34

Chapter 5. Sections and Relocation

Each subsection is zero padded up to a multiple of four bytes. (Subsections may be padded a different

amount on different flavors of

as

.)

Subsections appear in your object file in numeric order, lowest numbered to highest. (All this to be

compatible with other people's assemblers.) The object file contains no representation of subsections;

ld

and other programs that manipulate object files see no trace of them. They just see all your text

subsections as a text section, and all your data subsections as a data section.

To specify which subsection you want subsequent statements assembled into, use a numeric argument

to specify it, in a

.text expression

or a

.data expression

statement. When generating COFF

output, you can also use an extra subsection argument with arbitrary named sections:

.section

name, expression

. When generating ELF output, you can also use the

.subsection

directive (Sec 

tion 8.89

.subsection name

) to specify a subsection:

.subsection expression

.

Expression

should be an absolute expression. (Chapter 7 Expressions.) If you just say

.text

then

.text 0

is

assumed. Likewise

.data

means

.data 0

. Assembly begins in

text 0

. For instance:

.text 0

# The default subsection is text 0 anyway.

.ascii "This lives in the first text subsection. *"

.text 1

.ascii "But this lives in the second text subsection."

.data 0

.ascii "This lives in the data section,"

.ascii "in the first data subsection."

.text 0

.ascii "This lives in the first text section,"

.ascii "immediately following the asterisk (*)."

Each section has a location counter incremented by one for every byte assembled into that section.

Because subsections are merely a convenience restricted to

as

there is no concept of a subsection

location counter. There is no way to directly manipulate a location counter  but the

.align

directive

changes it, and any label definition captures its current value. The location counter of the section

where statements are being assembled is said to be the active location counter.

5.5. bss Section

The bss section is used for local common variable storage. You may allocate address space in the bss

section, but you may not dictate data to load into it before your program executes. When your program

starts running, all the contents of the bss section are zeroed bytes.

The

.lcomm

pseudo op defines a symbol in the bss section; see Section 8.52

.lcomm symbol,

length

.

The

.comm

pseudo op may be used to declare a common symbol, which is another form of uninitial 

ized symbol; see Section 8.8

.comm symbol, length

.

When assembling for a target which supports multiple sections, such as ELF or COFF, you may switch

into the

.bss

section and define symbols as usual; see Section 8.78

.section name

. You may only

assemble zero values into the section. Typically the section will only contain symbol definitions and

.skip

directives (Section 8.84

.skip size, fill

).