@node Assembler Internals
@chapter Assembler Internals
@cindex internals

@menu
* Data types::		Data types
@end menu

@node foo
@section foo

BFD_ASSEMBLER
BFD, MANY_SECTIONS, BFD_HEADERS


@node Data types
@section Data types
@cindex internals, data types

@subheading Symbols
@cindex internals, symbols
@cindex symbols, internal

... `local' symbols ... flags ...

The definition for @code{struct symbol}, also known as @code{symbolS},
is located in @file{struc-symbol.h}.  Symbol structures can contain the
following fields:

@table @code
@item sy_value
This is an @code{expressionS} that describes the value of the symbol.
It might refer to another symbol; if so, its true value may not be known
until @code{foo} is run.

More generally, however, ... undefined? ... or an offset from the start
of a frag pointed to by the @code{sy_frag} field.

@item sy_resolved
This field is non-zero if the symbol's value has been completely
resolved.  It is used during the final pass over the symbol table.

@item sy_resolving
This field is used to detect loops while resolving the symbol's value.

@item sy_used_in_reloc
This field is non-zero if the symbol is used by a relocation entry.  If
a local symbol is used in a relocation entry, it must be possible to
redirect those relocations to other symbols, or this symbol cannot be
removed from the final symbol list.

@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.)  These fields
should be accessed with @code{symbol_next} and @code{symbol_previous}.

@item sy_frag
This points to the @code{fragS} that this symbol is attached to.

@item sy_used
Whether the symbol is used as an operand or in an expression.  Note: Not
all the backends keep this information accurate; backends which use this
bit are responsible for setting it when a symbol is used in backend
routines.

@item bsym
If @code{BFD_ASSEMBLER} is defined, this points to the @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 symbol 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.

@item sy_tc
This processor-specific data is of type @code{TC_SYMFIELD_TYPE}.  If no
macro by that name is defined in @file{targ-cpu.h}, this field is not
defined.

@item TARGET_SYMBOL_FIELDS
If this macro is defined, it defines additional fields in the symbol
structure.  This macro is obsolete, and should be replaced when possible
by uses of @code{OBJ_SYMFIELD_TYPE} and @code{TC_SYMFIELD_TYPE}.

@end table

Access with S_SET_SEGMENT, S_SET_VALUE, S_GET_VALUE, S_GET_SEGMENT,
etc., etc.

@foo Expressions
@cindex internals, expressions
@cindex expressions, internal

Expressions are stored as a combination of operator, symbols, blah.

@subheading Fixups
@cindex internals, fixups
@cindex fixups

@subheading Frags
@cindex internals, frags
@cindex frags

@subheading Broken Words
@cindex internals, broken words
@cindex broken words
@cindex promises, promises

@node What Happens?
@section What Happens?

Blah blah blah, initialization, argument parsing, file reading,
whitespace munging, opcode parsing and lookup, operand parsing.  Now
it's time to write the output file.

In @code{BFD_ASSEMBLER} mode, processing of relocations and symbols and
creation of the output file is initiated by calling
@code{write_object_file}.

@node Target Dependent Definitions
@section Target Dependent Definitions

@subheader Format-specific definitions

@defmac obj_sec_sym_ok_for_reloc section
(@code{BFD_ASSEMBLER} only.)
Is it okay to use this section's section-symbol in a relocation entry?
If not, a new internal-linkage symbol is generated and emitted if such a
relocation entry is needed.  (Default: Always use a new symbol.)

@defmac EMIT_SECTION_SYMBOLS
(@code{BFD_ASSEMBLER} only.)
Should section symbols be included in the symbol list if they're used in
relocations?  Some formats can generate section-relative relocations,
and thus don't need 
(Default: 1.)

@node Source File Summary
@section Source File Summary

The code in the @file{obj-coff} back end assumes @code{BFD_ASSEMBLER} is
defined; the code in @file{obj-coffbfd} uses @code{BFD},
@code{BFD_HEADERS}, and @code{MANY_SEGMENTS}, but does a lot of the file
positioning itself.  This confusing situation arose from the history of
the code.

Originally, @file{obj-coff} was a purely non-BFD version, and
@file{obj-coffbfd} was created to use BFD for low-level byte-swapping.
When the @code{BFD_ASSEMBLER} conversion started, the first COFF target
to be converted was using @file{obj-coff}, and the two files had
diverged somewhat, and I didn't feel like first converting the support
of that target over to use the low-level BFD interface.

Currently, all COFF targets use one of the two BFD interfaces, so the
non-BFD code can be removed.  Eventually, all should be converted to
using one COFF back end, which uses the high-level BFD interface.