The IBM System/360 is the name of an important series of computers announced in 1964 and was in production from 1965 to 1977. The 360 series included a range of computers with different performances and capacities. They were designed to handle a range of applications, including commercial data processing and scientific computation. The name 360 refers to a vision of 360 degrees.
The IBM System/360 was based on a common architecture that allowed the machines in the series to have a common set of instructions, and was independent of technologies used in their implementation. In particular,
Machines in the 360 series were upwardly compatible; this meant that programs written for a smaller machine could run on any larger machine in the series.
Larger machines in the 360 series ran faster, had larger memories and shorter memory access times.
Peripherals - such as magnetic storage devices, visual display units, communication equipment, card readers and punches, printers and an optical character reader - were interchangeable between machines in the 360 series.
Thus, the principal idea of System/360 addressed the commercial issues of software portability and software legacy: customers of such a series could develop and upgrade their computing services more easily, have less software to rebuild and fewer peripherals to replace; thus, the long-term costs would be less. This portability is expressed in the original technical paper Amdahl, Blaauw and Brooks  as follows:
“Compatibility would ensure that the user's expanding needs be easily accommodated by any model. Compatibility would also ensure maximum utility of programming support prepared by the manufacturer, maximum sharing of programs generated by the user, ability to use small systems to back up large ones, and exceptional freedom in configuring systems for particular applications.”
The later IBM 370 series, announced in 1970 to replace the 360 series, was backward compatible with the 360 series; this meant that programs written for a 360 machine could run on a machine in the 370 series.
The chief architect of the 360 series was Gene Amdahl, who later left IBM and founded the Amdahl Corporation in Sunnyvale CA in 1970. Initially, the project was managed by Fred Brooks, who later became well known for writings on software engineering, The Mythical Man-Month, based upon his experiences with the design of the 360 operating system.
Machines of the period were highly specific, each with a particular architecture, instruction set, operating system and peripherals. The project was ambitious, difficult, expensive and risky. It aimed to replace all five of IBM’s computer product lines with one strictly compatible family of products and its cost was cited as $5 billion. But the System/360 proved to be successful for IBM, technically and commercially. There were many innovations and practices. The 360 popularised the 8-bit byte, 32-bit word and 8-bit character encoding; used emulators for backward compatibility; featured multiprogramming and timesharing. The project prompted the development of new programming ideas through the language PL/1.
At that time, hardware manufacturers were a major source of software, which was specific to their own machines and made available without cost. In the 1960s, IBM saw software as something to support hardware, and to be bundled with their hardware, rather than as product that had independent value. IBM software was given away.
There were attempts to make clones of the IBM 360. The RCA Spectra 70 was a series of machines established in 1965 whose system architecture and instruction-set were largely compatible with the IBM 360. The English Electric System 4 was a mainframe computer introduced in 1967 that derived from the RCA Spectra 70, and so was one step removed from the IBM 360. The Amdahl Corporation created Amdahl 470V/6 with different technologies, which was compatible with the 370 series, allowing the same IBM and customer software to be used.
J V Tucker
G.M.Amdahl, G. A. Blaauw and F. P. Brooks, Jr., Architecture of the IBM System/360, IBM Journal of Research and Development, 8 (2) (1964), 87-101.
C Boyer, The 360 Revolution, IBM, 2004.
F. P. Brooks, Jr., The Mythical Man-Month, Essays on Software Engineering, Anniversary Edition, Addison Wesley, 1995.
R L Glass, In the Beginning: Recollections of Software Pioneers, IEEE Computer Society Press, 1998.
E W Pugh, L R Johnson and J H Palmer, IBM’s 360 and Early 370 systems, MIT Press, 1986.
Announcement of the IBM System/360 on 7 April 1964 (link)
The 360 Revolution, Chuck Boyer, IBM (link)