View source for Relocatable Object Module Format

{{Infobox file format
| name                   = Relocatable Object Module Format
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| extension              = <code>.obj</code>
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| owner                  = Tool Interface Standards Committee
| released               = <!-- {{Start date|YYYY|mm|dd|df=yes}} -->
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| latest release date    = <!-- {{Start date and age|YYYY|mm|dd|df=yes}} -->
| genre                  = [[Object file]]
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The '''Relocatable Object Module Format (OMF)''' is an [[Object file|object file]] [[File format|format]] used primarily for software intended to run on Intel 80x86 [[Engineering:Microprocessor|microprocessor]]s. Version 4.0 was released by Intel in 1981 under the name ''Object Module Format'',<ref name="Intel_1981_ROMF"/> and is perhaps best known to [[Engineering:DOS|DOS]] users as an ''.OBJ file''. It has since been standardized by the Tool Interface Standards Committee.<ref name="TIS_1995_OMF"/>

== 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|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&nbsp;KiB. Records containing 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 library file format.

== Use ==
The file format is the most important object file format under DOS, 16-bit [[Software:Windows|Windows]], and 16-bit and 32-bit OS/2.{{citation needed|date=December 2022}}

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|a.out]]

==References==
{{Reflist|refs=
<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, Inc. |date=May 1995 |orig-year=1992 |url=http://pagesperso-orange.fr/pierrelib/exec_formats/OMF_v1.1.pdf |access-date=2020-02-07 |url-status=live |archive-url=https://web.archive.org/web/20200207210113/https://pierrelib.pagesperso-orange.fr/exec_formats/OMF_v1.1.pdf |archive-date=2020-02-07}} (87 pages)</ref>
<ref name="Intel_1981_ROMF">{{cite web |title=8086 Relocatable Object Module Formats |version=Version 4.0 |publisher=[[Company:Intel|Intel]] |date=1981 |id=Product Number 121748-001. ark:/13960/t07w7pz4b |url=https://archive.org/details/bitsavers_intel80861ableObjectModuleFormats1981_29998273/ |access-date=2020-02-08 }} (124 pages)</ref>
}}

==Further reading==
* {{cite book |author-last=Levine |author-first=John R. |title=Linkers and Loaders |date=2000 |orig-year=October 1999 |edition=1 |publisher=Morgan Kaufmann |series=The Morgan Kaufmann Series in Software Engineering and Programming |location=San Francisco, 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/] Errata: [https://linker.iecc.com/]

{{Executables}}

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

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