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{{Short description|Object file format}}
{{Use dmy dates|date=February 2020|cs1-dates=y}}
{{Use list-defined references|date=November 2023}}
{{Infobox file format
| name = (Relocatable) Object Module Format
| icon =
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| extension = <code>.omf</code>, <code>.obj</code>
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| owner = Tool Interface Standards Committee
| released = 
| latest release version =
| latest release date = 
| genre = [[Object file]]
| container for =
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}}
The '''Object Module Format''' ('''OMF''') is an [[object file]] [[file format|format]] used primarily for software intended to run on [[Intel 80x86]] [[microprocessor]]s. {{anchor|8080}}It was originally developed by [[Intel]] around 1975–1977 for [[ISIS-II]], targeting the 8-bit [[8080]]/[[8085]] processors.<ref name="Intel_1977_AOMF"/><ref name="Intel_1981_8080"/><ref name="Burgett_2017"/><ref name="Necasek_2020"/> This variant later became known as ''OMF-80''. {{anchor|8086}}As ''OMF-86'' it was adapted to the 16-bit [[8086]] processor in 1978.<ref name="Intel_1978"/>

Version 4.0 of OMF for the 8086 family was released in 1981 under the name '''Relocatable Object Module Format''',<ref name="Intel_1981_ROMF"/><ref name="Burgett_2017"/><ref name="Necasek_2020"/> and is perhaps best known to [[DOS]] users as an ''.OBJ file''. {{anchor|80286|80386}} Versions for the [[80286]] (''OMF-286'')<ref name="Intel_1981_80286"/><ref name="Intel_1983_80286"/> and the 32-bit [[80386]] processors (''OMF-386'')<ref name="Intel_1991_80386"/><ref name="Intel_1991_386-Boot"/><ref name="Burgett_2017"/> were introduced in 1981 and 1985, respectively. {{anchor|TIS}}It has since been standardized by the [[Tool Interface Standards Committee]]<ref name="TIS_1995_OMF"/> and was also extended by [[Microsoft]]<ref name="Microsoft_OBJ"/> and [[IBM]] (''IBM-OMF''). Intel also adapted the format to the [[8051]] microcontroller (''OMF-51''<ref name="Intel_1982_8051"/> and ''AOMF'').

== File format ==
Many object file formats consist of a set of tables, such as the relocation table, which are either stored on fixed positions in the file, like the [[a.out]] format, or are pointed to by the header, like the [[Executable and Linkable Format|ELF]] format. The "sections", code, data area, etc., are stored as contiguous areas of bytes within such files.

The Relocatable Object Module Format, however, was designed to require minimal memory when linking, and consists of a series of records that have the following format:
{| class="wikitable"
|-
! Size !! Contents
|-
| 1 byte || Record type, for example relocation information
|-
| 2 bytes || Data length ({{Var|N}}+1)
|-
| {{Var|N}} bytes || Data (varies depending on the record type)
|-
| 1 byte || [[Checksum]] or 0
|}

There is a wide variety of record types because of consolidation of OMF variants from several vendors, and because of adding such features as 32-bit code and dynamic linking. These are important record types:
* COMENT - (88h) Comment, which may also contain control information.
* EXTDEF - (8Ch) Defines external references
* PUBDEF - (90h/91h) Identifies external symbols in this module
* SEGDEF - (98h/99h) Identifies segments
* GRPDEF - (9Ah) Identifies groups of segments, for example MS-DOS <code>DGROUP</code>
* FIXUPP - (9Ch/9Dh) ''Fixup'' or relocation records
* LEDATA - (A0h/A1h) Contains text of a code or data section
* COMDEF - (B0h) Uninitialized common data
* COMDAT - (C2h/C3h) Initialized common data
* MODEND - (8Ah/8Bh) Indicates end of module

There is no header containing file offsets, such as a pointer to a symbol table, in the file; a [[Linker (computing)|linker]] must completely parse the object file to extract all the information.

In the OMF format the data of one section is not necessarily stored as contiguous bytes in the file, instead it ''can'' be represented by multiple records. The file format specification (version 1.1) says that this ''must'' be done for sections larger than 1 KiB. Records containing [[Relocation (computer science)|relocation information]] (fixups) must be stored immediately following the data records of the section they apply to, so the section data and the relocation information is "mixed" in the file.

