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* doc/internals.texi: Add loud disclaimer. Refill to 79 columns, specify

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fill-column in local-variables section.  Change subheadings to subsections so
they can be cross-referenced.  Describe broken words, frags, frag chains,
generic relaxation, relax table, m68k relaxation, m68k addressing modes, test
suite code.  Add a few words about various file formats.
This commit is contained in:
Ken Raeburn 1995-04-10 20:06:48 +00:00
parent 7f390875ca
commit ae6cd60f9e
1 changed files with 592 additions and 96 deletions
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gas/doc/internals.texi
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@ -1,7 +1,19 @@
\input texinfo
@setfilename internals.info
@node Assembler Internals @node Assembler Internals
@chapter Assembler Internals @chapter Assembler Internals
@cindex internals @cindex internals
This documentation is not ready for prime time yet. Not even close. It's not
so much documentation as random blathering of mine intended to be notes to
myself that may eventually be turned into real documentation.
I take no responsibility for any negative effect it may have on your
professional, personal, or spiritual life. Read it at your own risk. Caveat
emptor. Delete before reading. Abandon all hope, ye who enter here.
However, enhancements will be gratefully accepted.
@menu @menu
* Data types:: Data types * Data types:: Data types
@end menu @end menu
@ -17,162 +29,602 @@ BFD, MANY_SECTIONS, BFD_HEADERS
@section Data types @section Data types
@cindex internals, data types @cindex internals, data types
@subheading Symbols @subsection Symbols
@cindex internals, symbols @cindex internals, symbols
@cindex symbols, internal @cindex symbols, internal
... `local' symbols ... flags ... ... `local' symbols ... flags ...
The definition for @code{struct symbol}, also known as @code{symbolS}, The definition for @code{struct symbol}, also known as @code{symbolS}, is
is located in @file{struc-symbol.h}. Symbol structures can contain the located in @file{struc-symbol.h}. Symbol structures can contain the following
following fields: fields:
@table @code @table @code
@item sy_value @item sy_value
This is an @code{expressionS} that describes the value of the symbol. This is an @code{expressionS} that describes the value of the symbol. It might
It might refer to another symbol; if so, its true value may not be known refer to another symbol; if so, its true value may not be known until
until @code{foo} is run. @code{foo} is called.
More generally, however, ... undefined? ... or an offset from the start More generally, however, ... undefined? ... or an offset from the start of a
of a frag pointed to by the @code{sy_frag} field. frag pointed to by the @code{sy_frag} field.
@item sy_resolved @item sy_resolved
This field is non-zero if the symbol's value has been completely This field is non-zero if the symbol's value has been completely resolved. It
resolved. It is used during the final pass over the symbol table. is used during the final pass over the symbol table.
@item sy_resolving @item sy_resolving
This field is used to detect loops while resolving the symbol's value. This field is used to detect loops while resolving the symbol's value.
@item sy_used_in_reloc @item sy_used_in_reloc
This field is non-zero if the symbol is used by a relocation entry. If This field is non-zero if the symbol is used by a relocation entry. If a local
a local symbol is used in a relocation entry, it must be possible to symbol is used in a relocation entry, it must be possible to redirect those
redirect those relocations to other symbols, or this symbol cannot be relocations to other symbols, or this symbol cannot be removed from the final
removed from the final symbol list. symbol list.
@item sy_next @item sy_next
@itemx sy_previous @itemx sy_previous
These pointers to other @code{symbolS} structures describe a singly or These pointers to other @code{symbolS} structures describe a singly or doubly
doubly linked list. (If @code{SYMBOLS_NEED_BACKPOINTERS} is not linked list. (If @code{SYMBOLS_NEED_BACKPOINTERS} is not defined, the
defined, the @code{sy_previous} field will be omitted.) These fields @code{sy_previous} field will be omitted.) These fields should be accessed
should be accessed with @code{symbol_next} and @code{symbol_previous}. with @code{symbol_next} and @code{symbol_previous}.
