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BFD supports a number of different flavours of coff format. The major differences between formats are the sizes and alignments of fields in structures on disk, and the occasional extra field.
Coff in all its varieties is implemented with a few common
files and a number of implementation specific files. For
example, The 88k bcs coff format is implemented in the file
coff-m88k.c. This file #includes
coff/m88k.h which defines the external structure of the
coff format for the 88k, and coff/internal.h which
defines the internal structure. coff-m88k.c also
defines the relocations used by the 88k format
See Relocations.
The Intel i960 processor version of coff is implemented in coff-i960.c. This file has the same structure as coff-m88k.c, except that it includes coff/i960.h rather than coff-m88k.h.
The recommended method is to select from the existing
implementations the version of coff which is most like the one
you want to use. For example, we'll say that i386 coff is
the one you select, and that your coff flavour is called foo.
Copy i386coff.c to foocoff.c, copy
../include/coff/i386.h to ../include/coff/foo.h,
and add the lines to targets.c and Makefile.in
so that your new back end is used. Alter the shapes of the
structures in ../include/coff/foo.h so that they match
what you need. You will probably also have to add
#ifdefs to the code in coff/internal.h and
coffcode.h if your version of coff is too wild.
You can verify that your new BFD backend works quite simply by
building objdump from the binutils directory,
and making sure that its version of what's going on and your
host system's idea (assuming it has the pretty standard coff
dump utility, usually called att-dump or just
dump) are the same. Then clean up your code, and send
what you've done to Cygnus. Then your stuff will be in the
next release, and you won't have to keep integrating it.
The Coff backend is split into generic routines that are applicable to any Coff target and routines that are specific to a particular target. The target-specific routines are further split into ones which are basically the same for all Coff targets except that they use the external symbol format or use different values for certain constants.
The generic routines are in coffgen.c. These routines
work for any Coff target. They use some hooks into the target
specific code; the hooks are in a bfd_coff_backend_data
structure, one of which exists for each target.
The essentially similar target-specific routines are in coffcode.h. This header file includes executable C code. The various Coff targets first include the appropriate Coff header file, make any special defines that are needed, and then include coffcode.h.
Some of the Coff targets then also have additional routines in the target source file itself.
For example, coff-i960.c includes
coff/internal.h and coff/i960.h. It then
defines a few constants, such as I960, and includes
coffcode.h. Since the i960 has complex relocation
types, coff-i960.c also includes some code to
manipulate the i960 relocs. This code is not in
coffcode.h because it would not be used by any other
target.
In the standard Coff object format, section names are limited to
the eight bytes available in the s_name field of the
SCNHDR section header structure. The format requires the
field to be NUL-padded, but not necessarily NUL-terminated, so
the longest section names permitted are a full eight characters.
The Microsoft PE variants of the Coff object file format add
an extension to support the use of long section names. This
extension is defined in section 4 of the Microsoft PE/COFF
specification (rev 8.1). If a section name is too long to fit
into the section header's s_name field, it is instead
placed into the string table, and the s_name field is
filled with a slash ("/") followed by the ASCII decimal
representation of the offset of the full name relative to the
string table base.
Note that this implies that the extension can only be used in object files, as executables do not contain a string table. The standard specifies that long section names from objects emitted into executable images are to be truncated.
However, as a GNU extension, BFD can generate executable images that contain a string table and long section names. This would appear to be technically valid, as the standard only says that Coff debugging information is deprecated, not forbidden, and in practice it works, although some tools that parse PE files expecting the MS standard format may become confused; PEview is one known example.
The functionality is supported in BFD by code implemented under
the control of the macro COFF_LONG_SECTION_NAMES. If not
defined, the format does not support long section names in any way.
If defined, it is used to initialise a flag,
_bfd_coff_long_section_names, and a hook function pointer,
_bfd_coff_set_long_section_names, in the Coff backend data
structure. The flag controls the generation of long section names
in output BFDs at runtime; if it is false, as it will be by default
when generating an executable image, long section names are truncated;
if true, the long section names extension is employed. The hook
points to a function that allows the value of the flag to be altered
at runtime, on formats that support long section names at all; on
other formats it points to a stub that returns an error indication.
