IBM System/360 | 1964

 The IBM System/360 (S/360) is a family of mainframe computer systems announced by IBM on April 7, 1964. It was offered from 1965 to 1978. The System/360 was the first computer family designed to encompass both commercial and scientific applications, covering a complete range of applications from small to large. This design allowed IBM to release a collection of compatible designs at different price points by separating architecture from implementation. All systems, except for the partially compatible Model 44 and the most expensive systems, used microcode to implement the instruction set and featured 8-bit byte addressing, fixed-point binary, fixed-point decimal, and hexadecimal floating-point calculations.

The System/360 family introduced IBM's Solid Logic Technology (SLT), enabling the integration of more transistors on circuit cards, making it possible to create more powerful yet smaller computers.

The slowest System/360 model announced in 1964, the Model 30, could execute up to 34,500 instructions per second, with memory ranging from 8KB to 64KB. Higher-performance models were introduced later. In 1967, the IBM System/360 Model 91 could execute up to 16.6 million instructions per second. Larger 360 models could have up to 8MB of main memory, although such large memory was rare; large installations typically had 256KB of main storage, with 512KB, 768KB, or 1024KB being more common. Some models also offered up to 8 megabytes of slow (8 microsecond) large capacity storage (LCS).

The IBM 360 was very successful, allowing customers to purchase smaller systems and expand without needing to reprogram application software or replace peripherals as their needs grew. This had a lasting impact on computer design, and many consider it one of the most successful computers in history.

The chief designer of the System/360 was Gene Amdahl, and the project was managed by Fred Brooks, who reported to Thomas J. Watson Jr., the chairman. The commercial launch was led by Watson's other subordinate, John R. Opel, who managed the release of the IBM System 360 mainframe family in 1964.

Application-level compatibility (with some limitations) has been maintained to this day with the System z mainframe servers.

In the early 1960s, IBM was struggling to support and upgrade five separate computer lines, each targeting different market segments and being completely different from one another. Customers who had purchased machines like the IBM 1401 for accounting processing were now looking for machines like the IBM 7040 for engineering calculations, leaving little reason to choose IBM. The 7040 was not compatible with the 1401 and appeared as if it came from a different company. Customers were frustrated that they often needed to make large investments in entirely new machines and programs for small performance improvements.

In 1961, IBM formed a task force called SPREAD to plan developments for the 1960s. SPREAD stood for Systems Programming, Research, Engineering, and Development. At a meeting held at the New England Motor Hotel in Greenwich, Connecticut, SPREAD developed new concepts for the next generation of IBM machines. At that time, new technologies were emerging that replaced individual transistors with small-scale integrated circuits and transitioned from 6-bit oriented words to 8-bit bytes. This would lead to what is now known as the third generation of machines.

What set SPREAD apart from previous concepts was what functions would be supported. Instead of machines targeting different market niches, the new concept was a combination of all these designs. A single instruction set architecture (ISA) included instructions for binary, floating-point and decimal arithmetic, string processing, character set conversions (a major issue before the widespread use of ASCII), and extensive support for file processing.

This meant that IBM would introduce another line of machines that would once again not be compatible with previous machines. However, the new systems would be able to run all programs that previously required different machines. There was concern that customers might choose other vendors if faced with the need to purchase yet another new incompatible platform. Nevertheless, this concept steadily gained support, and six months after its formation, the company decided to implement the SPREAD concept.

A new team was organized under the leadership of Bob Evans, who personally persuaded CEO Thomas J. Watson Jr. to develop the new system. Gene Amdahl was the chief designer of the computer itself, Fred Brooks was responsible for the software project, and Erich Bloch led the development of IBM's hybrid integrated circuit design known as Solid Logic Technology.

Producing a single machine that supported all functions was nearly impossible. Instead, the SPREAD concept was based on separating the defined set of functions from internal operations, creating a family of machines with different performance and internal designs. Specifically, depending on the machine, some instructions might not be directly supported in hardware and could instead be completed using small programs written in internal machine-specific code stored in read-only memory. This is known today as microcode.

