IBM System/370

The IBM System/370 (S/370) is a range of computers, from entry-level to mainframes, announced as the successors to the System/360 family on June 30, 1970. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the product announcement. Like its predecessor, the System/370 became a dominant computing platform and was a major commercial success for IBM.

Early 370 systems differed from the 360 largely in their internal circuitry, moving from the Solid Logic Technologyhybrid integrated circuits containing separate transistors to more modern monolithic integrated circuits containing multiple transistors per integrated circuit, which IBM called Monolithic System Technology, or MST. The higher density packaging allowed several formerly optional features from the 360 line to be included as standard features of the machines, floating-point support for instance. The 370 also added a small number of new instructions.

At the time of its introduction, the development of virtual memory systems had become a major theme in the computer market, and the 370 was considered highly controversial as it lacked this feature. This was addressed in 1972 with the System/370 Advanced Function and its associated dynamic address translation (DAT) hardware. All future machines in the lineup received this option, along with several new operating systems that supported it. Smaller additions were made throughout the lifetime of the line, which led to a profusion of models that were generally referred to by the processor number. One of the last major additions to the line in 1988 were the ESA/370 extensions that allowed a machine to have multiple virtual address spaces and easily switch among them.

The 370 was IBM's primary large mainframe offering from the 1970s through the 1980s. Its level of success led to competition from plug-compatible manufacturers such as Amdahl Corporation, Hitachi, and others. IBM began a project to introduce a replacement for the 370 that would lock these vendors out, the Future Systems, but this was cancelled in 1975 and the line was instead updated with more powerful CPUs that were similar but added new features. On September 5, 1990, IBM replaced the System/370 line with the System/390, starting with the ES/9000. The S/390 was based on a new ESA/390 architecture, which slightly expanded the ESA/370 architecture.