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 (Section 8.78

.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

assem 

bler 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:

Where in the object file is the beginning of this reference to an address?

How long (in bytes) is this reference?

Which section does the address refer to? What is the numeric value of

(address)   (start address of section)?

Is the reference to an address "Program Counter relative"?

In fact, every address

as

ever uses is expressed as

(section) + (offset into section)