This chapter is not updated regularly, and it may be out of date.
@menu
-* GAS versions:: GAS versions
* Data types:: Data types
* GAS processing:: What GAS does when it runs
* Porting GAS:: Porting GAS
* Test suite:: Test suite
@end menu
-@node GAS versions
-@section GAS versions
-
-GAS has acquired layers of code over time. The original GAS only supported the
-a.out object file format, with three sections. Support for multiple sections
-has been added in two different ways.
-
-The preferred approach is to use the version of GAS created when the symbol
-@code{BFD_ASSEMBLER} is defined. The other versions of GAS are documented for
-historical purposes, and to help anybody who has to debug code written for
-them.
-
-The type @code{segT} is used to represent a section in code which must work
-with all versions of GAS.
-
-@menu
-* Original GAS:: Original GAS version
-* MANY_SEGMENTS:: MANY_SEGMENTS gas version
-* BFD_ASSEMBLER:: BFD_ASSEMBLER gas version
-@end menu
-
-@node Original GAS
-@subsection Original GAS
-
-The original GAS only supported the a.out object file format with three
-sections: @samp{.text}, @samp{.data}, and @samp{.bss}. This is the version of
-GAS that is compiled if neither @code{BFD_ASSEMBLER} nor @code{MANY_SEGMENTS}
-is defined. This version of GAS is still used for the m68k-aout target, and
-perhaps others.
-
-This version of GAS should not be used for any new development.
-
-There is still code that is specific to this version of GAS, notably in
-@file{write.c}. There is no way for this code to loop through all the
-sections; it simply looks at global variables like @code{text_frag_root} and
-@code{data_frag_root}.
-
-The type @code{segT} is an enum.
-
-@node MANY_SEGMENTS
-@subsection MANY_SEGMENTS gas version
-@cindex MANY_SEGMENTS
-
-The @code{MANY_SEGMENTS} version of gas is only used for COFF. It uses the BFD
-library, but it writes out all the data itself using @code{bfd_write}. This
-version of gas supports up to 40 normal sections. The section names are stored
-in the @code{seg_name} array. Other information is stored in the
-@code{segment_info} array.
-
-The type @code{segT} is an enum. Code that wants to examine all the sections
-can use a @code{segT} variable as loop index from @code{SEG_E0} up to but not
-including @code{SEG_UNKNOWN}.
-
-Most of the code specific to this version of GAS is in the file
-@file{config/obj-coff.c}, in the portion of that file that is compiled when
-@code{BFD_ASSEMBLER} is not defined.
-
-This version of GAS is still used for several COFF targets.
-
-@node BFD_ASSEMBLER
-@subsection BFD_ASSEMBLER gas version
-@cindex BFD_ASSEMBLER
-
-The preferred version of GAS is the @code{BFD_ASSEMBLER} version. In this
-version of GAS, the output file is a normal BFD, and the BFD routines are used
-to generate the output.
-
-@code{BFD_ASSEMBLER} will automatically be used for certain targets, including
-those that use the ELF, ECOFF, and SOM object file formats, and also all Alpha,
-MIPS, PowerPC, and SPARC targets. You can force the use of
-@code{BFD_ASSEMBLER} for other targets with the configure option
-@samp{--enable-bfd-assembler}; however, it has not been tested for many
-targets, and can not be assumed to work.
-
@node Data types
@section Data types
@cindex internals, data types
@item sy_next
@itemx sy_previous
-These pointers to other @code{symbolS} structures describe a singly or doubly
-linked list. (If @code{SYMBOLS_NEED_BACKPOINTERS} is not defined, the
-@code{sy_previous} field will be omitted; @code{SYMBOLS_NEED_BACKPOINTERS} is
-always defined if @code{BFD_ASSEMBLER}.) These fields should be accessed with
+These pointers to other @code{symbolS} structures describe a doubly
+linked list. These fields should be accessed with
the @code{symbol_next} and @code{symbol_previous} macros.
@item sy_frag
pseudo-op when assembling in MRI mode.
@item bsym
-If @code{BFD_ASSEMBLER} is defined, this points to the BFD @code{asymbol} that
+This points to the BFD @code{asymbol} that
will be used in writing the object file.
-@item sy_name_offset
-(Only used if @code{BFD_ASSEMBLER} is not defined.) This is the position of
-the symbol's name in the string table of the object file. On some formats,
-this will start at position 4, with position 0 reserved for unnamed symbols.
-This field is not used until @code{write_object_file} is called.
-
-@item sy_symbol
-(Only used if @code{BFD_ASSEMBLER} is not defined.) This is the
-format-specific symbol structure, as it would be written into the object file.