With input BFDs, the flag is set according to whether any long section
names are detected while reading the section headers. For a completely
new BFD, the flag is set to the default for the target format. This
information can be used by a client of the BFD library when deciding
what output format to generate, and means that a BFD that is opened
for read and subsequently converted to a writeable BFD and modified
in-place will retain whatever format it had on input.
If COFF_LONG_SECTION_NAMES is simply defined (blank), or is
defined to the value "1", then long section names are enabled by
default; if it is defined to the value zero, they are disabled by
default (but still accepted in input BFDs). The header coffcode.h
defines a macro, COFF_DEFAULT_LONG_SECTION_NAMES, which is
used in the backends to initialise the backend data structure fields
appropriately; see the comments for further detail.
Each flavour of coff supported in BFD has its own header file
describing the external layout of the structures. There is also
an internal description of the coff layout, in
coff/internal.h. A major function of the
coff backend is swapping the bytes and twiddling the bits to
translate the external form of the structures into the normal
internal form. This is all performed in the
bfd_swap_thing_direction routines. Some
elements are different sizes between different versions of
coff; it is the duty of the coff version specific include file
to override the definitions of various packing routines in
coffcode.h. E.g., the size of line number entry in coff is
sometimes 16 bits, and sometimes 32 bits. #defineing
PUT_LNSZ_LNNO and GET_LNSZ_LNNO will select the
correct one. No doubt, some day someone will find a version of
coff which has a varying field size not catered to at the
moment. To port BFD, that person will have to add more #defines.
Three of the bit twiddling routines are exported to
gdb; coff_swap_aux_in, coff_swap_sym_in
and coff_swap_lineno_in. GDB reads the symbol
table on its own, but uses BFD to fix things up. More of the
bit twiddlers are exported for gas;
coff_swap_aux_out, coff_swap_sym_out,
coff_swap_lineno_out, coff_swap_reloc_out,
coff_swap_filehdr_out, coff_swap_aouthdr_out,
coff_swap_scnhdr_out. Gas currently keeps track
of all the symbol table and reloc drudgery itself, thereby
saving the internal BFD overhead, but uses BFD to swap things
on the way out, making cross ports much safer. Doing so also
allows BFD (and thus the linker) to use the same header files
as gas, which makes one avenue to disaster disappear.
The simple canonical form for symbols used by BFD is not rich enough to keep all the information available in a coff symbol table. The back end gets around this problem by keeping the original symbol table around, "behind the scenes".
When a symbol table is requested (through a call to
bfd_canonicalize_symtab), a request gets through to
coff_get_normalized_symtab. This reads the symbol table from
the coff file and swaps all the structures inside into the
internal form. It also fixes up all the pointers in the table
(represented in the file by offsets from the first symbol in
the table) into physical pointers to elements in the new
internal table. This involves some work since the meanings of
fields change depending upon context: a field that is a
pointer to another structure in the symbol table at one moment
may be the size in bytes of a structure at the next. Another
pass is made over the table. All symbols which mark file names
(C_FILE symbols) are modified so that the internal
string points to the value in the auxent (the real filename)
rather than the normal text associated with the symbol
(".file").
At this time the symbol names are moved around. Coff stores all symbols less than nine characters long physically within the symbol table; longer strings are kept at the end of the file in the string table. This pass moves all strings into memory and replaces them with pointers to the strings.
The symbol table is massaged once again, this time to create
the canonical table used by the BFD application. Each symbol
is inspected in turn, and a decision made (using the
sclass field) about the various flags to set in the
asymbol. See Symbols. The generated canonical table
shares strings with the hidden internal symbol table.
Any linenumbers are read from the coff file too, and attached to the symbols which own the functions the linenumbers belong to.
Writing a symbol to a coff file which didn't come from a coff
file will lose any debugging information. The asymbol
structure remembers the BFD from which the symbol was taken, and on
output the back end makes sure that the same destination target as
source target is present.
When the symbols have come from a coff file then all the debugging information is preserved.