The file format provides special records (LIDATA) that allow compression of repeating data sequences in an object file. It also provides the possibility to store the symbol name of the entry point of the later executable file in one object file.

The file format can also be used as [[Static Library|library]] file format.

== Use ==
The file format is the most important object file format under DOS, 16-bit [[Windows]], {{anchor|IBM}}and 16-bit and 32-bit [[OS/2]].<ref name="IBM_1994"/><ref name="IBM_1996"/>

Few toolchains use the 32-bit version of the OMF format. For example, the [[Watcom C]] toolchain allows generating code for targets that use 32-bit [[X86 memory segmentation#Protected mode|segmented]] memory layouts; Iron Spring [[PL/I]] can generate code for OS/2 32-bit flat memory layouts.

The [[Embarcadero Delphi]] compiler used this format when generating <code>obj</code> files for C++. It was the only format of object files that could be linked to a program or unit written in Object Pascal prior to version XE2 of Delphi, which introduced support of [[COFF]] format, along with 64-bit Windows target.

==See also==
* [[COFF|Common Object File Format]] (COFF)
* [[Executable and Linkable Format]] (ELF)
* [[a.out]]
* [[Object file]]
* [[Object Module Format (VME)]] (OMF as used by ICL VME)
* [[Object Module Format (Apple)]] (OMF as used by ORCA / [[Apple IIGS]])

