IBM S/360

Operator Console of a System/360 Model 65, with registration valve, lamps and switches (photograph imitation), and up to the right, a red emergency switch.

The IBM S/360 (S/360) was a computer system of the mainframe family, which IBM announced on April 7, 1964. It was the first family of computers that was designed for cover applications, regardless of their size or environment (scientific or commercial). A clear distinction was made in the design between architecture and implementation (structure), allowing IBM to release a series of compatible models at differential prices. S/360 models announced in 1964 ranged in speed from 0.034 MIPS to 1.7 MIPS (50 times the speed) and between 8 Kb and 8 MB of main memory, although the latter capacity was highly unusual. The 360 was a huge success in the market, allowing customers to buy a smaller system knowing that they could always migrate to a higher capacity system. The design of this system is considered by many to be one of the most important in the history of computers as it influenced the design of computers in later years. The chief architect of the S/360 was Gene Amdahl.

The IBM 360 is one of the first commercial computers to use integrated circuits, and could perform both numerical analysis and file processing and/or administration tasks. The 360 is considered the starting point for the third generation of computers.

Application level compatibility for System/360 software is maintained, with some restrictions, with IBM System z servers.

History

Using the industry approach, IBM created a series of computers from small to large, high to low performance, high to low power, all using the same command set (with two exceptions for specific markets). This allowed customers to use cheaper models and then upgrade to more powerful systems as their needs increased without the excessive expense of rewriting their software. IBM made the first commercial use of microcode technology to achieve this compatibility, employing it in all but the most powerful models of its models.

This flexibility set IBM apart from the competition (with the possible exception of General Electric).

Models

IBM initially announced a family of six computers and forty peripherals. IBM eventually delivered fourteen models, including one-off models for NASA. The cheapest model was the S/360-20, with just 4KB of main memory, eight 16-bit registers instead of the original S/360's sixteen 32-bit registers, and an instruction set that was a subset of the instruction set. used by the rest of the range.

The initial announcement in 1964 included the 30, 40, 50, 60, 62, and 70 models. The first three were low-mid range systems; were released with the goal of getting the IBM 1400 series market. All three began shipping in mid-1965. The last three were intended to replace the 7000 series computers, but never sold and were replaced by the 65 and 75, whose first sales were during November 1965 and January 1966 respectively. Later there was an addition of cheaper models, which included the 20 (1966, intended for small businesses), 22 (1971), and 25 (1968). The model 22 was a model 30 with lower limitations. The Model 44 (1966) was a variant aimed at the mid-range scientific market; it had a floating point system but a limited instruction set. There was a succession of high-end machines including the 67 (1966), 85 (1969), 91 (1967), 95 (1968) and 195 (1971). The 195 was the intermediate between the System/360 family and its successor the System/370.

Although the implementations between models were substantial (for example, presence or not of microcode), the compatibility between them was very high. Except where specifically documented, the models had compatible architecture. New features could be added, without violating architecture definitions: the '65 had a dual-processor version (M65MP); the 85 introduced cache memory. The 44, 75, 91, 95, and 195 models were implemented with hardwired logic, rather than microcode like all the other models.

First sales of the 360-67 were in August 1966; it was the first IBM system to offer dynamic address conversion ("DAT", now commonly called MMU; the DAT hardware would reappear in the System/370 family in 1972, although it was initially not to be). Prior to '67, IBM had announced the 64 and 66 models, DAT versions of the 60 and 62, but they were almost immediately replaced by the 67 at the same time that the 60 and 62 were replaced by the 65. In announcing the 360-67 (August 1965), IBM also announced TSS/360, an operating system that allowed multitasking, although this project was later canceled in 1971. Instead of TSS/360, the operating system for the 360-67 was CP/CMS, the original virtual machine system. The CP/CMS was developed at IBM's Cambridge Science Center, in cooperation with MIT researchers. Little by little it gained wide acceptance, and led to the development of the IBM VM/CMS and z/VM.

All System/360 models were withdrawn from the market at the end of 1977.

Backward Compatibility

IBM's customers had a large investment in software that they managed to run on the second generation of computers. Many models offered the option of emulating microcode from the client's previous computer (eg IBM 1400 series on a 360/30 or IBM 7094 on a 360/65) so that old programs could run on the new machine. However customers had to stop the computer and reboot in emulation mode. The later S/370 retained the emulation options, but allowed them to run under operating system control alongside native programs.