Symbol tables are provided for writing to the back end in a vector of pointers to pointers. This allows applications like the linker to accumulate and output large symbol tables without having to do too much byte copying.
This function runs through the provided symbol table and
patches each symbol marked as a file place holder
(C_FILE) to point to the next file place holder in the
list. It also marks each offset field in the list with
the offset from the first symbol of the current symbol.
Another function of this procedure is to turn the canonical
value form of BFD into the form used by coff. Internally, BFD
expects symbol values to be offsets from a section base; so a
symbol physically at 0x120, but in a section starting at
0x100, would have the value 0x20. Coff expects symbols to
contain their final value, so symbols have their values
changed at this point to reflect their sum with their owning
section. This transformation uses the
output_section field of the asymbol's
asection See Sections.
coff_mangle_symbols
coff_write_symbols
coff_symbol_typeDescription
The hidden information for an asymbol is described in a
combined_entry_type:
typedef struct coff_ptr_struct
{
/* Remembers the offset from the first symbol in the file for
this symbol. Generated by coff_renumber_symbols. */
unsigned int offset;
/* Should the value of this symbol be renumbered. Used for
XCOFF C_BSTAT symbols. Set by coff_slurp_symbol_table. */
unsigned int fix_value : 1;
/* Should the tag field of this symbol be renumbered.
Created by coff_pointerize_aux. */
unsigned int fix_tag : 1;
/* Should the endidx field of this symbol be renumbered.
Created by coff_pointerize_aux. */
unsigned int fix_end : 1;
/* Should the x_csect.x_scnlen field be renumbered.
Created by coff_pointerize_aux. */
unsigned int fix_scnlen : 1;
/* Fix up an XCOFF C_BINCL/C_EINCL symbol. The value is the
index into the line number entries. Set by coff_slurp_symbol_table. */
unsigned int fix_line : 1;
/* The container for the symbol structure as read and translated
from the file. */
union
{
union internal_auxent auxent;
struct internal_syment syment;
} u;
} combined_entry_type;
/* Each canonical asymbol really looks like this: */
typedef struct coff_symbol_struct
{
/* The actual symbol which the rest of BFD works with */
asymbol symbol;
/* A pointer to the hidden information for this symbol */
combined_entry_type *native;
/* A pointer to the linenumber information for this symbol */
struct lineno_cache_entry *lineno;
/* Have the line numbers been relocated yet ? */
bfd_boolean done_lineno;
} coff_symbol_type;
bfd_coff_backend_data /* COFF symbol classifications. */
enum coff_symbol_classification
{
/* Global symbol. */
COFF_SYMBOL_GLOBAL,
/* Common symbol. */
COFF_SYMBOL_COMMON,
/* Undefined symbol. */
COFF_SYMBOL_UNDEFINED,
/* Local symbol. */
COFF_SYMBOL_LOCAL,
/* PE section symbol. */
COFF_SYMBOL_PE_SECTION
};
Special entry points for gdb to swap in coff symbol table parts:
typedef struct
{
void (*_bfd_coff_swap_aux_in)
(bfd *, void *, int, int, int, int, void *);
void (*_bfd_coff_swap_sym_in)
(bfd *, void *, void *);
void (*_bfd_coff_swap_lineno_in)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_aux_out)
(bfd *, void *, int, int, int, int, void *);
unsigned int (*_bfd_coff_swap_sym_out)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_lineno_out)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_reloc_out)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_filehdr_out)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_aouthdr_out)
