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Describe AT option of SECTIONS command, at long last.

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Roland Pesch 1994-02-02 02:27:55 +00:00
parent 03d2167461
commit 67c4333b27
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@ -1303,6 +1303,7 @@ big for the region, the linker will issue an error message.
@node SECTIONS @node SECTIONS
@section Specifying Output Sections @section Specifying Output Sections
@kindex SECTIONS @kindex SECTIONS
The @code{SECTIONS} command controls exactly where input sections are The @code{SECTIONS} command controls exactly where input sections are
placed into output sections, their order in the output file, and to placed into output sections, their order in the output file, and to
@ -1311,11 +1312,14 @@ which output sections they are allocated.
You may use at most one @code{SECTIONS} command in a script file, You may use at most one @code{SECTIONS} command in a script file,
but you can have as many statements within it as you wish. Statements but you can have as many statements within it as you wish. Statements
within the @code{SECTIONS} command can do one of three things: within the @code{SECTIONS} command can do one of three things:
@itemize @bullet @itemize @bullet
@item @item
define the entry point; define the entry point;
@item @item
assign a value to a symbol; assign a value to a symbol;
@item @item
describe the placement of a named output section, and which input describe the placement of a named output section, and which input
sections go into it. sections go into it.
@ -1327,7 +1331,7 @@ Point}, and @pxref{Assignment}. They are permitted here as well for
your convenience in reading the script, so that symbols and the entry your convenience in reading the script, so that symbols and the entry
point can be defined at meaningful points in your output-file layout. point can be defined at meaningful points in your output-file layout.
When no @code{SECTIONS} command is given, the linker places each input If you do not use a @code{SECTIONS} command, the linker places each input
section into an identically named output section in the order that the section into an identically named output section in the order that the
sections are first encountered in the input files. If all input sections sections are first encountered in the input files. If all input sections
are present in the first file, for example, the order of sections in the are present in the first file, for example, the order of sections in the
@ -1377,12 +1381,13 @@ sequence of characters, but any name which does not conform to the standard
@node Section Placement @node Section Placement
@subsection Section Placement @subsection Section Placement
@cindex contents of a section @cindex contents of a section
In a section definition, you can specify the contents of an output section by In a section definition, you can specify the contents of an output
listing particular input files, by listing particular input-file section by listing particular input files, by listing particular
sections, or by a combination of the two. You can also place arbitrary input-file sections, or by a combination of the two. You can also place
data in the section, and define symbols relative to the beginning of the arbitrary data in the section, and define symbols relative to the
section. beginning of the section.
The @var{contents} of a section definition may include any of the The @var{contents} of a section definition may include any of the
following kinds of statement. You can include as many of these as you following kinds of statement. You can include as many of these as you
@ -1512,15 +1517,16 @@ SECTIONS @{
@node Section Data Expressions @node Section Data Expressions
@subsection Section Data Expressions @subsection Section Data Expressions
@cindex expressions in a section @cindex expressions in a section
The foregoing statements The foregoing statements arrange, in your output file, data originating
arrange, in your output file, data originating from your input files. from your input files. You can also place data directly in an output
You can also place data directly in an output section from the link section from the link command script. Most of these additional
command script. Most of these additional statements involve statements involve expressions; @pxref{Expressions}. Although these
expressions; @pxref{Expressions}. Although these statements are shown statements are shown separately here for ease of presentation, no such
separately here for ease of presentation, no such segregation is needed segregation is needed within a section definition in the @code{SECTIONS}
within a section definition in the @code{SECTIONS} command; you can command; you can intermix them freely with any of the statements we've
intermix them freely with any of the statements we've just described. just described.
