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OBJASM: An Object Module Disassembler
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Original OBJASM Documentation
O B J A S M
An object file to assembly language file conversion utility
INTRODUCTION
------------
Under normal compilation procedures, most programs are converted into an
intermediate notation before being converted into executable code. This
notation simplifies the process of managing large programs by allowing the
programmer to divide a program into many modules. When changes are made
to a module, the programmer need only compile that module rather than
the entire program. For most 80x86 and compatible processors, this notation
is stored in a .OBJ file.
After compilation, the .OBJ file must be converted into a machine
executable form. At this stage the various modules must be combined and
any inter-module dependencies must be resolved. This process is called
linking and is accomplished by LINK, LINK86, or PLINK86 (depending on which
vendor you purchase it from). In order to accomplish this resolving, the
inter-module dependency information must be stored in the .OBJ file. This
information normally consists of routine names, variable names, and other
globally known quantities.
For the most part, once a program has been compiled into its .OBJ file,
there is no going back. There is no way to convert the .OBJ file back into
its appropriate language. It is for that reason that many companies who wish
to sell their proprietary routines to programmers, do so in the .OBJ file
form. Normally these companies send out a library file (.LIB). .LIB files
contain one or more .OBJ files. The program which groups .OBJ files into an
.LIB file is known as LIB, LIB86, or PLIB86. This method of distribution
has made routine vendors happy for many years.
But this distribution of encoded routines has meant headaches for
programmers. Programmers who discovered bugs in the routines, or wanted to
make enhancements had to contact the vendor and plead for help. The
programmers were at the mercy of the routine vendors.
Fortunately, OBJASM came along. OBJASM is a utility which can convert
.OBJ files back into assembly language, much like a dis-assembler. Unlike
a dis-assembler however, OBJASM is able to determine the names of the routines
that it is dis-assembling. Globally known data names can be determined as
well. OBJASM is also able to determine the segments of the original routines.
Segments are logical/physical divisions of sections of routines, normal
segments are code, data, stack, etc. With this additional knowledge, OBJASM
is able to produce an assembly language listing which is easier to read, easier
to understand, and easier to modify than normal dis-assembler output.
OBJASM output is compatible with MASM version 4.0 or later. It runs
under MS-DOS version 3.0 or later although it is able to process .OBJ files
(as specified by INTEL, document order no. 121748-001) from many other 80x86
operating systems such as CPM-86, XENIX 80x86, OS/2 and more. I believe that
the format for UNIX (it has .o files) is completely different.
COMPILING OBJASM
----------------
OBJASM was written for Microsoft C version 4.0 or later, and there is
a supplied 'make' file for that environment. To execute the 'make' file,
set-up your Microsoft C environment and go to the directory which contains
the OBJASM source files. Make sure that you have the Microsoft C utility
programs reachable using your 'PATH' environment variable, and type 'MAKE O'
from the dos prompt. The compilation will proceed to compile OBJASM for
the large model.
One of the modules (ouinsert.c) will give a compiler warning message, or
may fail depending how picky you have set your compiler. The message has
something to do with 'casting far pointer to long' or vice-versa. Make sure
your compiler gives only a warning message and proceeds with the compilation
process. The warning message can be ignored. Data structure fanatics: the
program uses threaded balanced binary trees for many internal lists and the
ouinsert module is the insert-into-a-list function. The root node of each tree
has the tree's height (an integer) stored into one of the structures
pointer type members. This is non-portable C code, but it's ok for most C
compilers. These threaded balanced binary tree routines were taken from a
set of generic list handling functions.
The modules contained in OBJASM are as follows:
omain.c main() processes command line and passes through input file
oprocess.c dis-assembly/dumping controlling routines
odisasm.c routines to dis-assembly the various kinds of 80x86
instructions
oreport.c routines to display some of the internal lists (publics and
externals)
ooutput.c routines to display various pieces of the dis-assembled .OBJ
routine
orXXXXXX.c (all except oreport.c) routines to process the various INTEL
standard .OBJ record formats
oubuff.c buffer I/O routines for the input file
ouinsert.c list insertion routine
oufind.c list searching routine
ounewtre.c list creation routine
ouinitre.c initialize all lists
oufmterr.c .OBJ format error handling routine
ouget.c input file formatting (into chars,ints,longs,etc) routines
oumalloc.c internal memory allocation routine (with check for out of
memory error)
oustruct.c assembly language structure routines
oextra.c additional information file processing routines
In addition, some of the modules (omain for example) have some debugging
statements conditionally compiled. If OBJASM exhibits strange behavior and
you wish to trace it down, re-compile these modules with the /DDEBUG option
(for MSC). This will define the preprocessor variable DEBUG, which will
enable the conditionally compiled debugging statements.