@item sy_frag @item sy_frag
This points to the @code{fragS} that this symbol is attached to. This points to the @code{fragS} that this symbol is attached to.
@item sy_used @item sy_used
Whether the symbol is used as an operand or in an expression. Note: Not Whether the symbol is used as an operand or in an expression. Note: Not all of
all the backends keep this information accurate; backends which use this the backends keep this information accurate; backends which use this bit are
bit are responsible for setting it when a symbol is used in backend responsible for setting it when a symbol is used in backend routines.
routines.
@item bsym @item bsym
If @code{BFD_ASSEMBLER} is defined, this points to the @code{asymbol} If @code{BFD_ASSEMBLER} is defined, this points to the @code{asymbol} that will
that will be used in writing the object file. be used in writing the object file.
@item sy_name_offset @item sy_name_offset
(Only used if @code{BFD_ASSEMBLER} is not defined.) (Only used if @code{BFD_ASSEMBLER} is not defined.) This is the position of
This is the position of the symbol's name in the symbol table of the the symbol's name in the symbol table of the object file. On some formats,
object file. On some formats, this will start at position 4, with this will start at position 4, with position 0 reserved for unnamed symbols.
position 0 reserved for unnamed symbols. This field is not used until This field is not used until @code{write_object_file} is called.
@code{write_object_file} is called.
@item sy_symbol @item sy_symbol
(Only used if @code{BFD_ASSEMBLER} is not defined.) (Only used if @code{BFD_ASSEMBLER} is not defined.) This is the
This is the format-specific symbol structure, as it would be written into format-specific symbol structure, as it would be written into the object file.
the object file.
@item sy_number @item sy_number
(Only used if @code{BFD_ASSEMBLER} is not defined.) (Only used if @code{BFD_ASSEMBLER} is not defined.) This is a 24-bit symbol
This is a 24-bit symbol number, for use in constructing relocation table number, for use in constructing relocation table entries.
entries.
@item sy_obj @item sy_obj
This format-specific data is of type @code{OBJ_SYMFIELD_TYPE}. If no This format-specific data is of type @code{OBJ_SYMFIELD_TYPE}. If no macro by
macro by that name is defined in @file{obj-format.h}, this field is not that name is defined in @file{obj-format.h}, this field is not defined.
defined.
@item sy_tc @item sy_tc
This processor-specific data is of type @code{TC_SYMFIELD_TYPE}. If no This processor-specific data is of type @code{TC_SYMFIELD_TYPE}. If no macro
macro by that name is defined in @file{targ-cpu.h}, this field is not by that name is defined in @file{targ-cpu.h}, this field is not defined.
defined.
@item TARGET_SYMBOL_FIELDS @item TARGET_SYMBOL_FIELDS
If this macro is defined, it defines additional fields in the symbol If this macro is defined, it defines additional fields in the symbol structure.
structure. This macro is obsolete, and should be replaced when possible This macro is obsolete, and should be replaced when possible by uses of
by uses of @code{OBJ_SYMFIELD_TYPE} and @code{TC_SYMFIELD_TYPE}. @code{OBJ_SYMFIELD_TYPE} and @code{TC_SYMFIELD_TYPE}.
@end table @end table
Access with S_SET_SEGMENT, S_SET_VALUE, S_GET_VALUE, S_GET_SEGMENT, Access with S_SET_SEGMENT, S_SET_VALUE, S_GET_VALUE, S_GET_SEGMENT, etc., etc.
etc., etc.
@foo Expressions @subsection Expressions
@cindex internals, expressions @cindex internals, expressions
@cindex expressions, internal @cindex expressions, internal
Expressions are stored as a combination of operator, symbols, blah. Expressions are stored as a combination of operator, symbols, blah.
@subheading Fixups @subsection Fixups
@cindex internals, fixups @cindex internals, fixups
@cindex fixups @cindex fixups
@subheading Frags @subsection Frags
@cindex internals, frags @cindex internals, frags
@cindex frags @cindex frags
@subheading Broken Words The frag is the basic unit for storing section contents.