@table @code @table @code
@item CREATE_OBJECT_SYMBOLS @item CREATE_OBJECT_SYMBOLS
@ -1649,16 +1655,17 @@ optional portions:
@smallexample @smallexample
SECTIONS @{ SECTIONS @{
@dots{} @dots{}
@var{secname} @var{start} BLOCK(@var{align}) (NOLOAD) : @{ @var{contents} @} =@var{fill} >@var{region} @var{secname} @var{start} BLOCK(@var{align}) (NOLOAD) : AT ( @var{ldadr} )
@{ @var{contents} @} =@var{fill} >@var{region}
@dots{} @dots{}
@} @}
@end smallexample @end smallexample
@var{secname} and @var{contents} are required. @xref{Section @var{secname} and @var{contents} are required. @xref{Section
Definition}, and @pxref{Section Placement} for details on @var{contents}. Definition}, and @pxref{Section Placement} for details on
The remaining elements---@var{start}, @code{BLOCK(@var{align)}}, @var{contents}. The remaining elements---@var{start},
@code{(NOLOAD)} @code{=@var{fill}}, and @code{>@var{region}}---are all @code{BLOCK(@var{align)}}, @code{(NOLOAD)}, @code{AT ( @var{ldadr} )},
optional. @code{=@var{fill}}, and @code{>@var{region}}---are all optional.
@table @code @table @code
@item @var{start} @item @var{start}
@ -1689,13 +1696,15 @@ the location counter @code{.} prior to the beginning of the section, so
that the section will begin at the specified alignment. @var{align} is that the section will begin at the specified alignment. @var{align} is
an expression. an expression.
@item (NOLOAD)
@kindex NOLOAD @kindex NOLOAD
@cindex prevent unnecessary loading @cindex prevent unnecessary loading
@cindex loading, preventing
@item (NOLOAD)
Use @samp{(NOLOAD)} to prevent a section from being loaded into memory Use @samp{(NOLOAD)} to prevent a section from being loaded into memory
each time it is accessed. For example, in the script sample below, the each time it is accessed. For example, in the script sample below, the
@code{ROM} segment is addressed at memory location @samp{0} and does not @code{ROM} segment is addressed at memory location @samp{0} and does not
need to be loaded into each object file: need to be loaded into each object file:
@example @example
SECTIONS @{ SECTIONS @{
ROM 0 (NOLOAD) : @{ @dots{} @} ROM 0 (NOLOAD) : @{ @dots{} @}
@ -1703,6 +1712,50 @@ SECTIONS @{
@} @}
@end example @end example
@kindex AT ( @var{ldadr} )
@cindex specify load address
@cindex load address, specifying
@item AT ( @var{ldadr} )
The expression @var{ldadr} that follows the @code{AT} keyword specifies
the load address of the section. The default (if you do not use the
@code{AT} keyword) is to make the load address the same as the
relocation address. This feature is designed to make it easy to build a
ROM image. For example, this @code{SECTIONS} definition creates two
output sections: one called @samp{.text}, which starts at @code{0x1000},
and one called @samp{.mdata}, which is loaded at the end of the
@samp{.text} section even though its relocation address is
@code{0x2000}. The symbol @code{_data} is defined with the value
@code{0x2000}:
@smallexample
SECTIONS
@{
.text 0x1000 : @{ *(.text) _etext = . ; @}
.mdata 0x2000 : AT ( ADDR(.text) + SIZEOF ( .text ) )
@{ _data = . ; *(.data); _edata = . ; @}
.bss 0x3000 : @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
@}
@end smallexample
The run-time initialization code (for C programs, usually @code{crt0})
for use with a ROM generated this way has to include something like
the following, to copy the initialized data from the ROM image to its runtime
address:
@example
/* ROM has data glommed at end of text; copy it. */
char *src = _etext;
char *dst = _data;
while (dst < _edata) @{
*dst++ = *src++;
@}
/* Zero bss */
for (dst = _bstart; dst< _bend; dst++)
*dst = 0;
@end example
@item =@var{fill} @item =@var{fill}
@kindex =@var{fill} @kindex =@var{fill}
@cindex section fill pattern @cindex section fill pattern