86REL(F) UNIX System V
Name
86rel - Intel 8086 Relocatable Format for Object Modules
Syntax
#include <sys/relsym86.h>
Description
Intel 8086 Relocatable Format, or 86rel, is the object
module format generated by masm(CP), and the input format
for the linker ld(CP). The include file relsym86.h
specifies appropriate definitions to access 86rel format
files from C. For the technical details of the 86rel
format, see Intel 8086 Object Module Format External
Product Specification.
An 86rel consists of one or more variable length records.
Each record has at least three fields: the record type,
length, and checksum. The first byte always denotes the
record type. There are thirty-one different record types.
Only eleven are used by ld(CP) and masm(CP). The word after
the first byte is the length of the record in bytes,
exclusive of the first three bytes. Following the length
word are typically one or more fields. Each record type has
a specific sequence of fields, some of which may be optional
or of varying length. The very last byte in each record is
a checksum. The checksum byte contains the sum modulo 256 of
all other bytes in the record. The sum modulo 256 of all
bytes in a record, including the checksum byte, should equal
zero.
With few exceptions, 86rel strings are length prefixed and
have no trailing null. The first byte contains a number
between 0 and 40, which is the remaining length of the
string in bytes. Although the Intel specification limits
the character set to upper case letters, digits, and the
characters ``?'', ``@'', ``:'', ``.'', and ``_'', masm(CP)
uses the complete ASCII character set.
The Intel Object Module Format (OMF) specification uses the
term ``index'' to mean a positive integer either in the
range 0 to 127, or 128 to 32,768. This terminology is
retained in this document and elsewhere in the 86rel
literature. An index has one or two bytes. If the first
byte has a leading 0 bit, the index is assumed to have only
one byte, and the remainder of the byte represents a
positive integer between 0 and 127. If the second byte has
a leading 1 bit, the index is assumed to take up two bytes,
and the remainder of the word represents a positive integer
between 128 and 32,768.
Following is a list of record types and the hexadecimal
value of their first byte, as defined in relsym86.h.
#define MRHEADR 0x6e /*rel module header/*
#define MREGINT 0x70 /*register initialization*/
#define MREDATA 0x72 /*explicit (enumerated) data image*/
#define MRIDATA 0x74 /*repeated (iterated) data image*/
#define MOVLDEF 0x76 /*overlay definition*/
#define MENDREC 0x78 /*block or overlay end record*/
#define MBLKDEF 0x7a /*block definition*/
#define MBLKEND 0x7c /*block end*/
#define MDEBSYM 0x7e /*debug symbols*/
#define MTHEADR 0x80 /*module header,
*usually first in a rel file*/
#define MLHEADR 0x82 /*link module header*/
#define MPEDATA 0x84 /*absolute data image*/
#define MPIDATA 0x86 /*absolute repeated (iterated)
*data image*/
#define MCOMENT 0x88 /*comment record*/
#define MMODEND 0x8a /*module end record*/
#define MEXTDEF 0x8c /*external definition*/
#define MTYPDEF 0x8e /*type definition*/
#define MPUBDEF 0x90 /*public definition*/
#define MLOCSYM 0x92 /*local symbols*/
#define MLINNUM 0x94 /*source line number*/
#define MLNAMES 0x96 /*name list record*/
#define MSEGDEF 0x98 /*segment definition*/
#define MGRPDEF 0x9a /*group definition*/
#define MFIXUPP 0x9c /*fix up previous data image*/
#define MNONE1 0x9e /*none*/
#define MLEDATA 0xa0 /*logical data image*/
#define MLIDATA 0xa2 /*logical repeated (iterated)
*data image*/
#define MLIBHED 0xa4 /*library header*/
#define MLIBNAM 0xa6 /*library names record*/
#define MLIBLOC 0xa8 /*library module locations*/
#define MLIBDIC 0xaa /*library dictionary*/
#define M386END 0x86 /*32 bit module end record*/
#define MPUB386 0x91 /*32 bit public definition*/
#define MLOC386 0x93 /*32 bit logical symbols*/
#define MLIN386 0x95 /*32 bit source line number*/
#define MSEG386 0x99 /*32 bit segment definition*/
#define MFIX386 0x9d /*fix up previous 32 bit data image*/
#define MLED386 0xa1 /*32 bit logical data image*/
#define MLID386 0xa3 /*32 bit logical repeated (iterated) data image*/
In the following discussion, the salient features of each
record type are given. If the record is not used by either
masm(CP) or ld(CP), it is not listed.
