In computing, the System Object Model (SOM) is a proprietary executable file format

File format

A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...

 developed by Hewlett-Packard

Hewlett-Packard

Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

 for its HP-UX

HP-UX

HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...

 and MPE/ix operating systems.
In particular, SOM is the native format used for 32-bit

32-bit

The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

 application executable

Executable

In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

s, object code

Object code

Object code, or sometimes object module, is what a computer compiler produces. In a general sense object code is a sequence of statements in a computer language, usually a machine code language....

, and shared libraries running under the PA-RISC

PA-RISC

PA-RISC is an instruction set architecture developed by Hewlett-Packard. As the name implies, it is a reduced instruction set computer architecture, where the PA stands for Precision Architecture...

 family of processors.

With the introduction of 64-bit

64-bit

64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...

 processors, Hewlett Packard adopted the Executable and Linkable Format

Executable and Linkable Format

In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...

 (ELF) format to represent the wider 64-bit program code, while still using SOM for applications running in 32-bit mode. Later, with the introduction of the Itanium

Itanium

Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...

 processor family, HP-UX has abandoned the SOM format in favor of ELF for both 32-bit and 64-bit application code.

In HP-UX the SOM file format is sometimes called the a.out format and is described by C programming language structures in the header file "/usr/include/a.out.h". However the SOM format is technically not the same as the standard a.out format used by many other Unix operating systems.

Overview of the SOM file format

A SOM file consists of a fixed-size header record followed by a number of sections, some of which are optional. The header always appears at the beginning of the file and contains the byte offsets and sizes of where the other sections are located within the file. Except for the header the other sections may appear anywhere in the file, although the typical layout of a SOM file (assuming all sections are present) is as follows:

• Header Record
• Auxiliary Header Record
• Space Records
• Subspace Records
• Loader Fixup Records
• Space Strings
• Symbol Records
• Fixup Records
• Symbol Strings
• Compiler Records
• Data for Loadable Spaces
• Data for Unloadable Spaces

Numeric fields are stored in big endian

Endianness

In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...

 byte order, the native byte order of the PA-RISC, with most being 32-bit wide. Character strings are generally encoded in 8-bit ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

 and both prefixed with a 32-bit length indicator as well as being null-terminated

Null-terminated string

In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...

, like C string

C string

In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...

s. Most records are word-aligned (start at even-byte offsets) with padding introduced as necessary.

External links

• HP-UX a.out(4) manual page, Hewlett-Packard
• The 32-bit PA-RISC Run-time Architecture Document, HP-UX 11.0 Version 1.0, Hewlett-Packard, 1997
• The 32-bit PA-RISC Run-time Architecture Document, HP-UX 10.20 version 3.0, Hewlett-Packard, 1997. Also available at parisc-linux.org
• HP-UX Software Transition Kit Glossary, Hewlett-Packard (online)
• PA-RISC 1.1 Architecture Specifications