(bfd *, void *, void *);
unsigned int (*_bfd_coff_swap_scnhdr_out)
(bfd *, void *, void *);
unsigned int _bfd_filhsz;
unsigned int _bfd_aoutsz;
unsigned int _bfd_scnhsz;
unsigned int _bfd_symesz;
unsigned int _bfd_auxesz;
unsigned int _bfd_relsz;
unsigned int _bfd_linesz;
unsigned int _bfd_filnmlen;
bfd_boolean _bfd_coff_long_filenames;
bfd_boolean _bfd_coff_long_section_names;
bfd_boolean (*_bfd_coff_set_long_section_names)
(bfd *, int);
unsigned int _bfd_coff_default_section_alignment_power;
bfd_boolean _bfd_coff_force_symnames_in_strings;
unsigned int _bfd_coff_debug_string_prefix_length;
void (*_bfd_coff_swap_filehdr_in)
(bfd *, void *, void *);
void (*_bfd_coff_swap_aouthdr_in)
(bfd *, void *, void *);
void (*_bfd_coff_swap_scnhdr_in)
(bfd *, void *, void *);
void (*_bfd_coff_swap_reloc_in)
(bfd *abfd, void *, void *);
bfd_boolean (*_bfd_coff_bad_format_hook)
(bfd *, void *);
bfd_boolean (*_bfd_coff_set_arch_mach_hook)
(bfd *, void *);
void * (*_bfd_coff_mkobject_hook)
(bfd *, void *, void *);
bfd_boolean (*_bfd_styp_to_sec_flags_hook)
(bfd *, void *, const char *, asection *, flagword *);
void (*_bfd_set_alignment_hook)
(bfd *, asection *, void *);
bfd_boolean (*_bfd_coff_slurp_symbol_table)
(bfd *);
bfd_boolean (*_bfd_coff_symname_in_debug)
(bfd *, struct internal_syment *);
bfd_boolean (*_bfd_coff_pointerize_aux_hook)
(bfd *, combined_entry_type *, combined_entry_type *,
unsigned int, combined_entry_type *);
bfd_boolean (*_bfd_coff_print_aux)
(bfd *, FILE *, combined_entry_type *, combined_entry_type *,
combined_entry_type *, unsigned int);
void (*_bfd_coff_reloc16_extra_cases)
(bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,
bfd_byte *, unsigned int *, unsigned int *);
int (*_bfd_coff_reloc16_estimate)
(bfd *, asection *, arelent *, unsigned int,
struct bfd_link_info *);
enum coff_symbol_classification (*_bfd_coff_classify_symbol)
(bfd *, struct internal_syment *);
bfd_boolean (*_bfd_coff_compute_section_file_positions)
(bfd *);
bfd_boolean (*_bfd_coff_start_final_link)
(bfd *, struct bfd_link_info *);
bfd_boolean (*_bfd_coff_relocate_section)
(bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,
struct internal_reloc *, struct internal_syment *, asection **);
reloc_howto_type *(*_bfd_coff_rtype_to_howto)
(bfd *, asection *, struct internal_reloc *,
struct coff_link_hash_entry *, struct internal_syment *,
bfd_vma *);
bfd_boolean (*_bfd_coff_adjust_symndx)
(bfd *, struct bfd_link_info *, bfd *, asection *,
struct internal_reloc *, bfd_boolean *);
bfd_boolean (*_bfd_coff_link_add_one_symbol)
(struct bfd_link_info *, bfd *, const char *, flagword,
asection *, bfd_vma, const char *, bfd_boolean, bfd_boolean,
struct bfd_link_hash_entry **);
bfd_boolean (*_bfd_coff_link_output_has_begun)
(bfd *, struct coff_final_link_info *);
bfd_boolean (*_bfd_coff_final_link_postscript)
(bfd *, struct coff_final_link_info *);
bfd_boolean (*_bfd_coff_print_pdata)
(bfd *, void *);
} bfd_coff_backend_data;
#define coff_backend_info(abfd) \
((bfd_coff_backend_data *) (abfd)->xvec->backend_data)
#define bfd_coff_swap_aux_in(a,e,t,c,ind,num,i) \
((coff_backend_info (a)->_bfd_coff_swap_aux_in) (a,e,t,c,ind,num,i))
#define bfd_coff_swap_sym_in(a,e,i) \
((coff_backend_info (a)->_bfd_coff_swap_sym_in) (a,e,i))
#define bfd_coff_swap_lineno_in(a,e,i) \
((coff_backend_info ( a)->_bfd_coff_swap_lineno_in) (a,e,i))
#define bfd_coff_swap_reloc_out(abfd, i, o) \
