OS/360 and successors

OS/360, officially called the IBM System/360 Operating System, was a special program that helped computers run tasks. It was made by IBM for their new System/360 mainframe computer and was announced in 1964. This operating system was one of the first to need a special kind of storage device called a direct access storage device to work properly.

Even though OS/360 is no longer used, later versions of it are still running today. These newer systems, like MVS and z/OS, can still use programs made for OS/360, which means they are very compatible. This shows how important OS/360 was in the history of computers.

Overview

IBM introduced three different versions of OS/360, all created from the same basic set of instructions. Over time, IBM changed the names and made some important updates to these versions. These versions were known by various nicknames like "Big OS" or "OS/MFT," but these were not official names.

OS/360 was provided as a collection of files on tapes that needed to be copied to a special type of storage called DASD before it could be used. IBM also allowed users to change parts of the system using special tapes and offered detailed listings of the system's code on microfiche. Later, IBM stopped sharing the source code and only provided the finished versions of the system.

Another major operating system for the same hardware was DOS/360. Today, OS/360 is available for free and can be run on old System/360 computers or using the free Hercules emulator, which works on many modern operating systems like UNIX, Unix-like systems including Linux, Solaris, and macOS, as well as Windows. There are also ready-to-use versions of OS/360 available on CDs for easy setup with Hercules.

Origin

See also: History of IBM mainframe operating systems

IBM planned to have just one operating system for its System/360 computers, called OS/360. This system was meant to work on even very small machines, but it needed more memory than was available. Because of this, IBM created a few simpler systems to keep sales going. These included Basic Programming Support, BOS/360 for the smallest machines, TOS/360 for machines with tape drives, and DOS/360, which became very popular and led to today's VSEⁿ.

IBM later made three versions of OS/360: PCP, which could only run one job at a time; MFT for mid-sized machines; and MVT for the largest machines. Both MFT and MVT were used for many years. OS/360 was first written in assembly language, and later some parts were written in a new language called Basic Systems Language. The timesharing system TSS/360 was delayed and unreliable, so IBM stopped it, but later offered a different version for some big customers.

OS/360 variants

The three versions of OS/360—PCP, MFT, and MVT—worked so similarly that moving programs between them needed little effort. Most software for IBM computers ran on all three. Though they acted like separate systems, they were actually just different ways to set up the same basic operating system.

PCP was made for machines with small memory. It was like MFT but could only run one program at a time. MFT could run several programs together, but only a fixed number. MVT was the most advanced, able to run many programs at once by sharing memory. All three versions shared the same ways for programs to talk to the computer and manage files, making it easier to move programs between them.

System/370 and virtual memory operating systems

When System/370 was announced in 1970, it offered similar features to System/360 but with faster processing speeds. In 1972, IBM introduced System/370 Advanced Functions, which included virtual memory capability for future sales and could also be added to existing systems. IBM committed to creating enhanced operating systems that could use this new virtual memory feature.

IBM provided an OS/360 SYSGEN option for S/370 support, which included support for control registers, enhanced I/O, a S/370 Machine Check Handler, and limited support for new timer facilities. OS/VS1 was the successor to MFT, offering similar features with additions like RES and virtual memory. It was designed for medium-sized workloads in the 1970s, consisting of batch processing applications managed by Job Entry Subsystem 1 (JES1). However, OS/VS1 could also support interactive applications and users by running IBM's CICS transaction processing monitor.

OS/VS2 Release 1 was MVT with virtual memory and VSAM, later renamed OS/VS2 SVS. Release 2, known as MVS, supported multiple virtual address spaces, eliminating memory fragmentation and improving security. MVS introduced workload management, allowing users to set performance targets for important jobs. It was IBM's first mainstream System/370 operating system to support tightly coupled multiprocessing, enabling multiple CPUs to share memory and operate more efficiently.

IBM's Virtual storage access method (VSAM) improved data access methods, offering Entry-Sequenced Datasets (ESDS), Key-Sequenced Datasets (KSDS), and Relative Record Datasets (RRDS). These became the foundation for IBM's database management systems, IMS/VS, and DB2. In 1974, IBM announced Systems Network Architecture, designed to make running large networks of terminals more efficient, available only with virtual memory operating systems.

Later MVS versions and enhancements

Main article: MVS

In 1977, IBM introduced MVS/System Extensions, an extra-cost program that made MVS work better and added new features.

Versions of the original MVS are still used today on the newest versions of System/360, called System/390 and zSeries. It was renamed to OS/390 for System/390, and the 64-bit version for the zSeries is called z/OS.

Structure, interface and logic

OS/360 is a system that manages computer tasks. It assigns processors to handle different jobs, similar to how some modern systems manage tasks. Each job has a control block and a stack of requests.

The system’s storage is divided into sections. The fixed area at the bottom contains important system parts, while the common area at the top holds shared resources. The middle private area is used for individual jobs.

When the system starts, it loads essential parts and sets up the environment for running jobs. It supports various types of work, processing job control language (JCL) to manage how jobs run.

OS/360 provides ways for programs to communicate with the system, using control blocks and specific instructions. It also includes methods for operators to manage and monitor the system through consoles.

Timeline

These facts come from records about when different versions of IBM's operating systems were released.