EXECUTING OBJASM
----------------
OBJASM is executed from the command line as:
OBJASM -options filename
OBJASM -options filename.OBJ
If you are dis-assembling a .LIB file, then you must use your LIB, LIB86, or
PLIB86 utility to extract the .OBJ files.
All output is sent to standard output (normally the console), but can be re-
directed using the '>' symbol. A normal OBJASM execution would look like this:
OBJASM OMAIN >OMAIN.ASM
The output would then be stored in the file omain.asm. Output contains tab
(control-I) characters to seperate the various assembly language columns.
A normal line format would appear as:
[LABEL]: (tab) [INSTRUCTION] (tab) [OPERANDS] (tab) ;[COMMENT]
start: mov ds,ax ; dummy comment
The options currently available for OBJASM are as follows:
Option Description
------ ---------------------------------------------------------------
-4 Make MASM v4.0 compatible output (No retf)
-a Add labels for better output (use with EXEOBJ)
-h Add hex comments showing bytes dis-assembled
-r Make RASM86 compatible output (special segment directives, etc)
-c Set the minimum size in a code segment, this option is followed
by a number which represents the new value. Eg. -c20
-s Set the minimum size in a data segment, this option is followed
by a number which represents the new value. Eg. -s10
-f Specifies an additional information file. The additional information
file must be given in parenthesis. Eg. -f(myfile) The additional
information file's default extension is ".add", as in the example
above, the file specified would be "myfile.add". For more
information see the ADDITIONAL INFORMATION FILE section.
Technical Information
---------------------
Most information used in writing OBJASM was taken from the following
public documents:
INTEL 8086 RELOCATABLE OBJECT MODULE FORMATS (Order No. 121748-001)
MICROSOFT OMF Specification (Dated February 18,1986; this is an edited
copy of the INTEL document)
MS-DOS ENCYCLOPEDIA
These documents describe the .OBJ file format. Here is a quick
summarization of the format. Each .OBJ file contains one or more records.
Each record has a record type (which specifies its purpose), a record length,
and record data (which depends on the record type). INTEL outlines 30
different record types, most of which are never used. MICROSOFT added to
these record types and removed the ones which it didn't need. The record
types which OBJASM supports are listed below.
Record Type Type Name Description
----------- ----------- -----------------------------------
80h THEADR Module Header Record
88h COMENT Comment Record
8Ah MODEND End of Module Record
8Ch EXTDEF External Dependency Definition
90h PUBDEF Public Value Definition
96h LNAMES List of Internally Referenced Names
98h GRPDEF Group Definition
9Ah SEGDEF Segment Definition
9Ch FIXUPP Dependency Fixing-up Record
A0h LEDATA (Logically) Enumerated Data Record
A2h LIDATA (Logically) Iterated Data Record
B0h COMDEF Communal Value Definition
B4h LEXTDEF Local External Definitions (C static routines)
B6h LPUBDEF Local Public Definitions (??)
B7h LPUBDF2 Local Public Definitions (?? Another case? )
B8h LCOMDEF Local Communal Value Definitions (??)
The C modules which handle the records of the above formats are named by
placing the characters "OR" before the type name and have a .C extension.
For example, the module named "ORSEGDEF.C" contains the C routine to handle
the segment definition records.
INTEL defines these other record types which are not supported by OBJASM.
Record Type Type Name Description
----------- ----------- -----------------------------------
6Eh RHEADR R-Module Header Record
70h REGINT Register Initialization Record
72h REDATA Relocatable Enumerated Data Record
74h RIDATA Relocatable Iterated Data Record
76h OVLDEF Overlay Definition Record
78h ENDREC End Record
7Ah BLKDEF Block Definition Record
7Ch BLKEND Block End Record
7Eh DEBSYM Debug Symbols Record
82h LHEADR L-Module Header Record
84h PEDATA Physically Enumerated Data Record
86h PIDATA Physically Iterated Data Record
8Eh TYPDEF Type Definition Record
92h LOCSYM Line Numbers Record
94h LINNUM Line Number Record
A4h LIBHED Library Header Record
A6h LIBNAM Library Module Names Record
A8h LIBLOC Library Module Locations Record
AAh LIBDIC Library Value Dictionary
MICROSOFT documents an obsolete method for generating communal records
using the TYPDEF record. This obsolete method is not handled by OBJASM.
