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