as.info: Secs Background

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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
Hitachi 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 output, `as' can also generate whatever other
named sections you specify using the `.section' directive (*note
`.section': Section.).  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' assembler 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)
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.