The dis-assembler portion of OBJASM is a two pass process. The first
determines where local labels and symbols need to be placed, and the second
outputs the dis-assembled instructions (with the labels and symbols).
To determine whether to dis-assemble instructions or data, OBJASM performs
a look ahead operation on each byte of the data records. If the byte is a
printable character, then successive bytes are checked as well. The main
module has a two variables called 'code_string' and 'data_string' which are
used to specify the minimum length of a string. The 'code_string' value
is used in code segments while the 'data_string' value is used in data
segments. If this minimum length of printable characters is exceeded, then
the bytes will be output as a string. OBJASM is shipped with a value of 20
for 'code_string' and 3 for 'data_string'. They work well for most .OBJ files.
Bytes which are not contained in strings are checked against an instruction
lookup and processing routine. If it is determined that this byte will
generate a valid instruction, then it is output as an instruction. All other
bytes are output as simple data bytes.
Some assemblers and compilers will generate .OBJ records which contain
a portion of a string or instruction in one record and the remainder in the
next .OBJ record. OBJASM does not handle this very well. Strings spanning
more than one record will be divided. Instructions spanning more than one
record will not be recognized. Output would look this this:
String:
db 'This is all o' it should be db 'This is all one string'
db 'ne string'
Instruction:
db 0BBh it should be mov bx,MYSYMBOL
dw MYSYMBOL
OBJASM attempts to accommodate for this by keeping a 16 byte overlap
area. This overlap area is checked before processing the end of a record.
This helps alleviate the problem for objects which are smaller that 16 bytes,
something which is true for all normal instructions.
For some other interesting quirks in OBJASM, please read the sections
titled "Differences in .OBJ files which cannot be detected" and "Features
allowable in .OBJ format which are not translatable into MASMable code".
ADDITIONAL INFORMATION FILE
---------------------------
The additional information file, specified with the -f option, is another
method of specifying information to OBJASM. Lines in this file take one of
the following formats:
Format: Example:
SEG segname segtype SEG DRIVER CODE
This specifies that the segment named "segname" should be considered
either data or code. The segtype should be either CODE or DATA indicating
which type is desired. Normally OBJASM determines the segment using the
segment's name, but specifying the segment name and segment type in this
way allows overriding this determination.
Format: Example:
var = segname : offset TableY = DRIVER : 128
TableX = DRIVER : 200h
This specifies that a label should be placed within the segment "segname"
at the offset "offset". This can be used to add labels to the dis-assembled
code, and can also be used to override the internal labels generated by OBJASM.
Format: Example:
segname : offset : datatype DRIVER : 200h : DW
DRIVER : 202h : DD
This directs OBJASM to avoid the instruction/string look-ahead process
and output data of a specified type. OBJASM outputs a data directive within
the specified segment named "segname", at the offset "offset". The "datatype"
can be any of these values: DB, DW, DD, DF, DQ, or DT. These correspond to
the MASM base data types.
Although this last format allows direct control of how OBJASM outputs
data, care must be taken to put OBJASM into a state where it can actually
use this information. If OBJASM is processing data as an instruction, it
cannot be directed to output a data directive in the middle of that
instruction. For example:
Additional information: .OBJ FILE:
SOME_SEGMENT: 1 : DW 8B360200 MOV SI,[0002]
this would direct OBJASM to output a word in the middle of the MOV instruction.
In-order to accomplish this, the additional information file would have to
direct that the first byte of the MOV instruction was also a data byte.
To get it to work, you would have to have this:
Additional information: .OBJ FILE:
SOME_SEGMENT: 0 : DB 8B db 08Bh
SOME_SEGMENT: 1 : DW 3602 dw 0236h
00 ...
Other
-----
Yes, there always is a section which defies categorization...
The OBJASM program being continually refined. If you have any comments about
its execution, wish to add features to it, or need to report a bug(s), please
contact:
Robert F. Day
19906 Filbert Drive
Bothell, WA 98012
(206) 481-8431
The following section is a list of bugs that could not be fixed before
shipping OBJASM. Please read the bug and problems lists to familiarize
yourself with the known bugs and problem situations. The design notes are
included to help you help me in refining OBJASM. If you need (or want) to
add features to OBJASM, please contact me. We may be working on a similar
addition and we may be able to save you some time.
