internals.texi

基于4个mips核的noc设计

@end table

@cindex frchainS structure
A chain of frags is built up for each subsection.  The data structure
describing a chain is called a @code{frchainS}, and contains the following
fields:

@table @code
@item frch_root
Points to the first frag in the chain.  May be NULL if there are no frags in
this chain.
@item frch_last
Points to the last frag in the chain, or NULL if there are none.
@item frch_next
Next in the list of @code{frchainS} structures.
@item frch_seg
Indicates the section this frag chain belongs to.
@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.
@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.
@item frch_frag_now
The current frag for this subsegment.
@end table

A @code{frchainS} corresponds to a subsection; each section has a list of
@code{frchainS} records associated with it.  In most cases, only one subsection
of each section is used, so the list will only be one element long, but any
processing of frag chains should be prepared to deal with multiple chains per
section.

After the input files have been completely processed, and no more frags are to
be generated, the frag chains are joined into one per section for further
processing.  After this point, it is safe to operate on one chain per section.

The assembler always has a current frag, named @code{frag_now}.  More space is
allocated for the current frag using the @code{frag_more} function; this
returns a pointer to the amount of requested space.  Relaxing is done using
variant frags allocated by @code{frag_var} or @code{frag_variant}
(@pxref{Relaxation}).

@node GAS processing
@section What GAS does when it runs
@cindex internals, overview

This is a quick look at what an assembler run looks like.

@itemize @bullet
@item
The assembler initializes itself by calling various init routines.

@item
For each source file, the @code{read_a_source_file} function reads in the file
and parses it.  The global variable @code{input_line_pointer} points to the
current text; it is guaranteed to be correct up to the end of the line, but not
farther.

@item
For each line, the assembler passes labels to the @code{colon} function, and
isolates the first word.  If it looks like a pseudo-op, the word is looked up
in the pseudo-op hash table @code{po_hash} and dispatched to a pseudo-op
routine.  Otherwise, the target dependent @code{md_assemble} routine is called
to parse the instruction.

@item
When pseudo-ops or instructions output data, they add it to a frag, calling
@code{frag_more} to get space to store it in.

@item
Pseudo-ops and instructions can also output fixups created by @code{fix_new} or
@code{fix_new_exp}.

@item
For certain targets, instructions can create variant frags which are used to
store relaxation information (@pxref{Relaxation}).

@item
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}).
@end itemize

@node Porting GAS
@section Porting GAS
@cindex porting

Each GAS target specifies two main things: the CPU file and the object format
file.  Two main switches in the @file{configure.in} file handle this.  The
first switches on CPU type to set the shell variable @code{cpu_type}.  The
second switches on the entire target to set the shell variable @code{fmt}.

The configure script uses the value of @code{cpu_type} to select two files in
the @file{config} directory: @file{tc-@var{CPU}.c} and @file{tc-@var{CPU}.h}.
The configuration process will create a file named @file{targ-cpu.h} in the
build directory which includes @file{tc-@var{CPU}.h}.

The configure script also uses the value of @code{fmt} to select two files:
@file{obj-@var{fmt}.c} and @file{obj-@var{fmt}.h}.  The configuration process
will create a file named @file{obj-format.h} in the build directory which
includes @file{obj-@var{fmt}.h}.

You can also set the emulation in the configure script by setting the @code{em}
variable.  Normally the default value of @samp{generic} is fine.  The
configuration process will create a file named @file{targ-env.h} in the build
directory which includes @file{te-@var{em}.h}.

There is a special case for COFF. For historical reason, the GNU COFF
assembler doesn't follow the documented behavior on certain debug symbols for
the compatibility with other COFF assemblers. A port can define
@code{STRICTCOFF} in the configure script to make the GNU COFF assembler
to follow the documented behavior.

Porting GAS to a new CPU requires writing the @file{tc-@var{CPU}} files.
Porting GAS to a new object file format requires writing the
@file{obj-@var{fmt}} files.  There is sometimes some interaction between these
two files, but it is normally minimal.

The best approach is, of course, to copy existing files.  The documentation
below assumes that you are looking at existing files to see usage details.