-
-@item sy_number
-(Only used if @code{BFD_ASSEMBLER} is not defined.) This is a 24-bit symbol
-number, for use in constructing relocation table entries.
-
@item sy_obj
This format-specific data is of type @code{OBJ_SYMFIELD_TYPE}. If no macro by
that name is defined in @file{obj-format.h}, this field is not defined.
@end table
-When @code{BFD_ASSEMBLER} is defined, GAS attempts to store local
+GAS attempts to store local
symbols--symbols which will not be written to the output file--using a
different structure, @code{struct local_symbol}. This structure can only
represent symbols whose value is an offset within a frag.
@cindex fix_new_exp
A fixup is created by a call to @code{fix_new} or @code{fix_new_exp}. Both
take a frag (@pxref{Frags}), a position within the frag, a size, an indication
-of whether the fixup is PC relative, and a type. In a @code{BFD_ASSEMBLER}
-GAS, the type is nominally a @code{bfd_reloc_code_real_type}, but several
+of whether the fixup is PC relative, and a type.
+The type is nominally a @code{bfd_reloc_code_real_type}, but several
targets use other type codes to represent fixups that can not be described as
relocations.
The next fixup in the section.
@item fx_r_type
-The type of the fixup. This field is only defined if @code{BFD_ASSEMBLER}, or
-if the target defines @code{NEED_FX_R_TYPE}.
+The type of the fixup.
@item fx_size
The size of the fixup. This is mostly used for error checking.
@item frch_subseg
Subsection (subsegment) number of this frag chain.
@item fix_root, fix_tail
-(Defined only if @code{BFD_ASSEMBLER} is defined). Point to first and last
-@code{fixS} structures associated with this subsection.
+Point to first and last @code{fixS} structures associated with this subsection.
@item frch_obstack
Not currently used. Intended to be used for frag allocation for this
subsection. This should reduce frag generation caused by switching sections.
When the input file is finished, the @code{write_object_file} routine is
called. It assigns addresses to all the frags (@code{relax_segment}), resolves
all the fixups (@code{fixup_segment}), resolves all the symbol values (using
-@code{resolve_symbol_value}), and finally writes out the file (in the
-@code{BFD_ASSEMBLER} case, this is done by simply calling @code{bfd_close}).
+@code{resolve_symbol_value}), and finally writes out the file.
@end itemize
@node Porting GAS
The macro has a default definition which returns 0 for all cases.
-@item md_reloc_size
-@cindex md_reloc_size
-This variable is only used in the original version of gas (not
-@code{BFD_ASSEMBLER} and not @code{MANY_SEGMENTS}). It holds the size of a
-relocation entry.
-
@item WORKING_DOT_WORD
@itemx md_short_jump_size
@itemx md_long_jump_size
@item S_FORCE_RELOC (@var{sym}, @var{strict})
@cindex S_FORCE_RELOC
-This macro (or function, for @code{BFD_ASSEMBLER} gas) returns true for symbols
+This function returns true for symbols
that should not be reduced to section symbols or eliminated from expressions,
because they may be overridden by the linker. ie. for symbols that are
undefined or common, and when @var{strict} is set, weak, or global (for ELF
@item tc_gen_reloc
@cindex tc_gen_reloc
-A @code{BFD_ASSEMBLER} GAS will call this to generate a reloc. GAS will pass
+GAS will call this to generate a reloc. GAS will pass
the resulting reloc to @code{bfd_install_relocation}. This currently works
poorly, as @code{bfd_install_relocation} often does the wrong thing, and
instances of @code{tc_gen_reloc} have been written to work around the problems,
@item tc_frob_section
@cindex tc_frob_section
-If you define this macro, a @code{BFD_ASSEMBLER} GAS will call it for each
+If you define this macro, GAS will call it for each
section at the end of the assembly.
@item tc_frob_file_before_adjust
The object format @file{.h} file must include @file{targ-cpu.h}.
-This section will only define the @code{BFD_ASSEMBLER} version of GAS. It is
-impossible to support a new object file format using any other version anyhow,
-as the original GAS version only supports a.out, and the @code{MANY_SEGMENTS}
-GAS version only supports COFF.
-
@table @code
@item OBJ_@var{format}
@cindex OBJ_@var{format}
@cindex SET_SECTION_RELOCS
If you define this, it will be called after the relocations have been set for
the section @var{sec}. The list of relocations is in @var{relocs}, and the
-number of relocations is in @var{n}. This is only used with
-@code{BFD_ASSEMBLER}.
+number of relocations is in @var{n}.
@end table
@node Emulations
projects / binutils-gdb.git / commitdiff