-------------------
Bugs to Dec 14 1990
-------------------
1. TYPDEF records are ignored (I had some Digital Research Libraries which
used TYPDEF records, but Microsoft doesn't use them).
2. Some languages generate segment names which are the same as the names
of labels within the segment. MASM won't allow this. Please rename
the segment, if possible.
--------------------------------------------------
Differences in .OBJ files which cannot be detected
--------------------------------------------------
(1) if label2 = label1 + 010h
MOV AX,label1 + 0010h ; Might have been like this
and
MOV AX,label2 ; OBJASM will generate this
; (Evaluates to equivalent address)
Reason: Public labels are resolved in local code before being sent to
the linker. Although the .OBJ specifications allow two places
to store symbol addition information (above, the 0010h), MASM
only uses one of them. This is a probable source of some other
.OBJ differences. MASM automatically computes the offset of
the public symbol and creates a fixup record indicating that
the offset of the segment added to the offset of the public
within the segment should be used (rather than just a fixup
indicating that the offset of the public should be used).
Handling: When resolving a reference to an address which is not equal to a
public symbol, a new local symbol is created.
-------------------------------------------------------------------------------
Features allowable in .OBJ format which are not translatable into MASMable code
-------------------------------------------------------------------------------
(1) A piece of code like:
DW _labeln - $ ; Relative (data form of local jmp/call)
Reason: MASM will not compile the above line. It is an equivalent data
representations for relative 'JMP's and 'CALL's
where _labeln is the label to jump or call to.
Handling: If OBJASM detects this type of code (almost all .OBJ files have
it) then the data output function will substitute an
actual value for the $ operator. This will be noted by a nasty
comment. The dis-assembly output function will still work if
they are truly 'JMP's or 'CALL's (Relative addressing is
proceeded by the jump short or call short opcodes).
-----------------
End of OBJASM.DOC
Directory of OBJASM 2.0 (Bin)
Volume in drive A is OBJASM_BIN
Directory of A:\
OBJASM DOC 20019 12-22-90 3:54p
OBJASM EXE 98915 8-13-93 10:51a
OBJUNARC EXE 125440 8-13-93 7:49a
READ ME 5986 1-28-91 12:59p
4 file(s) 250360 bytes
70656 bytes free
Directory of OBJASM 2.0 (Src)
Volume in drive A is OBJASM_SRC
Directory of A:\
O 2592 11-09-90 1:25p
O H 17544 12-12-90 2:29p
OBJASM LRF 335 11-09-90 1:26p
ODISASM C 59149 1-12-91 9:59a
ODISFP C 3553 11-15-90 6:36a
OEXTRA C 10583 12-19-90 11:28a
OMAIN C 17392 1-03-91 2:02p
OOUTPUT C 3573 10-02-90 1:05a
OPROCESS C 35990 12-19-90 11:35a
ORCOMDEF C 1823 7-02-90 2:25p
ORCOMENT C 5041 11-17-90 7:37a
OREPORT C 6802 11-03-90 7:26a
OREXTDEF C 1649 9-28-90 12:55a
ORFIXUPP C 6728 10-14-90 9:30a
ORGRPDEF C 4293 10-04-90 10:30a
ORLEDATA C 2567 11-02-90 10:04a
ORLIDATA C 1215 9-15-90 6:56a
ORLNAMES C 1018 2-25-90 8:03a
ORMODEND C 481 2-25-90 10:19a
ORPUBDEF C 2122 11-03-90 3:17a
ORSEGDEF C 5909 11-15-90 6:27a
ORTHEADR C 397 5-23-89 4:57p
OUBUFF C 7362 10-08-90 1:05a
OUFIND C 1752 11-15-90 8:41a
OUFMTERR C 194 2-25-90 7:40a
OUGET C 1409 2-25-90 7:33a
OUINITRE C 3547 12-05-90 4:05p
OUINSERT C 6585 5-18-90 3:48p
OUMALLOC C 324 2-25-90 7:36a
OUNEWTRE C 670 5-23-89 4:57p
OUSEGEXT C 1053 9-27-90 2:22p
OUSIZE C 2660 12-10-90 5:56p
OUSTRUCT C 996 11-03-90 3:20a
OUTABLE C 44081 1-03-91 1:33p
OUTARGET C 20940 11-10-90 4:15a
OUTRAVER C 826 5-23-89 4:57p
36 file(s) 283155 bytes
20480 bytes free