For example, a model designed for accounting could implement decimal arithmetic directly in hardware while choosing to handle floating-point instructions as subroutines. This means that floating-point operations on that system would execute (considerably) slower, but the important point is that execution is possible. Similarly, a company purchasing a system for engineering support could choose a model with floating-point hardware and sometimes use it for payroll processing. Using previous designs, systems performing floating-point operations generally lacked support for decimal arithmetic, requiring customers to either write such packages or purchase other machines.

This meant that a single lineup could have machines tailored to price and performance niches that previously required completely separate computer systems. This flexibility significantly lowered the barriers to entry. Along with most other vendors, customers had to choose between machines they could grow into and machines that were potentially too powerful and costly. In fact, this led many companies to refrain from purchasing computers. Now, customers could buy machines that addressed specific requirements and knew they could change models as needed without losing support for already running programs.

For instance, if a company that purchased an accounting system wanted to expand computer support for engineering, it meant they could develop and test engineering programs on the machine they were already using. If they needed more performance, they could purchase a machine with floating-point hardware, knowing that everything else would remain unchanged and simply be faster. The same peripherals could be used, allowing, for example, data from the engineering system to be recorded on tape and then printed using a high-speed line printer already connected to the accounting system. Alternatively, they could completely replace the accounting system with a system capable of performing both tasks.

The idea that a single design could address all the various ways a machine could be used gave rise to the name "360." This is a reference to the 360 degrees of a circle, and the circle of machines and components prominently featured in IBM's advertising.

IBM initially announced six computers and 40 common peripherals. Eventually, IBM offered 14 models, including a rare one-off model for NASA. The cheapest model was the Model 20, which had 4,096 bytes of core memory and eight 16-bit registers instead of the 32-bit registers used in other System/360 models, including a subset of the instruction set used in the other models.

The initial announcement in 1964 included models 30, 40, 50, 60, 62, and 70. The first three models were low to mid-range systems aimed at the IBM 1400 series market. All three of these models were shipped in mid-1965. The last three models were intended to replace the 7000 series machines but were not shipped and were instead replaced by the 65 and 75, which were first shipped in November 1965 and January 1966, respectively.

Subsequent additions to the low-cost models included the Model 20 (1966, mentioned above), 22 (1971), and 25 (1968). The Model 20 had several sub-models, with sub-model 5 being a higher version of the model. The Model 22 was a recycled Model 30 with some limitations, having a reduced maximum memory configuration and slower I/O channels, making it slower and limited to lower-capacity disk and tape devices than the 30.

The Model 44 (1966) was a specialized model designed for scientific computing, real-time computing, and process control, featuring some additional instructions while removing all storage inter-instruction and five complex instructions.

The advanced machines included the Model 67 (1966, mentioned below, briefly expected as 64 and 66), 85 (1969), 91 (1967, expected as 92), 95 (1968), and 195 (1971). The design of the 85 was an intermediate step between the System/360 line and the subsequent System/370, forming the basis for the 370/165. There was a System/370 version of the 195, but it did not include dynamic address translation (DAT).

Implementations varied significantly based on different basic data path widths and the presence of microcode, but they were highly compatible. Except in specially documented cases, the models were architecturally compatible. For example, the 91 was designed for scientific computing and provided out-of-order instruction execution (which could trigger "imprecise interrupts" if a program trap occurred while multiple instructions were being read), but it 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) for inter-CPU signaling expansion, and the 85 introduced cache memory. The Model 44, 75, 91, 95, and 195 were implemented with hardwired logic, unlike all other models.

The Model 67, announced in August 1965, was the first production IBM system to provide dynamic address translation (virtual memory) hardware to support time-sharing. "DAT" is now more commonly known as MMU. An experimental one-off device was built based on the Model 40. Before the 67, IBM announced models 64 and 66, which were DAT versions of the 60 and 62, but they were almost immediately replaced by the 67, and the 60 and 62 were replaced by the 65. DAT hardware reappeared in the S/370 series in 1972 but was initially omitted from the series. The 67, closely related to the 65, also provided dual CPUs.

IBM ceased marketing all System/360 models by the end of 1977.