These interfaces have grown over time, and have never been carefully thought
out or designed.  Nothing about the interfaces described here is cast in stone.
It is possible that they will change from one version of the assembler to the
next.  Also, new macros are added all the time as they are needed.

@menu
* CPU backend::                 Writing a CPU backend
* Object format backend::       Writing an object format backend
* Emulations::                  Writing emulation files
@end menu

@node CPU backend
@subsection Writing a CPU backend
@cindex CPU backend
@cindex @file{tc-@var{CPU}}

The CPU backend files are the heart of the assembler.  They are the only parts
of the assembler which actually know anything about the instruction set of the
processor.

You must define a reasonably small list of macros and functions in the CPU
backend files.  You may define a large number of additional macros in the CPU
backend files, not all of which are documented here.  You must, of course,
define macros in the @file{.h} file, which is included by every assembler
source file.  You may define the functions as macros in the @file{.h} file, or
as functions in the @file{.c} file.

@table @code
@item TC_@var{CPU}
@cindex TC_@var{CPU}
By convention, you should define this macro in the @file{.h} file.  For
example, @file{tc-m68k.h} defines @code{TC_M68K}.  You might have to use this
if it is necessary to add CPU specific code to the object format file.

@item TARGET_FORMAT
This macro is the BFD target name to use when creating the output file.  This
will normally depend upon the @code{OBJ_@var{FMT}} macro.

@item TARGET_ARCH
This macro is the BFD architecture to pass to @code{bfd_set_arch_mach}.

@item TARGET_MACH
This macro is the BFD machine number to pass to @code{bfd_set_arch_mach}.  If
it is not defined, GAS will use 0.

@item TARGET_BYTES_BIG_ENDIAN
You should define this macro to be non-zero if the target is big endian, and
zero if the target is little endian.

@item md_shortopts
@itemx md_longopts
@itemx md_longopts_size
@itemx md_parse_option
@itemx md_show_usage
@cindex md_shortopts
@cindex md_longopts
@cindex md_longopts_size
@cindex md_parse_option
@cindex md_show_usage
GAS uses these variables and functions during option processing.
@code{md_shortopts} is a @code{const char *} which GAS adds to the machine
independent string passed to @code{getopt}.  @code{md_longopts} is a
@code{struct option []} which GAS adds to the machine independent long options
passed to @code{getopt}; you may use @code{OPTION_MD_BASE}, defined in
@file{as.h}, as the start of a set of long option indices, if necessary.
@code{md_longopts_size} is a @code{size_t} holding the size @code{md_longopts}.
GAS will call @code{md_parse_option} whenever @code{getopt} returns an
unrecognized code, presumably indicating a special code value which appears in
@code{md_longopts}.  GAS will call @code{md_show_usage} when a usage message is
printed; it should print a description of the machine specific options.

@item md_begin
@cindex md_begin
GAS will call this function at the start of the assembly, after the command
line arguments have been parsed and all the machine independent initializations
have been completed.

@item md_cleanup
@cindex md_cleanup
If you define this macro, GAS will call it at the end of each input file.

@item md_assemble
@cindex md_assemble
GAS will call this function for each input line which does not contain a
pseudo-op.  The argument is a null terminated string.  The function should
assemble the string as an instruction with operands.  Normally
@code{md_assemble} will do this by calling @code{frag_more} and writing out
some bytes (@pxref{Frags}).  @code{md_assemble} will call @code{fix_new} to
create fixups as needed (@pxref{Fixups}).  Targets which need to do special
purpose relaxation will call @code{frag_var}.

@item md_pseudo_table
@cindex md_pseudo_table
This is a const array of type @code{pseudo_typeS}.  It is a mapping from
pseudo-op names to functions.  You should use this table to implement
pseudo-ops which are specific to the CPU.

@item tc_conditional_pseudoop
@cindex tc_conditional_pseudoop
If this macro is defined, GAS will call it with a @code{pseudo_typeS} argument.
It should return non-zero if the pseudo-op is a conditional which controls
whether code is assembled, such as @samp{.if}.  GAS knows about the normal
conditional pseudo-ops, and you should normally not have to define this macro.