@table @code
@item fr_address
The address of the frag. This is not set until the assembler rescans the list
of all frags after the entire input file is parsed. The function
@code{relax_segment} fills in this field.
@item fr_next
Pointer to the next frag in this (sub)section.
@item fr_fix
Fixed number of characters we know we're going to emit to the output file. May
be zero.
@item fr_var
Variable number of characters we may output, after the initial @code{fr_fix}
characters. May be zero.
@item fr_symbol
@itemx fr_offset
Foo.
@item fr_opcode
Points to the lowest-addressed byte of the opcode, for use in relaxation.
@item line
Holds line-number info.
@item fr_type
Relaxation state. This field indicates the interpretation of @code{fr_offset},
@code{fr_symbol} and the variable-length tail of the frag, as well as the
treatment it gets in various phases of processing. It does not affect the
initial @code{fr_fix} characters; they are always supposed to be output
verbatim (fixups aside). See below for specific values this field can have.
@item fr_subtype
Relaxation substate. If the macro @code{md_relax_frag} isn't defined, this is
assumed to be an index into @code{md_relax_table} for the generic relaxation
code to process. (@xref{Relaxation}.) If @code{md_relax_frag} is defined,
this field is available for any use by the CPU-specific code.
@item align_mask
@itemx align_offset
These fields are not used yet. They are intended to keep track of the
alignment of the current frag within its section, even if the exact offset
isn't known. In many cases, we should be able to avoid creating extra frags
when @code{.align} directives are given; instead, the number of bytes needed
may be computable when the @code{.align} directive is processed. Hmm. Is this
the right place for these, or should they be in the @code{frchainS} structure?
@item fr_pcrel_adjust
@itemx fr_bsr
These fields are only used in the NS32k configuration. But since @code{struct
frag} is defined before the CPU-specific header files are included, they must
unconditionally be defined.
@item fr_literal
Declared as a one-character array, this last field grows arbitrarily large to
hold the actual contents of the frag.
@end table
These are the possible relaxation states, provided in the enumeration type
@code{relax_stateT}, and the interpretations they represent for the other
fields:
@table @code
@item rs_align
The start of the following frag should be aligned on some boundary. In this
frag, @code{fr_offset} is the logarithm (base 2) of the alignment in bytes.
(For example, if alignment on an 8-byte boundary were desired, @code{fr_offset}
would have a value of 3.) The variable characters indicate the fill pattern to
be used. (More than one?)
@item rs_broken_word
This indicates that ``broken word'' processing should be done. @xref{Broken
Words,,Broken Words}. If broken word processing is not necessary on the target
machine, this enumerator value will not be defined.
@item rs_fill
The variable characters are to be repeated @code{fr_offset} times. If
@code{fr_offset} is 0, this frag has a length of @code{fr_fix}.
@item rs_machine_dependent
Displacement relaxation is to be done on this frag. The target is indicated by
@code{fr_symbol} and @code{fr_offset}, and @code{fr_subtype} indicates the
particular machine-specific addressing mode desired. @xref{Relaxation}.
@item rs_org
The start of the following frag should be pushed back to some specific offset
within the section. (Some assemblers use the value as an absolute address; the
@sc{gnu} assembler does not handle final absolute addresses, it requires that
the linker set them.) The offset is given by @code{fr_symbol} and
@code{fr_offset}; one character from the variable-length tail is used as the
fill character.
@end table
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.
@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.
@node Broken Words
@subsection Broken Words
@cindex internals, broken words @cindex internals, broken words
@cindex broken words @cindex broken words
@cindex promises, promises @cindex promises, promises
The ``broken word'' idea derives from the fact that some compilers, including
@code{gcc}, will sometimes emit switch tables specifying 16-bit @code{.word}
displacements to branch targets, and branch instructions that load entries from
that table to compute the target address. If this is done on a 32-bit machine,
there is a chance (at least with really large functions) that the displacement
will not fit in 16 bits. Thus the ``broken word'' idea is well named, since
there is an implied promise that the 16-bit field will in fact hold the
specified displacement.
