as: Ld Sections
4.2 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
customary 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 uninitialized 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.
*undefined section*
This "section" is a catch-all for address references to objects not
in the preceding sections.
An idealized example of three relocatable sections follows. The
example uses the traditional section names '.text' and '.data'. Memory
addresses are on the horizontal axis.
+-----+----+--+
partial program # 1: |ttttt|dddd|00|
+-----+----+--+
text data bss
seg. seg. seg.
+---+---+---+
partial program # 2: |TTT|DDD|000|
+---+---+---+
+--+---+-----+--+----+---+-----+~~
linked program: | |TTT|ttttt| |dddd|DDD|00000|
+--+---+-----+--+----+---+-----+~~
addresses: 0 ...
Info Catalog
as: Secs Background
as: Sections
as: As Sections