@item comment_chars
@cindex comment_chars
This is a null terminated @code{const char} array of characters which start a
comment.

@item tc_comment_chars
@cindex tc_comment_chars
If this macro is defined, GAS will use it instead of @code{comment_chars}.

@item tc_symbol_chars
@cindex tc_symbol_chars
If this macro is defined, it is a pointer to a null terminated list of
characters which may appear in an operand.  GAS already assumes that all
alphanumberic characters, and @samp{$}, @samp{.}, and @samp{_} may appear in an
operand (see @samp{symbol_chars} in @file{app.c}).  This macro may be defined
to treat additional characters as appearing in an operand.  This affects the
way in which GAS removes whitespace before passing the string to
@samp{md_assemble}.

@item line_comment_chars
@cindex line_comment_chars
This is a null terminated @code{const char} array of characters which start a
comment when they appear at the start of a line.

@item line_separator_chars
@cindex line_separator_chars
This is a null terminated @code{const char} array of characters which separate
lines (null and newline are such characters by default, and need not be
listed in this array).  Note that line_separator_chars do not separate lines
if found in a comment, such as after a character in line_comment_chars or
comment_chars.

@item EXP_CHARS
@cindex EXP_CHARS
This is a null terminated @code{const char} array of characters which may be
used as the exponent character in a floating point number.  This is normally
@code{"eE"}.

@item FLT_CHARS
@cindex FLT_CHARS
This is a null terminated @code{const char} array of characters which may be
used to indicate a floating point constant.  A zero followed by one of these
characters is assumed to be followed by a floating point number; thus they
operate the way that @code{0x} is used to indicate a hexadecimal constant.
Usually this includes @samp{r} and @samp{f}.

@item LEX_AT
@cindex LEX_AT
You may define this macro to the lexical type of the @kbd{@@} character.  The
default is zero.

Lexical types are a combination of @code{LEX_NAME} and @code{LEX_BEGIN_NAME},
both defined in @file{read.h}.  @code{LEX_NAME} indicates that the character
may appear in a name.  @code{LEX_BEGIN_NAME} indicates that the character may
appear at the beginning of a name.

@item LEX_BR
@cindex LEX_BR
You may define this macro to the lexical type of the brace characters @kbd{@{},
@kbd{@}}, @kbd{[}, and @kbd{]}.  The default value is zero.

@item LEX_PCT
@cindex LEX_PCT
You may define this macro to the lexical type of the @kbd{%} character.  The
default value is zero.

@item LEX_QM
@cindex LEX_QM
You may define this macro to the lexical type of the @kbd{?} character.  The
default value it zero.

@item LEX_DOLLAR
@cindex LEX_DOLLAR
You may define this macro to the lexical type of the @kbd{$} character.  The
default value is @code{LEX_NAME | LEX_BEGIN_NAME}.

@item NUMBERS_WITH_SUFFIX
@cindex NUMBERS_WITH_SUFFIX
When this macro is defined to be non-zero, the parser allows the radix of a
constant to be indicated with a suffix.  Valid suffixes are binary (B),
octal (Q), and hexadecimal (H).  Case is not significant.

@item SINGLE_QUOTE_STRINGS
@cindex SINGLE_QUOTE_STRINGS
If you define this macro, GAS will treat single quotes as string delimiters.
Normally only double quotes are accepted as string delimiters.

@item NO_STRING_ESCAPES
@cindex NO_STRING_ESCAPES
If you define this macro, GAS will not permit escape sequences in a string.

@item ONLY_STANDARD_ESCAPES
@cindex ONLY_STANDARD_ESCAPES
If you define this macro, GAS will warn about the use of nonstandard escape
sequences in a string.

@item md_start_line_hook
@cindex md_start_line_hook
If you define this macro, GAS will call it at the start of each line.

@item LABELS_WITHOUT_COLONS
@cindex LABELS_WITHOUT_COLONS
If you define this macro, GAS will assume that any text at the start of a line