IBM System/360

The IBM System/360 (S/360) is a family of computer systems announced by IBM on April 7, 1964,^[1] and delivered between 1965 and 1978.^[2] System/360 was the first family of computers designed to cover both commercial and scientific applications and a complete range of sizes from small, entry-level machines to large mainframes. The design distinguished between architecture and implementation, allowing IBM to release a suite of compatible designs at different prices. All but the only partially compatible Model 44 and the most expensive systems use microcode to implement the instruction set, which used 8-bit byte addressing with fixed-point binary, fixed-point decimal and hexadecimal floating-point calculations. The System/360 family introduced IBM's Solid Logic Technology (SLT), which packed more transistors onto a circuit card, allowing more powerful but smaller computers, but did not include integrated circuits, which IBM considered too immature.^[3]

System/360's chief architect was Gene Amdahl and the project was managed by Fred Brooks, responsible to Chairman Thomas J. Watson Jr.^[4][5] The commercial release was piloted by another of Watson's lieutenants, John R. Opel, who managed the launch of IBM's System/360 mainframe family in 1964.^[6] The slowest System/360 model announced in 1964, the Model 30, could perform up to 34,500 instructions per second, with memory from 8 to 64 KB.^[7] High-performance models came later. The 1967 IBM System/360 Model 91 could execute up to 16.6 million instructions per second.^[8] The larger 360 models could have up to 8 MB of main memory,^[5] though that much memory was unusual; a large installation might have as little as 256 KB of main storage, but 512 KB, 768 KB or 1024 KB was more common. Up to 8 megabytes of slower (8 microsecond) Large Capacity Storage (LCS) was also available for some models.

The IBM 360 was extremely successful, allowing customers to purchase a smaller system knowing they could expand it, if their needs grew, without reprogramming application software or replacing peripheral devices. It influenced computer design for years to come; many consider it one of history's most successful computers. Application-level compatibility (with some restrictions) for System/360 software is maintained to the present day with the IBM Z mainframe servers.