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+@c This summary of BFD is shared by the BFD and LD docs.
+When an object file is opened, BFD subroutines automatically determine
+the format of the input object file.  They then build a descriptor in
+memory with pointers to routines that will be used to access elements of
+the object file's data structures.
+
+As different information from the the object files is required,
+BFD reads from different sections of the file and processes them.
+For example, a very common operation for the linker is processing symbol
+tables.  Each BFD back end provides a routine for converting
+between the object file's representation of symbols and an internal
+canonical format. When the linker asks for the symbol table of an object
+file, it calls through a memory pointer to the routine from the
+relevant BFD back end which reads and converts the table into a canonical
+form.  The linker then operates upon the canonical form. When the link is
+finished and the linker writes the output file's symbol table,
+another BFD back end routine is called to take the newly
+created symbol table and convert it into the chosen output format.
+
+@menu
+* BFD information loss::	Information Loss
+* Canonical format::		The BFD	canonical object-file format 
+@end menu
+
+@node BFD information loss
+@subsection Information Loss
+
+@emph{Information can be lost during output.} The output formats
+supported by BFD do not provide identical facilities, and
+information which can be described in one form has nowhere to go in
+another format. One example of this is alignment information in
+@code{b.out}. There is nowhere in an @code{a.out} format file to store
+alignment information on the contained data, so when a file is linked
+from @code{b.out} and an @code{a.out} image is produced, alignment
+information will not propagate to the output file. (The linker will
+still use the alignment information internally, so the link is performed
+correctly).
+
+Another example is COFF section names. COFF files may contain an
+unlimited number of sections, each one with a textual section name. If
+the target of the link is a format which does not have many sections (e.g.,
+@code{a.out}) or has sections without names (e.g., the Oasys format), the
+link cannot be done simply. You can circumvent this problem by
+describing the desired input-to-output section mapping with the linker command
+language.
+
+@emph{Information can be lost during canonicalization.} The BFD
+internal canonical form of the external formats is not exhaustive; there
+are structures in input formats for which there is no direct
+representation internally.  This means that the BFD back ends
+cannot maintain all possible data richness through the transformation
+between external to internal and back to external formats.
+
+This limitation is only a problem when an application reads one
+format and writes another.  Each BFD back end is responsible for
+maintaining as much data as possible, and the internal BFD
+canonical form has structures which are opaque to the BFD core,
+and exported only to the back ends. When a file is read in one format,
+the canonical form is generated for BFD and the application. At the
+same time, the back end saves away any information which may otherwise
+be lost. If the data is then written back in the same format, the back
+end routine will be able to use the canonical form provided by the
+BFD core as well as the information it prepared earlier.  Since
+there is a great deal of commonality between back ends,
+there is no information lost when
+linking or copying big endian COFF to little endian COFF, or @code{a.out} to
+@code{b.out}.  When a mixture of formats is linked, the information is
+only lost from the files whose format differs from the destination.
+
+@node Canonical format
+@subsection The BFD canonical object-file format
+
+The greatest potential for loss of information occurs when there is the least
+overlap between the information provided by the source format, that
+stored by the canonical format, and that needed by the
+destination format. A brief description of the canonical form may help
+you understand which kinds of data you can count on preserving across
+conversions.
+@cindex BFD canonical format
+@cindex internal object-file format
+
+@table @emph
+@item files
+Information stored on a per-file basis includes target machine
+architecture, particular implementation format type, a demand pageable
+bit, and a write protected bit.  Information like Unix magic numbers is
+not stored here---only the magic numbers' meaning, so a @code{ZMAGIC}
+file would have both the demand pageable bit and the write protected
+text bit set.  The byte order of the target is stored on a per-file
+basis, so that big- and little-endian object files may be used with one
+another.
+
+@item sections
+Each section in the input file contains the name of the section, the
+section's original address in the object file, size and alignment
+information, various flags, and pointers into other BFD data
+structures.
+
+@item symbols
+Each symbol contains a pointer to the information for the object file
+which originally defined it, its name, its value, and various flag
+bits.  When a BFD back end reads in a symbol table, it relocates all
+symbols to make them relative to the base of the section where they were
+defined.  Doing this ensures that each symbol points to its containing
+section.  Each symbol also has a varying amount of hidden private data
+for the BFD back end.  Since the symbol points to the original file, the
+private data format for that symbol is accessible.  @code{ld} can
+operate on a collection of symbols of wildly different formats without
+problems.
+
+Normal global and simple local symbols are maintained on output, so an
+output file (no matter its format) will retain symbols pointing to
+functions and to global, static, and common variables.  Some symbol
+information is not worth retaining; in @code{a.out}, type information is
+stored in the symbol table as long symbol names.  This information would
+be useless to most COFF debuggers; the linker has command line switches
+to allow users to throw it away.
+
+There is one word of type information within the symbol, so if the
+format supports symbol type information within symbols (for example, COFF,
+IEEE, Oasys) and the type is simple enough to fit within one word
+(nearly everything but aggregates), the information will be preserved.
+
+@item relocation level
+Each canonical BFD relocation record contains a pointer to the symbol to
+relocate to, the offset of the data to relocate, the section the data
+is in, and a pointer to a relocation type descriptor. Relocation is
+performed by passing messages through the relocation type
+descriptor and the symbol pointer. Therefore, relocations can be performed
+on output data using a relocation method that is only available in one of the
+input formats. For instance, Oasys provides a byte relocation format.
+A relocation record requesting this relocation type would point
+indirectly to a routine to perform this, so the relocation may be
+performed on a byte being written to a 68k COFF file, even though 68k COFF
+has no such relocation type.
+
+@item line numbers
+Object formats can contain, for debugging purposes, some form of mapping
+between symbols, source line numbers, and addresses in the output file.
+These addresses have to be relocated along with the symbol information.
+Each symbol with an associated list of line number records points to the
+first record of the list.  The head of a line number list consists of a
+pointer to the symbol, which allows finding out the address of the
+function whose line number is being described. The rest of the list is
+made up of pairs: offsets into the section and line numbers. Any format
+which can simply derive this information can pass it successfully
+between formats (COFF, IEEE and Oasys).
+@end table