((coff_backend_info (abfd)->_bfd_coff_swap_reloc_out) (abfd, i, o))
#define bfd_coff_swap_lineno_out(abfd, i, o) \
((coff_backend_info (abfd)->_bfd_coff_swap_lineno_out) (abfd, i, o))
#define bfd_coff_swap_aux_out(a,i,t,c,ind,num,o) \
((coff_backend_info (a)->_bfd_coff_swap_aux_out) (a,i,t,c,ind,num,o))
#define bfd_coff_swap_sym_out(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_sym_out) (abfd, i, o))
#define bfd_coff_swap_scnhdr_out(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_out) (abfd, i, o))
#define bfd_coff_swap_filehdr_out(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_out) (abfd, i, o))
#define bfd_coff_swap_aouthdr_out(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_out) (abfd, i, o))
#define bfd_coff_filhsz(abfd) (coff_backend_info (abfd)->_bfd_filhsz)
#define bfd_coff_aoutsz(abfd) (coff_backend_info (abfd)->_bfd_aoutsz)
#define bfd_coff_scnhsz(abfd) (coff_backend_info (abfd)->_bfd_scnhsz)
#define bfd_coff_symesz(abfd) (coff_backend_info (abfd)->_bfd_symesz)
#define bfd_coff_auxesz(abfd) (coff_backend_info (abfd)->_bfd_auxesz)
#define bfd_coff_relsz(abfd) (coff_backend_info (abfd)->_bfd_relsz)
#define bfd_coff_linesz(abfd) (coff_backend_info (abfd)->_bfd_linesz)
#define bfd_coff_filnmlen(abfd) (coff_backend_info (abfd)->_bfd_filnmlen)
#define bfd_coff_long_filenames(abfd) \
(coff_backend_info (abfd)->_bfd_coff_long_filenames)
#define bfd_coff_long_section_names(abfd) \
(coff_backend_info (abfd)->_bfd_coff_long_section_names)
#define bfd_coff_set_long_section_names(abfd, enable) \
((coff_backend_info (abfd)->_bfd_coff_set_long_section_names) (abfd, enable))
#define bfd_coff_default_section_alignment_power(abfd) \
(coff_backend_info (abfd)->_bfd_coff_default_section_alignment_power)
#define bfd_coff_swap_filehdr_in(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_in) (abfd, i, o))
#define bfd_coff_swap_aouthdr_in(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_in) (abfd, i, o))
#define bfd_coff_swap_scnhdr_in(abfd, i,o) \
((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_in) (abfd, i, o))
#define bfd_coff_swap_reloc_in(abfd, i, o) \
((coff_backend_info (abfd)->_bfd_coff_swap_reloc_in) (abfd, i, o))
#define bfd_coff_bad_format_hook(abfd, filehdr) \
((coff_backend_info (abfd)->_bfd_coff_bad_format_hook) (abfd, filehdr))
#define bfd_coff_set_arch_mach_hook(abfd, filehdr)\
((coff_backend_info (abfd)->_bfd_coff_set_arch_mach_hook) (abfd, filehdr))
#define bfd_coff_mkobject_hook(abfd, filehdr, aouthdr)\
((coff_backend_info (abfd)->_bfd_coff_mkobject_hook)\
(abfd, filehdr, aouthdr))
#define bfd_coff_styp_to_sec_flags_hook(abfd, scnhdr, name, section, flags_ptr)\
((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook)\
(abfd, scnhdr, name, section, flags_ptr))
#define bfd_coff_set_alignment_hook(abfd, sec, scnhdr)\
((coff_backend_info (abfd)->_bfd_set_alignment_hook) (abfd, sec, scnhdr))
#define bfd_coff_slurp_symbol_table(abfd)\
((coff_backend_info (abfd)->_bfd_coff_slurp_symbol_table) (abfd))
#define bfd_coff_symname_in_debug(abfd, sym)\
((coff_backend_info (abfd)->_bfd_coff_symname_in_debug) (abfd, sym))
#define bfd_coff_force_symnames_in_strings(abfd)\
(coff_backend_info (abfd)->_bfd_coff_force_symnames_in_strings)
#define bfd_coff_debug_string_prefix_length(abfd)\
(coff_backend_info (abfd)->_bfd_coff_debug_string_prefix_length)
#define bfd_coff_print_aux(abfd, file, base, symbol, aux, indaux)\
((coff_backend_info (abfd)->_bfd_coff_print_aux)\
(abfd, file, base, symbol, aux, indaux))