If the ``broken word'' processing is enabled, and a situation like this is
encountered, the assembler will insert a jump instruction into the instruction
stream, close enough to be reached with the 16-bit displacement. This jump
instruction will transfer to the real desired target address. Thus, as long as
the @code{.word} value really is used as a displacement to compute an address
to jump to, the net effect will be correct (minus a very small efficiency
cost). If @code{.word} directives with label differences for values are used
for other purposes, however, things may not work properly. I think there is a
command-line option to turn on warnings when a broken word is discovered.
This code is turned off by the @code{WORKING_DOT_WORD} macro. It isn't needed
if @code{.word} emits a value large enough to contain an address (or, more
correctly, any possible difference between two addresses).
@node What Happens? @node What Happens?
@section What Happens? @section What Happens?
Blah blah blah, initialization, argument parsing, file reading, Blah blah blah, initialization, argument parsing, file reading, whitespace
whitespace munging, opcode parsing and lookup, operand parsing. Now munging, opcode parsing and lookup, operand parsing. Now it's time to write
it's time to write the output file. the output file.
In @code{BFD_ASSEMBLER} mode, processing of relocations and symbols and In @code{BFD_ASSEMBLER} mode, processing of relocations and symbols and
creation of the output file is initiated by calling creation of the output file is initiated by calling @code{write_object_file}.
@code{write_object_file}.
@node Target Dependent Definitions @node Target Dependent Definitions
@section Target Dependent Definitions @section Target Dependent Definitions
@subheader Format-specific definitions @subsection Format-specific definitions
@defmac obj_sec_sym_ok_for_reloc section @defmac obj_sec_sym_ok_for_reloc (section)
(@code{BFD_ASSEMBLER} only.) (@code{BFD_ASSEMBLER} only.)
Is it okay to use this section's section-symbol in a relocation entry? Is it okay to use this section's section-symbol in a relocation entry? If not,
If not, a new internal-linkage symbol is generated and emitted if such a a new internal-linkage symbol is generated and emitted if such a relocation
relocation entry is needed. (Default: Always use a new symbol.) entry is needed. (Default: Always use a new symbol.)
@end defmac
@defmac obj_adjust_symtab
(@code{BFD_ASSEMBLER} only.)
If this macro is defined, it is invoked just before setting the symbol table of
the output BFD. Any finalizing changes needed in the symbol table should be
done here. For example, in the COFF support, if there is no @code{.file}
symbol defined already, one is generated at this point. If no such adjustments
are needed, this macro need not be defined.
@end defmac
@defmac EMIT_SECTION_SYMBOLS @defmac EMIT_SECTION_SYMBOLS
(@code{BFD_ASSEMBLER} only.) (@code{BFD_ASSEMBLER} only.)
Should section symbols be included in the symbol list if they're used in Should section symbols be included in the symbol list if they're used in
relocations? Some formats can generate section-relative relocations, relocations? Some formats can generate section-relative relocations, and thus
and thus don't need don't need symbols emitted for them. (Default: 1.)
(Default: 1.) @end defmac
@defmac obj_frob_file
Any final cleanup needed before writing out the BFD may be done here. For
example, ECOFF formats (and MIPS ELF format) may do some work on the MIPS-style
symbol table with its integrated debug information. The symbol table should
not be modified at this time.
@end defmac
@subsection CPU-specific definitions
@node Relaxation
@subsubsection Relaxation
@cindex Relaxation
If @code{md_relax_frag} isn't defined, the assembler will perform some
relaxation on @code{rs_machine_dependent} frags based on the frag subtype and
the displacement to some specified target address. The basic idea is that many
machines have different addressing modes for instructions that can specify
different ranges of values, with successive modes able to access wider ranges,
including the entirety of the previous range. Smaller ranges are assumed to be
more desirable (perhaps the instruction requires one word instead of two or
three); if this is not the case, don't describe the smaller-range, inferior
mode.
