System/360

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.

Table of contents

1 A family of computers 2 System software 3 Basic hardware 4 The project and its impact 5 References 6 External links

A family of computers

Unlike most systems, IBM created an entire line of computers (or CPUss) from small to large, low to high performance, all (with but one exception) 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. Some 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 buy "low end" machines without fear.

The S/360 family initially consisted of six computers and forty common peripherals; there were thirteen models in all. 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 to offer virtual machine computing to its users through its CP-67 operating system.

System software

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 TSS/360 (Time-Sharing System, a Multics copy), 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.

Basic hardware

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 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 of moon exploration; IBM's System/360 was the second most expensive. 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. ($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 8-bit byte (against financial pressure during development to reduce the byte to 4 or 6 bits)
• byte-addressable memory (as opposed to word-addressable memory)
• 32-bit words
• segmented and paged memory
• the EBCDIC character set
• commercial use of microcoded CPUs
• the IBM Floating Point Architecture (until superseded by the IEEE 754-1985 floating-point standard, 20 years later)

The S/360 was replaced by the compatible System/370 range in 1971. 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.

"Hardened" and otherwise somewhat modified S/360s, in the form of the AP-101, are used as computers in the Space Shuttle.

See also: List of IBM products

References

• IBM Corp. (1964). IBM System/360 Principles of Operation. IBM Systems Reference Library, File No. S360-01, Form A22-6821-0. Poughkeepsie, N.Y.

External links

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

• Architecture of the IBM System/360 by Gene Amdahl, Fred Brooks and G. A. Blaauw
• Solid Logic Technology by E. M. Davis, W. E. Harding, R. S. Schwartz and J. J. Corning

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This article (or an earlier version of it) contains material from FOLDOC, used with permission.