Object Module Format (Intel)
| Filename extension | .omf, .obj |
|---|---|
| Developed by | Tool Interface Standards Committee |
| Type of format | Object file |
The Relocatable Object Module Format (OMF) is an object file format used primarily for software intended to run on Intel 80x86 microprocessors. It was originally developed by Intel around 1975–1977 for ISIS, targetting their 8080/8085 processors.[1][2] Version 4.0 for the 8086 processor family was released by Intel in 1981 under the name Object Module Format,[3][1][2] and is perhaps best known to DOS users as an .OBJ file. It has since been standardized by the Tool Interface Standards Committee.[4]
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 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:
| Size | Contents |
|---|---|
| 1 byte | Record type, for example relocation information |
| 2 bytes | Data length (N+1) |
| 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
DGROUP - 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 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 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 Windows, and 16-bit and 32-bit OS/2.[citation needed]
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 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 obj 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
- Common Object File Format (COFF)
- Executable and Linkable Format (ELF)
- a.out
- Object file
- Wavefront .obj file
References
- ^ a b Burgett, Ken (2017-11-10). "Development of Intel ISIS Operating System - An interview with Ken Burgett". Archived from the original on 2023-11-24. Retrieved 2023-11-25.
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 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 prom-burner.
[1][2] - ^ a b Necasek, Michal (2020-07-25) [2020-07-09]. "How Old Is OMF?". OS/2 Museum. Archived from the original on 2023-11-25. Retrieved 2023-11-25.
- ^ "8086 Relocatable Object Module Formats" (PDF). Version 4.0. Santa Clara, California, USA: Intel Corporation. November 1981. Product Number 121748-001. ark:/13960/t07w7pz4b. Archived (PDF) from the original on 2023-11-25. Retrieved 2020-02-08. [3][4][5] (124 pages)
- ^ "Tool Interface Standards (TIS) Relocatable Object Module Format (OMF) Specification" (PDF). Version 1.1. TIS Committee / UNIX International, Inc. May 1995 [1992]. Archived from the original (PDF) on 2020-02-07. Retrieved 2020-02-07. (87 pages)
Further reading
- Levine, John R. (2000) [October 1999]. "Chapter 3: Object files". Linkers and Loaders. The Morgan Kaufmann Series in Software Engineering and Programming (1 ed.). San Francisco, california, USA: Morgan Kaufmann. ISBN 1-55860-496-0. OCLC 42413382. Archived from the original on 2013-01-25. Retrieved 2020-01-12. Code: [6][7] Errata: [8]
- External Product Specification, 8086 Object Module Formats. Intel Corporation. 1978-12-19.
- MCS 80/85 Relocatable Object Module Formats (PDF) (Technical specification). Santa Clara, California, USA: Intel Corporation. 1981. Product Number 121747-001. A33S/381/S00 IP. Archived (PDF) from the original on 2023-11-25. Retrieved 2023-11-25. (42 pages)
- MCS 80/85 Absolute Object File Formats (PDF) (Technical specification). Santa Clara, California, USA: Intel Corporation. 1977. Product Number 9800183B. Archived from the original (PDF) on 2017-06-27. (16 pages)
- External Product Specification for the MCS-51 Object Module Format (PDF). Version 5.0 Revision A. Intel Corporation. 1982-09-05. Product Number 9800183B. Archived (PDF) from the original on 2023-11-25. (24 pages)