The @code{fr_subtype} and the field of a frag is an index into a CPU-specific
relaxation table. That table entry indicates the range of values that can be
stored, the number of bytes that will have to be added to the frag to
accomodate the addressing mode, and the index of the next entry to examine if
the value to be stored is outside the range accessible by the current
addressing mode. The @code{fr_symbol} field of the frag indicates what symbol
is to be accessed; the @code{fr_offset} field is added in.
If the @code{fr_pcrel_adjust} field is set, which currently should only happen
for the NS32k family, the @code{TC_PCREL_ADJUST} macro is called on the frag to
compute an adjustment to be made to the displacement.
The value fitted by the relaxation code is always assumed to be a displacement
from the current frag. (More specifically, from @code{fr_fix} bytes into the
frag.) This seems kinda silly. What about fitting small absolute values? I
suppose @code{md_assemble} is supposed to take care of that, but if the operand
is a difference between symbols, it might not be able to, if the difference was
not computable yet.
The end of the relaxation sequence is indicated by a ``next'' value of 0. This
is kinda silly too, since it means that the first entry in the table can't be
used. I think -1 would make a more logical sentinel value.
The table @code{md_relax_table} from @file{targ-cpu.c} describes the relaxation
modes available. Currently this must always be provided, even on machines for
which this type of relaxation isn't possible or practical. Probably fewer than
half the machines gas supports used it; it ought to be made conditional on some
CPU-specific macro. Currently, also that table must be declared ``const;'' on
some machines, though, it might make sense to keep it writeable, so it can be
modified depending on which CPU of a family is specified. For example, in the
m68k family, the 68020 has some addressing modes that are not available on the
68000.
The relaxation table type contains these fields:
@table @code
@item long rlx_forward
Forward reach, must be non-negative.
@item long rlx_backward
Backward reach, must be zero or negative.
@item rlx_length
Length in bytes of this addressing mode.
@item rlx_more
Index of the next-longer relax state, or zero if there is no ``next''
relax state.
@end table
The relaxation is done in @code{relax_segment} in @file{write.c}. The
difference in the length fields between the original mode and the one finally
chosen by the relaxing code is taken as the size by which the current frag will
be increased in size. For example, if the initial relaxing mode has a length
of 2 bytes, and because of the size of the displacement, it gets upgraded to a
mode with a size of 6 bytes, it is assumed that the frag will grow by 4 bytes.
(The initial two bytes should have been part of the fixed portion of the frag,
since it is already known that they will be output.) This growth must be
effected by @code{md_convert_frag}; it should increase the @code{fr_fix} field
by the appropriate size, and fill in the appropriate bytes of the frag.
(Enough space for the maximum growth should have been allocated in the call to
frag_var as the second argument.)
If relocation records are needed, they should be emitted by
@code{md_estimate_size_before_relax}.
These are the machine-specific definitions associated with the relaxation
mechanism:
@deftypefun int md_estimate_size_before_relax (fragS *@var{frag}, segT @var{sec})
This function should examine the target symbol of the supplied frag and correct
the @code{fr_subtype} of the frag if needed. When this function is called, if
the symbol has not yet been defined, it will not become defined later; however,
its value may still change if the section it is in gets relaxed.
Usually, if the symbol is in the same section as the frag (given by the
@var{sec} argument), the narrowest likely relaxation mode is stored in
@code{fr_subtype}, and that's that.
If the symbol is undefined, or in a different section (and therefore moveable
to an arbitrarily large distance), the largest available relaxation mode is
specified, @code{fix_new} is called to produce the relocation record,
@code{fr_fix} is increased to include the relocated field (remember, this
storage was allocated when @code{frag_var} was called), and @code{frag_wane} is
called to convert the frag to an @code{rs_fill} frag with no variant part.
Sometimes changing addressing modes may also require rewriting the instruction.
It can be accessed via @code{fr_opcode} or @code{fr_fix}.