==References==
{{Reflist|refs=
<ref name="Intel_1978">{{cite book |title=External Product Specification, 8086 Object Module Formats |version= |publisher=[[Intel Corporation]] |publication-place= |date=1978-12-19 |id= |url= |access-date= }}</ref>
<ref name="TIS_1995_OMF">{{cite web |title=Tool Interface Standards (TIS) Relocatable Object Module Format (OMF) Specification |version=Version 1.1 |publisher=TIS Committee / [[Unix International]] |date=May 1995 |orig-date=1993, 1992 |url=http://pagesperso-orange.fr/pierrelib/exec_formats/OMF_v1.1.pdf |access-date=2020-02-07 |url-status=dead |archive-url=https://web.archive.org/web/20200207210113/https://pierrelib.pagesperso-orange.fr/exec_formats/OMF_v1.1.pdf |archive-date=2020-02-07}} [https://web.archive.org/web/20231126142755/http://www.azillionmonkeys.com/qed/Omfg.pdf][https://web.archive.org/web/20231126151214/http://sininenankka.dy.fi/~sami/watcom_mirror/devel/omf.pdf] (87 pages)</ref>
<ref name="Intel_1981_ROMF">{{cite web |title=8086 Relocatable Object Module Formats |version=Version 4.0 |publisher=[[Intel Corporation]] |publication-place=Santa Clara, California, USA |date=November 1981 |id=Product Number 121748-001. ark:/13960/t07w7pz4b |url=http://www.bitsavers.org/pdf/intel/ISIS_II/121748-001_8086_Relocatable_Object_Module_Formats_Nov81.pdf |access-date=2020-02-08 |url-status=live |archive-url=https://web.archive.org/web/20231125143127/http://www.bitsavers.org/pdf/intel/ISIS_II/121748-001_8086_Relocatable_Object_Module_Formats_Nov81.pdf |archive-date=2023-11-25}} [https://archive.org/details/bitsavers_intel80861ableObjectModuleFormats1981_29998273/][https://mark-ogden.uk/files/intel/publications/121748-001%208086%20Relocatable%20Object%20Module%20Formats-%23Nov81.pdf][https://mark-ogden.uk/files/intel/publications/121748-001%208086%20Relocatable%20Object%20Module%20Formats-%23Oct82.pdf] (124 pages)</ref>
<ref name="Intel_1981_80286">{{cite book |title=iAPX 286 System Builder User's Guide |date=September 1981 |id=Order Number 121711-001 |publisher=[[Intel Corporation]] |publication-place=Santa Clara, California, USA |url=http://www.bitsavers.org/pdf/intel/MDS3/121711-001_iAPX_286_Evaluation_Builder_Users_Guide_Sep81.pdf |access-date=2023-11-26 |url-status=live |archive-url=https://web.archive.org/web/20231126155806/http://www.bitsavers.org/pdf/intel/MDS3/121711-001_iAPX_286_Evaluation_Builder_Users_Guide_Sep81.pdf |archive-date=2023-11-26}} [https://web.archive.org/web/20231126155830/https://www.ardent-tool.com/CPU/docs/Intel/286/manuals/121711-001.pdf] (90 pages)</ref>
<ref name="Intel_1983_80286">{{cite book |title=iAPX 286 Operating Systems Writer's Guide |chapter=iAPX 286 Object Module Format |publisher=[[Intel Corporation]] |date=1983 |pages=11-10 – 11-11 |id=Order Number 121960-001 |url=http://www.bitsavers.org/components/intel/80286/121960-001_iAPX_286_Operating_Systems_Writers_Guide_1983.pdf |access-date=2023-11-26 |url-status=live |archive-url=https://web.archive.org/web/20231126152747/http://www.bitsavers.org/components/intel/80286/121960-001_iAPX_286_Operating_Systems_Writers_Guide_1983.pdf |archive-date=2023-11-26}} (2 of 251 pages)</ref>
<ref name="Intel_1991_80386">{{cite book |title=Specification 386 Object Modules Format |date=1991 |orig-date=1990 |id=Order Number 482991-002 |publisher=[[Intel Corporation]]}}</ref>
<ref name="Intel_1991_386-Boot">{{cite book |title=Simple Bootloadable Files in 386 Object Modules Format |date=1991 |orig-date=1990 |id=Order Number 483164-001 |publisher=[[Intel Corporation]]}}</ref>
<ref name="Burgett_2017">{{cite web |title=Development of Intel ISIS Operating System - An interview with Ken Burgett |author-first=Kenneth "Ken" |author-last=Burgett |date=2017-11-10 |url=https://www.rogerarrick.com/osiris/burgett.txt |access-date=2023-11-25 |url-status=live |archive-url=https://web.archive.org/web/20231124165914/https://www.rogerarrick.com/osiris/burgett.txt |archive-date=2023-11-24 |quote=A good friend of mine, Bruce, got the job of developing the Object Module Format (OMF) for the [[8080]], and the upcoming [[8086]]. This piece of work would live on for years, since [[MS-DOS]] used it with little or no modifications […] It was a good piece of work and it was updated in 1985 to support the [[80386]] 32-bit flat address mode. […] The OMF gave us a good roadmap for the design of the linker, which knitted object modules together, and the locator, which processed the [[relocatable code]] into a fixed memory image, with a defined entry point, since that was how [[ISIS (operating system)|ISIS]] handled load modules. The locator was also built to enable mapping of a linked module into a set of [[EPROM]] images, so they could be programmed by a [[Programmable ROM|prom-]]burner.}} [https://web.archive.org/web/20231125032320/https://00362628054895325210.googlegroups.com/attach/1cc719e8305ed/burgett.txt?part=0.1&view=1&vt=ANaJVrFYjqMX6NCUFa_l-LUDCL-Mu3EqJS1xO4wZczmxImZg2kP3F8E1qEWYTXI5rHv6URyeMi3pcc1oH5LeyEkzHUSeSnUncsii1D7SyIa5EcFQPzu1fzM][https://web.archive.org/web/20231125121523/https://www.rogerarrick.com/kenburgett/]</ref>