THEADR The record type byte is 0x80. The THEADR
record specifies the name of the source module
at assembly-time (see Notes). The sole field is
the T-MODULE NAME which contains a length-
prefixed string derived from the base name of
the source module.
COMENT The record type byte is 0x88. The COMENT record
may contain a remark generated by the compiler
system. masm(CP) inserts the string ``UNIX
8086 ASSEMBLER .''
MODEND The record type byte is 0x8a. The MODEND
record terminates a module. It can specify
whether the current module is to be used as the
entry point to the linked executable. If the
module is an entry point, the MODEND record can
then specify the address of the entry point
within the executable.
EXTDEF The record type byte is 0x8c. The EXTDEF
record contains the names and types of symbols
defined in other modules by a PUBDEF record
(see below). This corresponds to the C storage
class ``extern.'' The fields consist of one or
more length-prefixed strings, each with a
following type index. The indices reference a
TYPDEF record seen earlier in the module.
masm(CP) generates only one EXTDEF per exterior
symbol.
TYPDEF The record type byte is 0x8e. The TYPDEF
record gives a description of the type (size
and storage attributes) of an object or
objects. This description can then be
referenced by EXTDEF , PUBDEF , and other
records.
PUBDEF The record type byte is 0x90. The PUBDEF record
gives a list of one or more names that may be
referenced by other modules at link-time
(``publics''). The list of names is preceded
by a group and segment index, which reference
the location of the start of the list of
publics within the current segment and group.
If the segment and group indices are zero, a
frame number is given to provide an absolute
address in the module. The list consists of
one or more of length-prefixed strings, each
associated with a 16-bit offset within the
current segment and a type index referring to a
TYPDEF .
LNAMES The record type byte is 0x96. The LNAMES record
gives a series of length-prefixed strings which
are associated with name indices within the
current module. Each name is indexed in
sequence given starting with 1. The names may
then be referenced within the current module by
successive SEGDEF and GRPDEF records to provide
strings for segments, classes, overlays or
groups.
SEGDEF The record type byte is 0x98. The SEGDEF record
provides an index to reference a segment, and
information concerning segment addressing and
attributes. This index may be used by other
records to refer to the segment. The first
byte in the record after the length field gives
information about the alignment, and about
combination attributes of the segment. The
next word is the segment length in bytes. Note
that this restrains segments to a maximum
65,536 bytes in length. Following this word is
an index (see above) for the segment. Lastly,
the SEGDEF may optionally contain class and/or
overlay index fields.
GRPDEF The record type is 0x9a. The GRPDEF record
provides a name to reference several segments.
The group name is implemented as an index (see
above).
FIXUPP The record byte is 0x9c. The FIXUPP record
specifies one or more load-time address
modifications (``fixups''). Each fixup refers
to a location in a preceding LEDATA (see below)
record. The fixup is specified by four data; a
location, a mode, a target and a frame. The
frame and target may be specified explicitly or
by reference to an already defined fixup.
LEDATA The record type byte is 0xa0. This record
provides a contiguous text or data image which
the loader ld(CP) uses to construct a portion
of an 8086 run-time executable. The image
might require additional processing (see
FIXUPP) before being loaded into the
executable. The image is preceded by two
fields, a segment index and an enumerated data
offset. The segment index (see INDEX)
specifies a segment given by a previously seen
SEGDEF . The enumerated data offset (a word)
specifies the offset from the start of this
segment.
See Also
as(CP), ld(CP)
Notes
If you attempt to load a number of modules assembled under
the same basename, the loader will try to put them all in
one big segment. In 286 programs, segment size is limited
to 64K. In a large program the resulting segment size can
easily exceed 64K. A large model code executable results
from the link of one or more modules, composed of segments
that aggregate into greater than 64K of text.
Hence, be sure that the assembly-time name of the module has
the same basename as the source. This can occur if the
source module is preprocessed not by cc(CP), but, for
example, by hand or shell script, prior to assembly. The
following example is incorrect:
#incorrect
cc -E module1.c | filter > x.c
cc x.c
mv x.o module1.o
cc -E module2.c | filter > x.c
cc x.c
mv x.o module2.o
cc -E module3.c | filter > x.c
cc x.c
mv x.o module3.o
ld module1.o module2.o module3.o
To avoid this, each of the modules should have a unique name
when assembled, as follows:
#correct
cc -E module1.c | filter > x.c
cc -S x.c
mv x.s module1.s
as module1.s
.
.
.
ld module1.o module2.o module3.o
Value Added
86rel is an extension of AT&T System V provided by the Santa
Cruz Operation.
(printed 8/23/89) 86REL(F)