Sometimes @code{fr_var} is increased instead, and @code{frag_wane} is not
called. I'm not sure, but I think this is to keep @code{fr_fix} referring to
an earlier byte, and @code{fr_subtype} set to @code{rs_machine_dependent} so
that @code{md_convert_frag} will get called.
@end deftypefun
@deftypevar relax_typeS md_relax_table []
This is the table.
@end deftypevar
@defmac md_relax_frag (@var{frag})
This macro, if defined, overrides all of the processing described above. It's
only defined for the MIPS target CPU, and there it doesn't do anything; it's
used solely to disable the relaxing code and free up the @code{fr_subtype}
field for use by the CPU-specific code.
@end defmac
@defmac tc_frob_file
Like @code{obj_frob_file}, this macro handles miscellaneous last-minute
cleanup. Currently only used on PowerPC/POWER support, for setting up a
@code{.debug} section. This macro should not cause the symbol table to be
modified.
@end defmac
@node Source File Summary @node Source File Summary
@section Source File Summary @section Source File Summary
The code in the @file{obj-coff} back end assumes @code{BFD_ASSEMBLER} is @subsection File Format Descriptions
defined; the code in @file{obj-coffbfd} uses @code{BFD},
@code{BFD_HEADERS}, and @code{MANY_SEGMENTS}, but does a lot of the file @subheading a.out
positioning itself. This confusing situation arose from the history of
the code. The @code{a.out} format is described by @file{obj-aout.*}.
@subheading b.out
The @code{b.out} format, described by @file{obj-bout.*}, is similar to
@code{a.out} format, except for a few additional fields in the file header
describing section alignment and address.
@subheading COFF
Originally, @file{obj-coff} was a purely non-BFD version, and Originally, @file{obj-coff} was a purely non-BFD version, and
@file{obj-coffbfd} was created to use BFD for low-level byte-swapping. @file{obj-coffbfd} was created to use BFD for low-level byte-swapping. When
When the @code{BFD_ASSEMBLER} conversion started, the first COFF target the @code{BFD_ASSEMBLER} conversion started, the first COFF target to be
to be converted was using @file{obj-coff}, and the two files had converted was using @file{obj-coff}, and the two files had diverged somewhat,
diverged somewhat, and I didn't feel like first converting the support and I didn't feel like first converting the support of that target over to use
of that target over to use the low-level BFD interface. the low-level BFD interface.
Currently, all COFF targets use one of the two BFD interfaces, so the So @file{obj-coff} got converted, and to simplify certain things,
non-BFD code can be removed. Eventually, all should be converted to @file{obj-coffbfd} got ``merged'' in with a brute-force approach.
using one COFF back end, which uses the high-level BFD interface. Specifically, preprocessor conditionals testing for @code{BFD_ASSEMBLER}
effectively split the @file{obj-coff} files into the two separate versions. It
isn't pretty. They will be merged more thoroughly, and eventually only the
higher-level interface will be used.
@subheading ECOFF
All ECOFF configurations use BFD for writing object files.
@subheading ELF
ELF is a fairly reasonable format, without many of the deficiencies the other
object file formats have. (It's got some of its own, but not as bad as the
others.) All ELF configurations use BFD for writing object files.
@subheading EVAX
This is the format used on VMS. Yes, someone has actually written BFD support
for it. The code hasn't been integrated yet though.
@subheading HP300?
@subheading IEEE?
@subheading SOM
@subheading XCOFF
The XCOFF configuration is based on the COFF cofiguration (using the
higher-level BFD interface). In fact, it uses the same files in the assembler.
@subheading VMS
This is the old Vax VMS support. It doesn't use BFD.
@subsection Processor Descriptions
Foo: a29k, alpha, h8300, h8500, hppa, i386, i860, i960, m68k, m88k, mips,
ns32k, ppc, sh, sparc, tahoe, vax, z8k.
@node M68k
@subsubsection M68k
The operand syntax handling is atrocious. There is no clear specification of
the operand syntax. I'm looking into using a Bison grammar to replace much of
it.
