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The IBM System/360 (S/360) is a 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.
Unlike past practice, IBM created an entire line of computers (or CPUs) from small to large, low to high performance, all (with two exceptions) running the same command set. This allowed customers to use a low-cost version of the family, and upgrade to larger systems if their needs grew. 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 still be run on the new machine.
This flexibility greatly lowered barriers to entry. With other machines customers had to choose between being able to afford a machine that might not have the power you needed, or instead purchasing one that guaranteed the power but cost so much as to be unattractive. The result was that many companies simply didn't buy computers. The S/360 changed the entire nature of the market, and companies could now choose "low end" machines without fear (IBM rented or leased computers at that time rather that selling them).
The S/360 family initially consisted of six computers and forty common peripherals; there were fourteen models that were actually delivered. The cheapest model was the 360/20 with 24K of memory, only half the registers of other models, and an instruction set that was not binary-compatible with the rest of the range. The most significant model was the 360/67 (first shipped in August 1966), which was the first IBM computer to offer virtual machine computing to its users through 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 and shipped in mid-1965. The last three, intended to replace 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), 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), 85 (1969), 91 (1967), 95 (1968) and 195 (1971). The last bridged into the System/370 era. All 360's were withdrawn by the end of 1977.
Operating System/360 (OS/360) was developed for the mid-range System/360 computers. The smaller machines used DOS/360 and the larger were supposed to use OS/ 360 MVT. 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 Time-sharing System), TSO (Time-Sharing Option for OS/360) or one of several others. 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.
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 a "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 S/360 was the most expensive CPU project in history. The most expensive project of the 1960s was the Apollo program of moon exploration; IBM's System/360 was the second most expensive. (Incidentally, S/360 machines were also heavily used in the Apollo project). Fortune magazine at the time referred to it as IBM's "$5 billion gamble" and they were right; IBM absolutely bet the company on this machine (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:
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-seventies 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 the IBM z/Series. The S/360 was also successfully cloned in the Soviet Union, under the name 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 (five nodes).
This article was originally based on material from the Free On-line Dictionary of Computing and is used under the GFDL.
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