System Object Model (file format)

In computing, the System Object Model (SOM) is a proprietary executable file format developed by Hewlett-Packard for its HP-UX and MPE/ix operating systems. In particular, SOM is the native format used for 32-bit application executables, object code, and shared libraries running under the PA-RISC family of processors.

With the introduction of 64-bit processors, Hewlett Packard adopted the Executable and Linkable Format (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 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.