OS/360
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OS/360 was a batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964. OS/360 was amongst the earliest operating systems to make direct access storage devices a prerequisite for its operation.
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Origins
OS/360 was developed as a family of three control programs, which increased in size as well as functionality. Initially, the single task PCP (Primary Control Program) processed jobs sequentially; the next, MFT (Multiprogramming with a Fixed number of Tasks) added multitasking, but only allowed a fixed number of concurrent tasks, each one having a preset memory allocation. Finally MVT (Multiprogramming with a Variable number of Tasks) allowed varying numbers of tasks whose memory size could change dynamically. OS/360 also introduced IBM's batch-scripting language JCL (Job Control Language).
OS/360 was late being delivered, due to a combination of organizational disarray inside IBM and lack of experience with the pitfalls of large software projects, as well as the significant technical challenges. Originally scheduled for delivery in 1965 (for the simplest versions) and 1966 (for the more complex ones), it ended up being a year late — it was not released in its first production versions until 1966 and 1967 (PCP and MVT, respectively).
A family of simple alternative systems, BOS (Basic Operating System, for the smallest machines), TOS (Tape Operating System, for machines with only tape drives), and DOS (Disk Operating System), was therefore hurriedly put together to allow customers to use the new machines.
Later development
Although intended as a temporary interim tool, DOS survives today, having made the transition to virtual storage as DOS/VS, which later became DOS/VSE, then just VSE, and finally its current form, VSE/ESA.
When virtual addressing hardware was later developed for the System/370, the OS/360 systems were upgraded and renamed; MFT became OS/VS1 and MVT became OS/VS2. OS/VS2 was announced in two forms: OS/VS2 Release 1, aka SVS (Single Virtual Storage) and OS/VS2 Release 2, aka MVS (Multiple Virtual Storage). SVS was intended as a stepping stone from MVT to MVS, and is only of historical interest today. MVS is still used on the latest members of the IBM 360 Series and its successors, the System/370, System/390 and zSeries, although it has been known by several other names as time went by, most recently OS/390, now known as z/OS.
As of the turn of the millennium, the original OS/360 is in the public domain and can be downloaded freely. As well as being run on actual System/360 hardware, it can be executed on the free Hercules emulator which runs under GNU/Linux, Windows, and Mac OS X.
Lessons
After his experience running the System/360 project, and later specifically in charge of the (by then long overdue) OS/360, Frederick P. Brooks wrote his acclaimed and famous book, The Mythical Man-Month, based on the problems encountered (and lessons learned) during the project. He cited OS/360 as a very realistic and comprehensive example of the second-system effect.
Further reading
- Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and Early 370 Systems. MIT Press, Cambridge, MA. (pp. 291-345)
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
