss section

This 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 in

the object file. The bss section was invented to eliminate those explicit zeros from object files.

absolute section

Address 0 of this section is always "relocated" to runtime address 0. This is useful if you want to

refer to an address that

ld

must not change when relocating. In this sense we speak of absolute

addresses being "unrelocatable": they do not change during relocation.