Operands on the 68k series processors can have two displacement values
specified, plus a base register and a (possibly scaled) index register of which
only some bits might be used. Thus a single 68k operand requires up to two
expressions, two register numbers, and size and scale factors. The
@code{struct m68k_op} type also includes a field indicating the mode of the
operand, and an @code{error} field indicating a problem encountered while
parsing the operand.
An instruction on the 68k may have up to 6 operands, although most of them have
to be simple register operands. Up to 11 (16-bit) words may be required to
express the instruction.
A @code{struct m68k_exp} expression contains an @code{expressionS}, pointers to
the first and last characters of the input that produced the expression, an
indication of the section to which the expression belongs, and a size field.
I'm not sure what the size field describes.
@subsubheading M68k addressing modes
Many instructions used the low six bits of the first instruction word to
describe the location of the operand, or how to compute the location. The six
bits are typically split into three for a ``mode'' and three for a ``register''
value. The interpretation of these values is as follows:
@example
Mode Register Operand addressing mode
0 Dn data register
1 An address register
2 An indirect
3 An indirect, post-increment
4 An indirect, pre-decrement
5 An indirect with displacement
6 An indirect with optional displacement and index;
may involve multiple indirections and two
displacements
7 0 16-bit address follows
7 1 32-bit address follows
7 2 PC indirect with displacement
7 3 PC indirect with optional displacements and index
7 4 immediate 16- or 32-bit
7 5,6,7 Reserved
@end example
On the 68000 and 68010, support for modes 6 and 7.3 are incomplete; the
displacement must fit in 8 bits, and no scaling or index suppression is
permitted.
@subsubheading M68k relaxation modes
The relaxation modes used on the 68k are:
@table @code
@item ABRANCH
Case @samp{g} except when @code{BCC68000} is applicable.
@item FBRANCH
Coprocessor branches.
@item PCREL
Mode 7.2 -- program counter indirect with 16-bit displacement. This is
available on all processors. Widens to 32-bit absolute. Used only if the
original code used @code{ABSL} mode, and the CPU is not a 68000 or 68010.
(Why? Those processors support mode 7.2.)
@item BCC68000
A conditional branch instruction, on the 68000 or 68010. These instructions
support only 16-bit displacements on these processors. If a larger
displacement is needed, the condition is negated and turned into a short branch
around a jump instruction to the specified target. This jump will have an
long absolute addressing mode.
@item DBCC
Like @code{BCC68000}, but for @code{dbCC} (decrement and branch on condition)
instructions.
@item PCLEA
Not currently used?? Short form is mode 7.2 (program counter indirect, 16-bit
displacement); long form is 7.3/0x0170 (program counter indirect, suppressed
index register, 32-bit displacement). Used in progressive-930331 for mode
@code{AOFF} with a PC-relative addressing mode and a displacement that won't
fit in 16 bits, or which is variable and is not specified to have a size other
than long.
@item PCINDEX
Newly added. PC indirect with index. An 8-bit displacement is supported on
the 68000 and 68010, wider displacements on later processors.
Well, actually, I haven't added it yet. I need to soon, though. It fixes a
bug reported by a customer.
@end table
@subsection ``Emulation'' Descriptions
These are the @file{te-*.h} files.
@node Foo @node Foo
@section Foo @section Foo
@ -182,12 +634,12 @@ using one COFF back end, which uses the high-level BFD interface.
@deftypefun int had_warnings (void) @deftypefun int had_warnings (void)
@deftypefunx int had_errors (void) @deftypefunx int had_errors (void)
Returns non-zero if any warnings or errors, respectively, have been Returns non-zero if any warnings or errors, respectively, have been printed
printed during this invocation. during this invocation.
@end deftypefun @end deftypefun
@deftypefun void as_perror (const char *@var{gripe}, const char *@var{filename}) @deftypefun void as_perror (const char *@var{gripe}, const char *@var{filename})
Displays a BFD or system error, then clears the error status. Displays a BFD or system error, then clears the error status.