<ref name="Necasek_2020">{{cite web |title=How Old Is OMF? |author-first=Michal |author-last=Necasek |work=OS/2 Museum |date=2020-07-25 |orig-date=2020-07-09 |url=https://www.os2museum.com/wp/how-old-is-omf/ |access-date=2023-11-25 |url-status=live |archive-url=https://web.archive.org/web/20231125143921/https://www.os2museum.com/wp/how-old-is-omf/ |archive-date=2023-11-25}}</ref>
<ref name="IBM_1994">{{cite book |title=IBM OS/2 16/32-bit Object Module Format (OMF) and Linear eXecutable Module Format (LX) |version=Version 8 |date=1994-06-30 |orig-date=1991, 1993 |publisher=[[IBM Corporation]] |publication-place=Boca Raton, Florida, USA |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/pc/os2/OS2_OMF_and_LX_Object_Formats_Revision_8_199406.pdf |access-date=2023-11-26 |url-status=live |archive-url=https://web.archive.org/web/20231126100114/http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/pc/os2/OS2_OMF_and_LX_Object_Formats_Revision_8_199406.pdf |archive-date=2023-11-26}} (v+87 pages)</ref>
<ref name="IBM_1996">{{cite book |title=IBM OS/2 16/32-bit Object Module Format (OMF) and Linear eXecutable Module Format (LX) |version=Version/Revision 10 |date=1996-10-09 |orig-date=1991 |publisher=[[IBM Corporation]] |publication-place=Boca Raton, Florida, USA |url=https://www.yumpu.com/en/document/read/31229113/ibm-os-2-16-32-bit-object-module-format-omf |access-date=2023-11-26 |url-status=live |archive-url=https://web.archive.org/web/20231126100740/https://www.yumpu.com/en/document/read/31229113/ibm-os-2-16-32-bit-object-module-format-omf |archive-date=2023-11-26}} [https://web.archive.org/web/20231126151317/http://sininenankka.dy.fi/~sami/watcom_mirror/devel/lxomf.pdf] (vi+95 pages)</ref>
<ref name="Intel_1982_8051">{{anchor|8051}}{{cite book |title=External Product Specification for the MCS-51 Object Module Format |date=1982-09-05 |version=Version 5.0 Revision A |publisher=[[Intel Corporation]] |id=Product Number 9800183B. Document Control Center Number 481984 |url=https://mark-ogden.uk/files/intel/publications/481984A%20MCS-51%20Object%20Module%20Format%20v5.0-Sep82.pdf |url-status=live |archive-url=https://web.archive.org/web/20231125161702/https://mark-ogden.uk/files/intel/publications/481984A%20MCS-51%20Object%20Module%20Format%20v5.0-Sep82.pdf |archive-date=2023-11-25}} (24 pages) (NB. This is the version shipped with Intel's RL51 2.0 and PL/M-51 1.0. There was an earlier version X207 dated 1982-04-12.)</ref>
<ref name="Microsoft_OBJ">{{cite web |title=The Microsoft OBJ File Format |date= |website=FileFormat.Info |url=https://www.fileformat.info/format/ms-obj/corion.htm |access-date=2023-11-26 |url-status=live |archive-url=https://web.archive.org/web/20231125170744/https://www.fileformat.info/format/ms-obj/corion.htm |archive-date=2023-11-25}}</ref>
<ref name="Intel_1977_AOMF">{{cite book |title=MCS 80/85 Absolute Object File Formats |date=1977 |type=Technical specification |publisher=[[Intel Corporation]] |publication-place=Santa Clara, California, USA |id=Product Number 9800183B |url=http://www.nj7p.org/Manuals/PDFs/Intel/9800183B.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170627062910/http://www.nj7p.org/Manuals/PDFs/Intel/9800183B.pdf |archive-date=2017-06-27}} (16 pages)</ref>
<ref name="Intel_1981_8080">{{cite book |title=MCS 80/85 Relocatable Object Module Formats |date=March 1981 |type=Technical specification |publisher=[[Intel Corporation]] |publication-place=Santa Clara, California, USA |id=Product Number 121747-001. A33S/381/S00 IP |url=http://www.bitsavers.org/pdf/intel/ISIS_II/121747-001_MCS_80_85_Relocatable_Object_Formats_Mar81.pdf |access-date=2023-11-25 |url-status=live |archive-url=https://web.archive.org/web/20231125144402/http://www.bitsavers.org/pdf/intel/ISIS_II/121747-001_MCS_80_85_Relocatable_Object_Formats_Mar81.pdf |archive-date=2023-11-25}} (42 pages)</ref>
}}

==Further reading==
* {{cite book |author-last=Levine |author-first=John R. |author-link=John R. Levine |title=Linkers and Loaders |date=2000 |orig-date=October 1999 |edition=1 |publisher=[[Morgan Kaufmann]] |series=The Morgan Kaufmann Series in Software Engineering and Programming |location=San Francisco, California, USA |isbn=1-55860-496-0 |oclc=42413382 |chapter=Chapter 3: Object files |chapter-url=http://www.iecc.com/linker/linker03.html |access-date=2020-01-12 |url-status=dead |archive-url=https://archive.today/20130125220014/http://www.iecc.com/linker/linker03.html |archive-date=2013-01-25}} Code: [https://archive.today/20200114225034/https://linker.iecc.com/code.html][ftp://ftp.iecc.com/pub/linker/]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}} Errata: [https://linker.iecc.com/]

{{Executables}}

[[Category:Executable file formats]]
[[Category:Computer file formats]]

Object Module Format (Intel) - Revision history

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