#define bfd_coff_reloc16_extra_cases(abfd, link_info, link_order,\
reloc, data, src_ptr, dst_ptr)\
((coff_backend_info (abfd)->_bfd_coff_reloc16_extra_cases)\
(abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr))
#define bfd_coff_reloc16_estimate(abfd, section, reloc, shrink, link_info)\
((coff_backend_info (abfd)->_bfd_coff_reloc16_estimate)\
(abfd, section, reloc, shrink, link_info))
#define bfd_coff_classify_symbol(abfd, sym)\
((coff_backend_info (abfd)->_bfd_coff_classify_symbol)\
(abfd, sym))
#define bfd_coff_compute_section_file_positions(abfd)\
((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\
(abfd))
#define bfd_coff_start_final_link(obfd, info)\
((coff_backend_info (obfd)->_bfd_coff_start_final_link)\
(obfd, info))
#define bfd_coff_relocate_section(obfd,info,ibfd,o,con,rel,isyms,secs)\
((coff_backend_info (ibfd)->_bfd_coff_relocate_section)\
(obfd, info, ibfd, o, con, rel, isyms, secs))
#define bfd_coff_rtype_to_howto(abfd, sec, rel, h, sym, addendp)\
((coff_backend_info (abfd)->_bfd_coff_rtype_to_howto)\
(abfd, sec, rel, h, sym, addendp))
#define bfd_coff_adjust_symndx(obfd, info, ibfd, sec, rel, adjustedp)\
((coff_backend_info (abfd)->_bfd_coff_adjust_symndx)\
(obfd, info, ibfd, sec, rel, adjustedp))
#define bfd_coff_link_add_one_symbol(info, abfd, name, flags, section,\
value, string, cp, coll, hashp)\
((coff_backend_info (abfd)->_bfd_coff_link_add_one_symbol)\
(info, abfd, name, flags, section, value, string, cp, coll, hashp))
#define bfd_coff_link_output_has_begun(a,p) \
((coff_backend_info (a)->_bfd_coff_link_output_has_begun) (a, p))
#define bfd_coff_final_link_postscript(a,p) \
((coff_backend_info (a)->_bfd_coff_final_link_postscript) (a, p))
#define bfd_coff_have_print_pdata(a) \
(coff_backend_info (a)->_bfd_coff_print_pdata)
#define bfd_coff_print_pdata(a,p) \
((coff_backend_info (a)->_bfd_coff_print_pdata) (a, p))
/* Macro: Returns true if the bfd is a PE executable as opposed to a
PE object file. */
#define bfd_pei_p(abfd) \
(CONST_STRNEQ ((abfd)->xvec->name, "pei-"))
To write relocations, the back end steps though the
canonical relocation table and create an
internal_reloc. The symbol index to use is removed from
the offset field in the symbol table supplied. The
address comes directly from the sum of the section base
address and the relocation offset; the type is dug directly
from the howto field. Then the internal_reloc is
swapped into the shape of an external_reloc and written
out to disk.
Creating the linenumber table is done by reading in the entire coff linenumber table, and creating another table for internal use.
A coff linenumber table is structured so that each function is marked as having a line number of 0. Each line within the function is an offset from the first line in the function. The base of the line number information for the table is stored in the symbol associated with the function.
Note: The PE format uses line number 0 for a flag indicating a new source file.
The information is copied from the external to the internal table, and each symbol which marks a function is marked by pointing its...
How does this work ?
Coff relocations are easily transformed into the internal BFD form
(arelent).
Reading a coff relocation table is done in the following stages:
bfd_canonicalize_symtab. The back end will call that
routine and save the result if a canonicalization hasn't been done.
r_type to directly produce an index
into a howto table vector; the 88k subtracts a number from the
r_type field and creates an addend field.