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System/360

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The IBM System/360 was a mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 system family first announced by IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific. The design made a clear distinction between architecture
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....

 and implementation, allowing IBM to release a suite of compatible designs at different prices. All but the most expensive systems used microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

 to implement the instruction set, which featured 8-bit byte addressing and binary, decimal and floating-point calculations.

The slowest System/360 models announced in 1964 ranged in speed from 0.0018 to 0.034 MIPS; the fastest System/360 models were approximately 50 times as fast with 8 kB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

 and up to 8 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...

 of internal main memory, though the latter was unusual, and up to 8 megabytes of slower Large Core Storage (LCS)
IBM 2361 Large Capacity Storage
The IBM 2361 Large Capacity Storage was a component of the IBM System/360 models 50, 65 , and 75 computers. Storage was implemented using magnetic cores, the cycle time was 8 microseconds and the access time was 3.6 microseconds...

. A large system might have as little as 256 kB of main storage, but 512 kB, 768 kB or 1024 kB was more common.

The 360s were extremely successful in the market, allowing customers to purchase a smaller system with the knowledge they would always be able to migrate upward if their needs grew, without reprogramming of application software. The design is considered by many to be one of the most successful computers in history, influencing computer design for years to come.

The chief architect of the S/360 was Gene Amdahl
Gene Amdahl
Gene Myron Amdahl is a Norwegian-American computer architect and high-tech entrepreneur, chiefly known for his work on mainframe computers at IBM and later his own companies, especially Amdahl Corporation...

, and the project was managed by Fred Brooks
Fred Brooks
Frederick Phillips Brooks, Jr. is a software engineer and computer scientist, best known for managing the development of IBM's System/360 family of computers and the OS/360 software support package, then later writing candidly about the process in his seminal book The Mythical Man-Month...

, responsible to Chairman Thomas J. Watson Jr. The 360's commercial release was piloted by another of Watson's lieutenants John R. Opel who managed the launch of IBM’s System 360 mainframe family in 1964.

Application level compatibility (with some restrictions) for System/360 software is maintained until present day with the IBM zSeries computers.

System/360 history



A family of computers


Contrasting with at-the-time normal industry practice, IBM created an entire series of computers (or CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

s) from small to large, low to high performance, all using the same instruction set (with two exceptions for specific markets). This feat allowed customers to use a cheaper model and then upgrade to larger systems as their needs increased without the time and expense of rewriting software. IBM was the first manufacturer to exploit microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

 technology to implement a compatible range of computers of widely differing performance, although the largest, fastest, models had hard-wired logic instead.

This flexibility greatly lowered barriers to entry. With other vendors (with the notable exception of ICT
International Computers and Tabulators
International Computers and Tabulators or ICT was formed in 1959 by a merger of the British Tabulating Machine Company and Powers-Samas. In 1963 it also added the business computer divisions of Ferranti...

), 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 did not buy computers.

Models


IBM initially announced a series of six computers and forty common peripherals. IBM eventually delivered fourteen models, including rare one-off models for NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

. The cheapest model was the S/360-20 with as little as 4 K of core memory, eight 16-bit registers instead of the sixteen 32-bit registers of real 360s, and an instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...

 that was a subset of that used by the rest of the range. (The Model 20 was suited for smaller businesses — yet it had the IBM name and salesforce.)

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
IBM 1400 series
The IBM 1400 series were second generation mid-range business decimal computers that IBM sold in the early 1960s. They could be operated as an independent system, in conjunction with IBM punched card equipment, or as auxiliary equipment to other computer systems.1400-series machines stored...

 market. All three were sold first during mid-1965. The last three, intended to replace the 7000 series
IBM 700/7000 series
The IBM 700/7000 series was a series of large-scale computer systems made by IBM through the 1950s and early 1960s. The series included several different, incompatible processor architectures. The 700s used vacuum tube logic and were made obsolete by the introduction of the transistorized 7000s...

 machines, were never sold and were replaced by the 65 and 75, which was first delivered during November 1965, and January 1966, respectively.

Later additions to cheaper models included the 20 (1966, mentioned above), 22 (1971), and 25 (1968). The Model 22 was a recycled Model 30 with minor limitations: a smaller maximum memory configuration, and slower I/O channels which limited it to slower and lower-capacity disk and tape devices than on the 30.

The Model 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, briefly anticipated as the 64 and 66), 85 (1969), 91 (1967, anticipated as the 92), 95 (1968), and 195 (1971). The 85 design was intermediate between the System/360 line and the follow-on System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 and was the basis for the 370/165. There was a System/370 version of the 195, but it did not include Dynamic Address Translation.

The implementations differed substantially, using different native data path widths, presence or absence of microcode, yet were extremely compatible. Except where specifically documented, the models were architecturally compatible. The 91, for example, was designed for scientific computing and provided out-of-order instruction execution
Out-of-order execution
In computer engineering, out-of-order execution is a paradigm used in most high-performance microprocessors to make use of instruction cycles that would otherwise be wasted by a certain type of costly delay...

 (and could yield "imprecise interrupts" if a program trap occurred while several instructions were being read), but 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) with extensions for inter-CPU signalling; the 85 introduced cache memory. Models 44, 75, 91, 95, and 195 were implemented with hardwired logic, rather than microcoded as all other models.

The S/360-67, announced in August 1965, was the first production IBM system to offer dynamic address translation hardware to support time-sharing
Time-sharing
Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...

. "DAT" is now more commonly referred to as an MMU
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...

. An experimental one-off unit was built based on a model 40. Before the 67, IBM had announced models 64 and 66, DAT versions of the 60 and 62, but they were almost immediately replaced by the 67 at the same time that the 60 and 62 were replaced by the 65. DAT hardware would reappear in the S/370 series in 1972, though it was initially absent from the series. Like the 65 to which it was closely related, the 67 also had a dual-CPU implementation.

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 executed on second generation machines. Many models offered the option of emulation
Emulator
In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

 of the customer's previous computer (e.g. the IBM 1400 series
IBM 1400 series
The IBM 1400 series were second generation mid-range business decimal computers that IBM sold in the early 1960s. They could be operated as an independent system, in conjunction with IBM punched card equipment, or as auxiliary equipment to other computer systems.1400-series machines stored...

 on a 360-30 or the IBM 7094 on a 360-65) using a combination of special hardware, special microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

 and an emulation program that used the emulation instructions to simulate the target system, so that old programs could run on the new machine. However, customers had to halt the computer and load the emulation program. The 360/85 and 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 (excepting the 360/20) was replaced by the compatible System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 range in 1970 and 360/20 users were targeted to move to the IBM System/3. (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 ES/9000
IBM ES/9000 family
IBM ES/9000 is the family of IBM mainframes, introduced in 1990, as the first implementations of the ESA/390 architecture, and developed to accommodate VSE/ESA, VM/ESA and MVS/ESA operating systems. New hardware features included implementation of ESCON fiber optic channels...

 family, 9672 (System/390 family), the zSeries
ZSeries
IBM System z, or earlier IBM eServer zSeries, is a brand name designated by IBM to all its mainframe computers.In 2000, IBM rebranded the existing System/390 to IBM eServer zSeries with the e depicted in IBM's red trademarked symbol, but because no specific machine names were changed for...

, System z9
System z9
IBM System z9 is a line of IBM mainframe. It was announced on July 25, 2005 and the first models were available on September 16, 2005. The System z9 also marks the end of the previously used eServer zSeries naming convention, and it is the last z/Architecture 1 machine.- Background :System z9 is a...

, System z10 and IBM zEnterprise System
IBM zEnterprise System
IBM zEnterprise System is the latest line of IBM mainframes, introduced on July 22, 2010. It consists of four components, zEnterprise 196 , zEnterprise BladeCenter Extension and zEnterprise Unified Resource Manager...

.

Computers that were mostlyThe RCA Spectra 70 had radically different architecture for interrupts and I/O. There were compatibility packages to allow operating systems for the S/360 to run on a Spectra/70 and vice versa. identical or compatible in terms of the machine code or architecture of the System/360 included Amdahl
Amdahl Corporation
Amdahl Corporation is an information technology company which specializes in IBM mainframe-compatible computer products. Founded in 1970 by Dr. Gene Amdahl, a former IBM employee, it has been a wholly owned subsidiary of Fujitsu since 1997...

's 470 family (and its successors), Hitachi
Hitachi, Ltd.
is a Japanese multinational conglomerate headquartered in Marunouchi 1-chome, Chiyoda, Tokyo, Japan. The company is the parent of the Hitachi Group as part of the larger DKB Group companies...

 mainframes, the UNIVAC
UNIVAC
UNIVAC is the name of a business unit and division of the Remington Rand company formed by the 1950 purchase of the Eckert-Mauchly Computer Corporation, founded four years earlier by ENIAC inventors J. Presper Eckert and John Mauchly, and the associated line of computers which continues to this day...

 9200/9300/9400 series, the English Electric
English Electric
English Electric was a British industrial manufacturer. Founded in 1918, it initially specialised in industrial electric motors and transformers...

 System 4
English Electric System 4
The English Electric System 4 was a mainframe computer introduced in the mid-1960s. It was derived from the RCA Spectra 70 range, itself a clone of the IBM System 360 range. The models in the range included the System 4-10, 4-30, 4-50 , 4-70 and 4-75. ICL documentation also mentions a model 4-40...

, and the RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...

 Spectra 70 series, which was sold to what was then UNIVAC
UNIVAC
UNIVAC is the name of a business unit and division of the Remington Rand company formed by the 1950 purchase of the Eckert-Mauchly Computer Corporation, founded four years earlier by ENIAC inventors J. Presper Eckert and John Mauchly, and the associated line of computers which continues to this day...

 to become the UNIVAC 90/60
UNIVAC 90/60
The Univac 90/60 series computer was a mainframe class computer manufactured by Sperry Corporation as a competitor to the IBM System 360 series of mainframe computers...

 and later releases. The Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 produced an S/360 clone named the ES EVM
ES EVM
ES EVM was a series of clones of IBM's System/360 and System/370 mainframes, released in the Comecon countries under the initiative of the Soviet Union since the 1960s. Production continued until 1998...

.

The IBM 5100
IBM 5100
The IBM 5100 Portable Computer was a portable computer introduced in September 1975, six years before the IBM PC. It was the evolution of a prototype called the SCAMP that was developed at the IBM Palo Alto Scientific Center in 1973. In January 1978 IBM announced the IBM 5110, its larger cousin,...

 portable computer, introduced in 1975, offered an option to execute the System/360's APL.SV programming language
APL programming language
APL is an interactive array-oriented language and integrated development environment, which is available from a number of commercial and noncommercial vendors and for most computer platforms. It is based on a mathematical notation developed by Kenneth E...

 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
System/4 Pi
The IBM System/4 Pi is a family of radiation hardened avionics computers used, in various versions, on the B-52 Stratofortress bomber, the F-15 Eagle fighter, E-3 Sentry, NASA's Skylab space station, MOL, and the Space Shuttle, as well as other aircraft. It descends from the System/360 mainframe...

 avionics
Avionics
Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...

 computer, are used in several fighter and bomber jet aircraft. In the complete 32-bit AP-101 version, 4 Pi machines are used as the replicated computing nodes of the fault-tolerant Space Shuttle
Space Shuttle program
NASA's Space Shuttle program, officially called Space Transportation System , was the United States government's manned launch vehicle program from 1981 to 2011...

 computer system (in five nodes). The U.S. Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

 operated the IBM 9020
IBM 9020
The IBM 9020 refers to IBM System/360-family computers adapted into a multiprocessor system for use by the U.S. FAA for en route Air Traffic Control in its 20 Air Route Traffic Control Centers beginning in the late 1960s...

, 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
Emulator
In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

 on newer hardware.)

Influential features



The System/360 introduced a number of industry standards to the marketplace, such as:
  • The 8-bit
    8-bit
    The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...

     byte
    Byte
    The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

     (against financial pressure during development to reduce the byte to 4 or 6 bits), rather than adopting the 7030
    IBM 7030
    The IBM 7030, also known as Stretch, was IBM's first transistorized supercomputer. The first one was delivered to Los Alamos National Laboratory in 1961....

     concept of accessing bytes of variable size at arbitrary bit addresses.
  • Byte-addressable
    Memory address
    A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...

     memory (as opposed to bit-addressable or word-addressable memory)
  • 32-bit
    32-bit
    The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

     words
  • The bus and tag I/O channel standardized in FIPS-60
  • Commercial use of microcode
    Microcode
    Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

    d CPUs
  • The IBM Floating Point Architecture
    IBM Floating Point Architecture
    IBM System/360 computers, and subsequent machines based on that architecture , support a hexadecimal floating-point format.In comparison to IEEE 754 floating-point, the IBM floating point format has a longer significand, and a shorter exponent....

     (until superseded by the IEEE 754-1985 floating-point standard, 20 years later)
  • The EBCDIC
    EBCDIC
    Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....

     character setIBM was a major advocate of the ASCII
    ASCII
    The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

     standardization, supporting the approval of the American Standard Code for Information Interchange, ASA X3.4-1963. This code was first implemented in the Teletype Model 33 machines used in American Telephone & Telegraph's TWX (TeletypeWriter eXchange) network. A proposed American National Standard for the representation of ASCII in 80-column, 12-row punched cards was in the approval process when System/360 was announced. In the System/360 architecture bit 12 of the Program status word (PSW), see IBM Corp (1964), p.70, controlled selection of the EBCDIC
    EBCDIC
    Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....

     or the ASCII mode signed decimal data as described on p.35. Thus the System/360 processor architecture provided support of this proposed ASCII standard in anticipation of its approval. When the user community rejected this proposed standard, the requisite peripheral devices were never made available and this System/360 capability was not included in System/370. Bit 12 of the PSW was redefined to switch between System/360 and System/370 modes.


These second-generation systems supported “national use” characters in the IBM Binary-Coded Decimal
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

 standard to accommodate the accented characters in the alphabets of French speaking Canada, Central and South America and Western Europe. See Western Latin character sets (computing)
Western Latin character sets (computing)
Several binary representations of character sets for common Western European languages are compared in this article. These encodings were designed for representation of Italian, Spanish, Portuguese, French, German, Dutch, English, Danish, Swedish, Norwegian, and Icelandic, which use the Latin...

 for a discussion of the problems involved and the various solutions, including EBCDIC, that have been used over the years. When designing System/360, ASCII had yet to be expanded for international use, a work that began later under the name Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

.
  • Nine track magnetic tape

Architectural overview


The System/360 series had a computer system architecture
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....

 specification. This specification does not make any assumptions on the implementation itself, but rather describes the interfaces and the expected behavior of an implementation. The architecture describes mandatory interfaces that must be available on all implementations and optional interfaces which may or may not be implemented.

Some of the aspects of this architecture are:
  • Big endian byte ordering
  • A processor with
    • 16 32-bit
      32-bit
      The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

       General purpose registers (R0-R15)
    • A 64-bit Program status word (PSW) which describes (among other things)
      • Interrupt
        Interrupt
        In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

         masks
      • Privilege states
      • A condition code
      • A 24-bit instruction address
        Program counter
        The program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...

    • An interruption mechanism, maskable
      Interrupt
      In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

       and unmaskable interruption classes and subclasses
    • An instruction set
      Instruction set
      An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...

      . Each instruction is wholly described and also defines the conditions under which an exception is recognized in the form of program interruption.
  • A memory (called storage) subsystem with
    • 8 bits per byte
    • A special processor communication area starting at address 0
    • 24-bit addressing
  • Manual control operations that allow
    • A bootstrap
      Booting
      In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...

       process (a process called Initial Program Load or IPL)
    • Operator-initiated interrupts
    • Resetting the system
    • Basic debugging facilities
    • Manual display and modifications of the system's state (memory and processor)
  • An Input/Output mechanism - which does not describe the devices themselves


Some of the optional features are:
  • Binary-coded decimal
    Binary-coded decimal
    In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

     instructions
  • Floating point
    Floating point
    In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...

     instructions
  • Timing facilities (interval timer)
  • Key-controlled memory protection


All models of System/360, except for the Model 20, implemented that specification.

Binary arithmetic and logical operations are performed as register-to-register and as memory-to-register/register-to-memory as a standard feature. If the Commercial Instruction Set option was installed, packed decimal
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

 arithmetic could be performed as memory-to-memory with some memory-to-register operations. The Scientific Instruction Set feature, if installed, provided access to four floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...

 registers that could be programmed for either 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

 or 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...

 floating point operations. The Models 85 and 195 could also operate on 128-bit extended-precision floating point numbers stored in pairs of floating point registers, and software provided emulation in other models. The System/360 used an 8-bit byte, 32-bit word, 64-bit double-word, and 4-bit nibble. Machine instructions had operators with operands, which could contain register numbers or memory addresses. This complex combination of instruction options resulted in a variety of instruction lengths and formats.

Memory addressing was accomplished using a base-plus-displacement scheme, with registers 1 through F (15). A displacement was encoded in 12 bits, thus allowing a 4096-byte displacement (0-4095), as the offset from the address put in a base register. Register 0 could not be used as a base register, as "0" was reserved to indicate an address in the first 4 KB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

 of memory. This permitted initial execution of the interrupt routines, since base registers would not necessarily be set to 0 during the first few instruction cycles of an interrupt routine. It isn't needed for IPL ("Initial Program Load" or boot), as one can always clear a register without the need to save it.

With the exception of the Model 67, all addresses were real memory addresses. Virtual memory was not available in most IBM mainframes until the System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 series. The Model 67 introduced a virtual memory architecture which was used by MTS
Michigan Terminal System
The Michigan Terminal System is one of the first time-sharing computer operating systems. Initially developed in 1967 at the University of Michigan for use on IBM S/360-67, S/370 and compatible mainframe computers, it was developed and used by a consortium of eight universities in the United...

, CP-67
CP-67
CP-67 was the control program portion of CP/CMS, a virtual machine operating system developed for the IBM System/360-67 by IBM's Cambridge Scientific Center. It was a reimplementation of their earlier research system CP-40, which ran on a one-off customized S/360-40...

, and TSS/360
TSS/360
The IBM Time Sharing System TSS/360 was an early time-sharing operating system designed exclusively for a special model of the System/360 line of mainframes, the Model 67. Made available on a trial basis to a limited set of customers in 1967, it was never officially released as a supported product...

, but not by IBM's mainline System/360 operating systems.

The System/360 machine-code instructions are 2 (no memory operands), 4 (one operand), or 6 bytes (two operands) long. Instructions are always situated on 2-byte boundaries.

Operations like the MVC (Move-Character) (Hex: D2) can only move at most 256 bytes of information. Moving more than 256 bytes of data required multiple MVC operations. (The System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 series introduced a family of more powerful instructions such as the MVCL "Move-Character-Long" instruction, which allows 16 MB to be moved at once.)

An operand is two bytes long, typically representing an address as a 4-bit nibble denoting a base register and a 12-bit displacement relative to the contents of that register, in the range 000–FFF (shown here as hexadecimal
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

 numbers). The address corresponding to that operand is the contents of the specified general-purpose register plus the displacement. For example, an MVC instruction that moves 256 bytes (with length code 255 in hexadecimal as FF) from base register 7, plus displacement 000, to base register 8, plus displacement 001, would be coded as the 6-byte instruction "D2FF 8001 7000" (operator/length/address1/address2).

The System/360 was designed to separate the "system state" from the "problem state". This provided a basic level of security and recoverability from programming errors. Problem (user) programs could not modify data or program storage associated with the system state. Addressing, data, or operation exception errors caused the system state to be entered through a controlled routine allowing the operating system to attempt to correct or terminate the program in error. Similarly, certain processor hardware errors could be recovered through the "machine check" routines.

Channels


Peripherals interfaced to the system via channels. A channel was a specialized processor with the instruction set optimized for transferring data between a peripheral and main memory. In modern terms, this could be compared to direct memory access
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....

 (DMA).

There were initially two types of channels; byte-multiplexer channels, for connecting "slow speed" devices such as card readers and punches, line printer
Line printer
The line printer is a form of high speed impact printer in which one line of type is printed at a time. They are mostly associated with the early days of computing, but the technology is still in use...

s, and communications controllers, and selector channels for connecting high speed devices, such as disk drives, tape drive
Tape drive
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...

s, data cells and drums
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....

. Every S/360 (except for the Model 20, which was not a standard S/360) had a byte-multiplexer channel and 1 or more selector channels. The smaller models (up to the model 50) had integrated channels, while for the larger models (model 65 and above) the channels were large separate units in separate cabinets, such as the IBM 2860 and 2870.

