System/360

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.

System/360 history

A family of computers

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. IBM made the first commercial use of microcode technology to accomplish this compatibility, employing it in all but the largest models.

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). This meant that many companies simply didn't buy computers. The System/360 changed the entire nature of the market as companies could now lease "low end" machines without fear and at a lower initial cost. (At that time, IBM leased computers instead of selling them.)

The project's size and gravity

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 $30 billion in 2005 dollars), which paid off.

Models

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 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 below), 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.

The 360/67, first shipped in August, 1966, was the first IBM system to offer dynamic address translation ("DAT," now more commonly referred to as an MMU) and virtual machine capabilities to its users in conjunction with its CP-67 operating system.

All System/360 models were withdrawn from marketing by the end of 1977.

Backward compatibility

IBM's existing customers had a large investment in software that ran on second generation machines. Many models offered the option of microcode emulation of the customer's previous computer (e.g. the IBM 1400 series on a 360/30 or the IBM 7094 on a 360/65) so that old programs could run on the new machine. However customers had to halt the computer, throw a switch, and restart to enter emulation mode. The later System/370 retained the emulation options, but allowed them to be executed under operating system control alongside native programs.

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 for cost-effectiveness and continuity reasons.) Later compatible IBM systems include the 3090, the System/390 family, and most recently (and currently) the System z9 and zSeries.

Computers which were identical or compatible in terms of the machine code or architecture of the System/360 included Amdahl's 470 family (and its successors), Hitachi mainframes, and the RCA Spectra 70 series, which was sold to what was then UNIVAC to become the Univac 9000 series of computers including the Univac 90/60 and later releases. The Soviet Union produced an S/360 clone called the ES EVM. IBM's other competitors during the System/360 and 370 era—producing non-compatible mainframes—included Burroughs, UNIVAC (later Sperry), NCR, CDC, and General Electric (later Honeywell).

The IBM 5100 portable computer, introduced in 1975, offered an option to run the System/360's programming language|APL\SV programming language" target="_blank" >^* through a hardware emulator. IBM used this approach in order to avoid the costs and delay in creating a version of APL specific to the 5100.

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.)

Technical description

Key features of lasting impact

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
Two's complement arithmetic
Segmented and paged memory
Commercial use of microcoded CPUs
The IBM Floating Point Architecture (until superseded by the IEEE 754-1985 floating-point standard, 20 years later)
The EBCDIC character setThe System/360 was originally to use the ASCII character set, and IBM was a major advocate of the ASCII standardization process. However, IBM did not have enough ASCII-based peripherals ready for the System/360's launch, and decided instead on EBCDIC, a derivation of the earlier Binary-Coded Decimal (BCD) system. There was also pressure from large commercial and government customers who had massive BCD files, which could not be converted context-free into ASCII. 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.

Architectural overview

Most System/360s had 16 general purpose 32-bit registers (R0–R15). They also had four floating point registers that could be programmed for either 32-bit or 64-bit floating point operations.

Memory addressing was accomplished using a base plus displacement scheme using registers 1 through F (15). A displacement was encoded in 12 bits, thus allowing a 4096-byte displacement (0–4095). Register 0 could not be used as a base register, as "0" was reserved to indicate an address in the first 4 KB of memory. This permitted initial execution of the IPL (boot) since base registers would not necessarily be set to 0 during the first few instruction cycles.

Instructions were always 1 byte (8 bits) followed by at least a 1-byte immediate operand. Instructions were always situated on 2-byte boundaries. There were three types of instructions: those that took no operands (2 bytes), one operand (4 bytes), and two operands (6 bytes).

Operations like the MVC (Move-Character) (Hex: D2) could only move at most 256 bytes of information. Moving more than 256 bytes of data required multiple MVC operations. (The System/370 series introduced a family of more powerful instructions such as the MVCL "Move-Long" instruction.)

An operand was two bytes long: a 4-bit nibble denoting the base register, plus 1 1/2 bytes (3 nibbles) for the displacement: 000–FFF (shown here as hexadecimal numbers). As an example, a 6-byte MVC instruction that moved 256 bytes (actually represented as 255, coded in hexadecimal as FF) from base register 7 plus displacement 000 to base register 8 plus displacement 001 would be coded D2FF 7000 8001.

Basic hardware components

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 "Solid Logic Technology" (SLT) 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.

Operating 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), Card Operating System (COS/360), Tape Operating System (TOS/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.

The /360 Model 20 offered a simplified and rarely used tape-based system called TPS (Tape Processing System), and also DPS (Disk Processing System) that provided support for the 2311 disk drive. TPS could run on a machine with 8K of memory, and DPS required 12K, which was pretty hefty for a Model 20. Many customers ran quite happily with 4K and CPS (Card Processing System).

With TPS and DOS, the card reader was used (a) to define the stack of jobs to be run (Job Control Language), and (b) to feed in transaction data, like customer payments. But the operating system was held on tape or disk, and results (master files!) could also be stored on the tapes or hard drives. Stacked job processing became an exciting possibility for the small but adventurous computer user.

Remaining machines

Despite being sold in very large numbers for a mainframe system of its era, only a few System/360 computers are known to exist today in working condition. Most machines were scrapped when they could no longer profitably be leased, in part for the gold content of their circuits. As with all mainframe systems, complete System/360 computers were prohibitively large to be held in storage, and too expensive to maintain. The Smithsonian Institution owns a System/360 Model 65, although it is no longer on public display. The Computer History Museum in Mountain View, California has a non-operable System/360 Model 30 on display.

Notes

References

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

General:

IBM's announcement of the System/360
SHARE – The professional association of IBM mainframe users, founded 1955
http://www.s390.ru – A System/390 related site in Russia(n) (location+language) (many language-independent photos and diagrams)
Lawrence Wilkinson's IBM 360/30 Saga
Pictures of an IBM 360/67 at Newcastle (UK) University

IBM hardware | Computing platforms | Mainframe computers

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