The IBM System/360 (S/360) is a family of computer systems announced by IBM on April 7, 1964, and delivered between 1965 and 1978. 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.
System/360's chief architect was Gene Amdahl and the project was managed by Fred Brooks, responsible to Chairman Thomas J. Watson Jr. 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. 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. High-performance models came later. The 1967 IBM System/360 Model 91 could execute up to 16.6 million instructions per second. The larger 360 models could have up to 8 MB of main memory, 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.
System/360 history
Background By the early 1960s, IBM was struggling with the load of supporting and upgrading five separate lines of computers. These were aimed at different market segments and were entirely different from each other. A customer who purchased a machine to handle accounting, such as the IBM 1401, that was now looking for a machine for engineering calculations, such as the IBM 7040, had no reason to select IBM – the 7040 was incompatible with the 1401 and they might as well have been from different companies. Customers were frustrated that major investments, often entirely new machines and programs, were required when seemingly small performance improvements were needed.
In 1961, IBM assembled a task force to chart their developments for the 1960s, known as SPREAD, for Systems Programming, Research, Engineering and Development. In meetings at the New Englander Motor Hotel in Greenwich, Connecticut, SPREAD developed a new concept for the next generation of IBM machines. At the time, new technologies were coming into the market including the introduction of replacement of individual transistors with small-scale integrated circuits and the move to an 8-bit byte from the former 6-bit oriented words. These were going to lead to a new generation of machines, today known as the third generation, from all of the existing vendors.
Where SPREAD differed significantly from previous concepts was what features would be supported. Instead of machines aimed at different market niches, the new concept was effectively the union of all of these designs. A single instruction set architecture (ISA) included instructions for binary, floating-point, and decimal arithmetic, string processing, conversion between character sets (a major issue before the widespread use of ASCII) and extensive support for file handling, among many other features.
This would mean IBM would be introducing yet another line of machines, once again incompatible with their earlier machines. But the new systems would be able to run all of the programs that formerly required different machines. A concern was that there was a risk that their customers, facing the purchase of yet another new and incompatible platform, would simply choose some other vendor. Yet the concept steadily gained support, and six months after being formed, the company decided to implement the SPREAD concept.
A new team was organized under the direction of Bob Evans, who personally persuaded CEO Thomas J. Watson Jr. to develop the new system. Gene Amdahl was the chief architect of the computers themselves, while Fred Brooks was the project lead for the software and Erich Bloch led the development of IBM's hybrid integrated circuit designs, Solid Logic Technology.
"Family" concept Producing a single system design with support for all of these features, at a price acceptable to low-end customers and with a performance level acceptable to high-end customers, would border on impossible. Instead, the SPREAD concept was based on the separation of the defined feature set from its internal operation, with a family of machines with different performance and different internal designs.
Specifically, depending on the machine, some components might not be directly implemented in hardware, and would instead be completed using small programs referred to as microcode or microprograms. These small programs, or subprograms, would usuallyThe 360/85 had a small writable control store (WCS) for compatibility features and diagnostics. be stored in read-only memory (ROM)The original models used transformer read-only storage (TROS) and capacitor read-only storage (CROS) inside the machine. Some models360/20, 360/22, 360/25, 360/30, 360/40, 360/50, 360/65, 360/67, 360/85 use microcode in the central processing unit (CPU) to implement instructions while others360/44, 360/75, 360/91, 360/95, 360/195 use only hardware. Some models360/20, 360/22, 360/25, 360/30, 360/40, 360/50 use cycle-stealing microcode in the CPU to implement I/O channels while others360/44, 360/65, 360/75, 360/85, 360/91, 360/95, 360/195 use only hardware in separateOriginally 2860 and 2870, but IBM later announced the 2880 to support the 2835/2305 on the 360/85 and 360/195. units. Today this approach is known as microcode.
This meant that a single lineup could have machines tailored to match the price and performance niches that formerly demanded entirely separate computer systems, where software was specific to each system. This flexibility greatly lowered barriers to entry. With most other vendors customers had to choose between machines they might outgrow or machines that were potentially too powerful and thus too costly. In practice, this meant that many companies simply did not buy computers. Now, a customer could purchase a machine that solved a particular requirement, knowing they could switch models as their needs changed, without losing support for the programs they were already running.
For instance, in the case of a firm that purchased an accounting system and was now looking to expand their computer support into engineering, this meant they could develop and test their engineering program on the machine they already used. If they ever needed more performance, they could purchase a machine with floating-point hardware, knowing that nothing else would change, it would simply get faster. Even the same peripherals could be used, allowing, for instance, data from the engineering system to be written to tape and then printed using a high-speed line printer already connected to their accounting system. Or they might replace the accounting system outright with a system with the performance to run both tasks.
The idea that a single design could address all the myriad ways that the machines could be used gave rise to the name, "360" is a reference to 360 degrees in a circle, and circles of machines and components featured prominently in IBM's advertising.
Models IBM initially announced a series of six computers and forty common peripherals. IBM eventually delivered fourteen models, including rare one-off models for NASA. The least expensive model was the Model 20 with as little as 4096 bytes of core memory, eight 16-bit registers instead of the sixteen 32-bit registers of other System/360 models, and an instruction set that was a subset of that used by the rest of the range.
The initial announcement in 1964 included Models 30, 40, 50, 60, 62, and 70. The first three were low- to middle-range systems aimed at the IBM 1400 series market. All three first shipped in mid-1965. The last three, intended to replace the 7000 series machines, never shipped and were replaced with the 65 and 75, which were first delivered in November 1965, and January 1966, respectively.
Later additions to the low-end included models 20 (1966, mentioned above), 22 (1971), and 25 (1968). The Model 20 had several sub-models; sub-model 5 was at the higher end of the model. The Model 22 was a recycled Model 30 with minor limitations: a smaller maximum memory configuration, and slower I/O channels, which limited it to slower and lower-capacity disk and tape devices than on the 30.
The Model 44 (1966) was a specialized model, designed for scientific computing and for real-time computing and process control, featuring some additional instructions, and with all storage-to-storage instructions and five other complex instructions eliminated.
A succession of high-end machines included the Model 67 (1966, mentioned below, briefly anticipated as the 64 and 66), 85 (1969), 91 (1967, anticipated as the 92), 95 (1968), and 195 (1971). The 85 design was intermediate between the System/360 line and the follow-on System/370 and was the basis for the 370/165. There was a System/370 version of the 195, but it did not include Dynamic Address Translation.
The implementations differed substantially, using different native data path widths, presence or absence of microcode, yet were extremely compatible. Except where specifically documented, the models were architecturally compatible. The 91, for example, was designed for scientific computing and provided out-of-order instruction execution (and could yield "imprecise interrupts" if a program trap occurred while several instructions were being read), but lacked the decimal instruction set used in commercial applications. New features could be added without violating architectural definitions: the 65 had a dual-processor version (M65MP) with extensions for inter-CPU signalling; the 85 introduced cache memory. Models 44, 75, 91, 95, and 195 were implemented with hardwired logic, rather than microcoded as all other models.
The Model 67, announced in August 1965, was the first production IBM system to offer dynamic address translation (virtual memory) hardware to support time-sharing. "DAT" is now more commonly referred to as an MMU. An experimental one-off unit was built based on a model 40. Before the 67, IBM had announced models 64 and 66, DAT versions of the 60 and 62, but they were almost immediately replaced with the 67 at the same time that the 60 and 62 were replaced with the 65. DAT hardware would reappear in the S/370 series in 1972, though it was initially absent from the series. Like its close relative, the 65, the 67 also offered dual CPUs.
IBM stopped marketing all System/360 models by the end of 1977. shows the announcement, ship and withdrawal dates for all S/360 models other than the transient models 64 and 66
Backward compatibility