@ -198,33 +650,77 @@ Displays a BFD or system error, then clears the error status.
@deftypefunx void as_bad (const char *@var{format}, ...) @deftypefunx void as_bad (const char *@var{format}, ...)
@deftypefunx void as_fatal (const char *@var{format}, ...) @deftypefunx void as_fatal (const char *@var{format}, ...)
These functions display messages about something amiss with the input These functions display messages about something amiss with the input file, or
file, or internal problems in the assembler itself. The current file internal problems in the assembler itself. The current file name and line
name and line number are printed, followed by the supplied message, number are printed, followed by the supplied message, formatted using
formatted using @code{vfprintf}, and a final newline. @code{vfprintf}, and a final newline.
An error indicated by @code{as_bad} will result in a non-zero exit status when
the assembler has finished. Calling @code{as_fatal} will result in immediate
termination of the assembler process.
@end deftypefun @end deftypefun
@deftypefun void as_warn_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...) @deftypefun void as_warn_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...)
@deftypefunx void as_bad_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...) @deftypefunx void as_bad_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...)
These variants permit specification of the file name and line number, These variants permit specification of the file name and line number, and are
and are used when problems are detected when reprocessing information used when problems are detected when reprocessing information saved away when
saved away when processing some earlier part of the file. For example, processing some earlier part of the file. For example, fixups are processed
fixups are processed after all input has been read, but messages about after all input has been read, but messages about fixups should refer to the
fixups should refer to the original filename and line number that they original filename and line number that they are applicable to.
are applicable to.
@end deftypefun @end deftypefun
@deftypefun void fprint_value (FILE *@var{file}, valueT @var{val}) @deftypefun void fprint_value (FILE *@var{file}, valueT @var{val})
@deftypefunx void sprint_value (char *@var{buf}, valueT @var{val}) @deftypefunx void sprint_value (char *@var{buf}, valueT @var{val})
These functions are helpful for converting a @code{valueT} value into These functions are helpful for converting a @code{valueT} value into printable
printable format, in case it's wider than modes that @code{*printf} can format, in case it's wider than modes that @code{*printf} can handle. If the
handle. If the type is narrow enough, a decimal number will be type is narrow enough, a decimal number will be produced; otherwise, it will be
produced; otherwise, it will be in hexadecimal (FIXME: currently without in hexadecimal (FIXME: currently without `0x' prefix). The value itself is not
`0x' prefix). The value itself is not examined to make this examined to make this determination.
determination.
@end deftypefun @end deftypefun
@node Writing a new target
@section Writing a new target
@node Test suite
@section Test suite
@cindex test suite
The test suite is kind of lame for most processors. Often it only checks to
see if a couple of files can be assembled without the assembler reporting any
errors. For more complete testing, write a test which either examines the
assembler listing, or runs @code{objdump} and examines its output. For the
latter, the TCL procedure @code{run_dump_test} may come in handy. It takes the
base name of a file, and looks for @file{@var{file}.d}. This file should
contain as its initial lines a set of variable settings in @samp{#} comments,
in the form:
@example
#@var{varname}: @var{value}
@end example
The @var{varname} may be @code{objdump}, @code{nm}, or @code{as}, in which case
it specifies the options to be passed to the specified programs. Exactly one
of @code{objdump} or @code{nm} must be specified, as that also specifies which
program to run after the assembler has finished. If @var{varname} is
@code{source}, it specifies the name of the source file; otherwise,
@file{@var{file}.s} is used. If @var{varname} is @code{name}, it specifies the
name of the test to be used in the @code{pass} or @code{fail} messages.
The non-commented parts of the file are interpreted as regular expressions, one
per line. Blank lines in the @code{objdump} or @code{nm} output are skipped,
as are blank lines in the @code{.d} file; the other lines are tested to see if
the regular expression matches the program output. If it does not, the test
fails.
Note that this means the tests must be modified if the @code{objdump} output
style is changed.
@bye
@c Local Variables:
@c fill-column: 79
@c End: