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The IBM System/360 (S/360) is a mainframe computer system family announced by International Business Machines on April 7 1964. It was the first family of computers making a clear distinction between architecture and implementation. The chief architect of the S/360 was Gene Amdahl.


A family of computers

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System 360/65 operator's console

Unlike past practice, IBM created an entire line of computers (or CPUs) from small to large, low to high performance, all running the same command set (with two exceptions for specific markets). This feat allowed customers to use a lower cost model and then upgrade to larger systems as their needs grew — without the time and expense of rewriting software. Many models (e.g. the 360/30) even offered the option of microcode emulation of the customer's previous computer (e.g. the IBM 1401 or the IBM 1620) so that old programs could run on the new machine.

This flexibility greatly lowered barriers to entry. With other vendors (with the possible and notable exception of General Electric), customers had to choose between machines they could outgrow and machines that were potentially overpowered (and thus too expensive). The result was that many companies simply didn't buy computers. The System/360 changed the entire nature of the market, and companies could now lease "low end" machines without fear and at a lower initial cost. (At that time, IBM leased computers rather than sold them.)

IBM initially announced a family of six computers and forty common peripherals. IBM actually delivered fourteen models, including rare one-off models for NASA. The cheapest model was the 360/20 with 24K of core memory, only half the registers of other models, and an instruction set that was not binary-compatible with the rest of the range. (The Model 20 was well-suited to smaller businesses — it had the IBM name and salesforce but not all the "bells and whistles.") The most historically significant model was the 360/67, first shipped in August, 1966. The 67 was the first computer to offer primitive virtual machine capabilities to its users in conjunction with its CP-67 operating system.

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 began shipping in mid-1965. The last three, intended to replace the 7000 series machines, never shipped and were replaced by the 65 and 75, which shipped in November, 1965, and January, 1966, respectively. Later additions on the low end included the 20 (1966, mentioned above), 22 (1971), and 25 (1968). The 44 (1966) was a variant aimed at the mid-range scientific market with hardware floating point but an otherwise limited instruction set. A succession of high-end machines included the 67 (1966, mentioned above), 85 (1969), 91 (1967), 95 (1968), and 195 (1971). The 195 bridged the gap between the System/360 line and the follow-on System/370. All 360s were withdrawn by the end of 1977.

System software

Operating System/360 (OS/360) was developed for the mid-range System/360 computers. The smaller machines used Basic Operating System/360 (BOS/360) or Disk Operating System/360 (DOS/360, which evolved into VSE) and the larger were supposed to use OS/360 MVT (which evolved into MVS). MVT took a long time to develop into a usable system, and the less ambitious MFT was widely used. TSS/360 (Time-Sharing System, a Multics copy) was promised, but it never worked properly and was replaced with either CP-67 (made to run on the S/360 Model 67, as mentioned above), MTS (Michigan Terminal System), TSO (Time Sharing Option for OS/360), or one of several other time-sharing systems. CP-67 was eventually developed into VM/370, later known as VM/CMS, which turned out to become a very popular and long-lasting OS among users of the S/370 range and later successors to the higher-end S/360 models.

Basic hardware

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SLT card frame. Click to enlarge.

Being somewhat uncertain of the reliability and availability of the then new monolithic integrated circuits, IBM chose instead to design custom hybrid integrated circuits using discrete flip chip mounted glass encapsulated transistors and diodes with silk screened resistors on a ceramic substrate, then either encapsulated in plastic or covered with a metal lid. Several of these were then mounted on a small multi-layer printed circuit board to make an SLT ("Solid Logic Technology") module. Each "SLT" module had a socket on one edge that plugged into pins on the computer's backplane (the exact reverse of how most other company's modules were mounted). The IBM 1130 was a low cost computer system based on the System/360 SLT approach, which was sometimes used as a remote job entry system for the System/360.

The project and its impact

The S/360 was the most expensive CPU project in history. (The most expensive project of the 1960s was the Apollo program for moon exploration. IBM's System/360 was the second most expensive. S/360 machines were also heavily used in the Apollo project.) Fortune Magazine at the time referred to the project as IBM's "$5 billion gamble," and they were right. IBM absolutely bet the company on the System/360. (US$5 billion in 1964 dollars translates to about $28 billion in 2002 dollars.) The bet paid off.

The System/360 introduced a number of industry standards to the marketplace, such as:

Originally the System/360 was to use the ASCII character set. (IBM was a major proponent of the ASCII standardization process.) However, IBM ran out of time and didn't have enough ASCII-based peripherals ready for the system's launch. EBCDIC had been used in some earlier systems, but the System/360 turned EBCDIC into an industry standard for mainframe computing due to its own success and the subsequent need to maintain backward compatibility.

Successors and variants

The S/360 was replaced by the compatible System/370 range in 1971. (The idea of a major breakthrough with FS technology was dropped in the mid-1970s by IBM for cost, efficiency, and continuity reasons.) Later compatible systems include the competing Amdahl 470 family, IBM's own 3090, the IBM System/390 family, and most recently (and currently) the IBM zSeries. The Soviet Union produced a S/360 clone (the ES EVM).

Special radiation-hardened and otherwise somewhat modified S/360s, in the form of the System/4 Pi avionics computer, are used in several fighter and bomber jet aircraft. In the full 32-bit AP-101 version, 4 Pi machines are used as the replicated computing nodes of the fault-tolerant Space Shuttle computer system (in five nodes). The U.S. Federal Aviation Administration operated the IBM 9020, a special cluster of modified System/360s for air traffic control, from 1970 until the 1990s. (Some 9020 software is apparently still used via emulation on newer hardware.)

IBM's competitors during the System/360 era included Burroughs, UNIVAC (later Sperry), NCR, CDC, and General Electric (later Honeywell), among others.

See also


  • Emerson W. Pugh, Lyle R. Johnson, John H. Palmer, IBM's 360 and Early 370 Systems (MIT Press, Cambridge, 1991) is the definite reference work on the early history of the family.
  • IBM Corp (1964). IBM System/360 Principles of Operation. Poughkeepsie, NY: IBM Systems Reference Library, File No. S360-01, Form A22-6821-0.

External links

From the IBM Journal of Research and Development (PDF files):

  • Architecture of the IBM System/360 ( – By S/360 architects Gene Amdahl (HW), Fred Brooks (OS), and G. A. Blaauw (HW)
  • Solid Logic Technology ( – By E. M. Davis, W. E. Harding, R. S. Schwartz and J. J. Corning

Independent websites:

  • Isham Research ( – A British industry analysis and consultancy service focusing on IBM mainframe related matters
  • SHARE ( – The professional association of IBM mainframe users, founded 1955 (thus the world's first society of computing professionals)
  • – A System/390 related site in Russia(n) (location+language) (many language-independent photos and diagrams)

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

fr:IBM 360 et 370 it:System/360 ja:システム/360


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