UNIX System Services
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UNIX System Services (USS) is a component of z/OS. USS is a certified UNIX implementation (XPG4 UNIX 95) optimized for mainframe architecture. It is the first UNIX 95 which was not derived from the AT&T source code. Through integration with the rest of z/OS, additional time share option (TSO) commands are available alongside the usual UNIX services, making it possible to process UNIX files using ISPF. Extensions in JCL make it possible to use these files in batch processing.
UNIX System Services allows UNIX applications from other platforms to run on IBM mainframes. In many cases only a recompile is necessary, although additional effort may be advisable for z/OS integration (such as SMP/E installation). Binary programs using hardcoded ASCII numerical values need adjustment — z/OS architecture is based on EBCDIC. Programs running under USS have full access to the internal functions of z/OS. Database access (DB2 via Call Attach) is one example of how USS can access services found elsewhere in z/OS. Naturally such programs cannot be ported to non-mainframe platforms without rewriting should they use these z/OS-specific services.
The file systems for USS (HFS and zFS), which support UNIX-style long filenames, appear as special datasets to the rest of the z/OS. Numerous core zSeries subsystems and applications now rely on UNIX System Services, including Java.
USS's predecessor was an operating system component called Open MVS, first implemented in MVS/ESA 4.3. Open MVS only supported the POSIX standards.