The byte-multiplexer channel was able to handle I/O to/from several devices simultaneously at the device's highest rated speeds, hence the name, as it multiplex
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

ed I/O from those devices onto a single data path to main memory. Devices connected to a byte-multiplexer channel were configured to operate in 1-byte, 2-byte, 4-byte, or "burst" mode. The larger "blocks" of data were used to handle progressively faster devices. For example, a 2501 card reader operating at 600 cards per minute would be in 1-byte mode, while a 1403-N1 printer would be in burst mode. Also, the byte-multiplexer channel had an optional sub-selector section that would accommodate tape drives. The byte-multiplexor's channel address was typically "0" and the sub-selector addresses were from "C0" to "FF." Thus, tape drives on S/360 were commonly addressed at 0C0-0C7. Other common byte-multiplexer addresses were: 00A: 2501 Card Reader, 00C/00D: 2540 Reader/Punch, 00E/00F: 1403-N1 Printers, 010-013: 3211 Printers, 020-0BF: 2701/2703 Telecommunications Units. These addresses are still commonly used in z/VM virtual machines.

The S/360 models 30, 40, and 50 had an integrated 1052-7 console that was usually addressed as 01F, however, this was not connected to the byte-multiplexer channel, but rather, had a direct internal connection to the mainframe.

Selector channels enabled I/O to high speed devices. These storage devices were attached to a control unit and then to the channel. The control unit enabled clusters of devices to be attached to the channels. On higher speed S/360 models, multiple selector channels, which could operate simultaneously or in parallel, improved overall performance.
Control units were connected to the channels with gray "bus and tag" cable pairs. The bus cables carried the address and data information and the tag cables identified what data was on the bus. The general configuration of a channel was to connect the devices in a chain, like this: Mainframe—Control Unit X—Control Unit Y—Control Unit Z. Each control unit was assigned a "capture range" of addresses that it serviced. For example, control unit X might capture addresses 40-4F, control unit Y: C0-DF, and control unit Z: 80-9F. The capture ranges had to be a multiple of 8, 16, 32, 64, or 128 devices and be aligned on appropriate boundaries. Each control unit in turn had one or more devices attached to it. For example, you could have control unit Y with 6 disks, that would be addressed as C0-C5.

The cable ordering of the control units on the channel was also significant. Each control unit was "strapped" as High or Low priority. When a device selection was sent out on a mainframe's channel, the selection was sent from X->Y->Z->Y->X. If the control unit was "high" then the selection was checked in the outbound direction, if "low" then the inbound direction. Thus, control unit X was either 1st or 5th, Y was either 2nd or 4th, and Z was 3rd in line. It was also possible to have multiple channels attached to a control unit from the same or multiple mainframes, thus providing a rich high-performance, multiple-access, and backup capability.

Typically the total cable length of a channel was limited to 200 feet, less being preferred. Each control unit accounted for about 10 "feet" of the 200-foot limit.

Block multiplexer channel


IBM introduced a new type of I/O channel on the 360/85 and 360/195: the 2880 block multiplexer channel. The channel allowed a device to suspend a channel program, pending the completion of an I/O operation and thus to free the channel for use by another device. The initial use for this was the 2305 fixed-head disk, which had 8 "exposures" (alias addresses) and rotational position sensing (RPS).

These channels could support either standard 1.5 MB/second connections or, with the 2-byte interface feature, 3 MB/second; the later used one tag cable and two bus cables.

Basic hardware components


Being somewhat uncertain of the reliability and availability of the then new monolithic integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s, IBM chose instead to design custom hybrid integrated circuits using discrete flip chip
Flip chip
Flip chip, also known as Controlled Collapse Chip Connection or its acronym, C4, is a method for interconnecting semiconductor devices, such as IC chips and Microelectromechanical systems , to external circuitry with solder bumps that have been deposited onto the chip pads...

 mounted glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 encapsulated transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

s and diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

s with silk screened
Screen-printing
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...

 resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

s on a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 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
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

 board to make a "Solid Logic Technology
Solid Logic Technology
Solid Logic Technology was IBM's method for packaging electronic circuitry introduced in 1964 with the IBM System/360 series and related machines. IBM chose to design custom hybrid circuits using discrete, flip chip-mounted, glass-encapsulated transistors and diodes, with silk screened resistors...

" (SLT) module. Each SLT module had a socket on one edge that plugged into pins on the computer's backplane (the reverse of how most other company's modules were mounted).

Operating system software


The smaller S/360 models used Basic Operating System/360 (BOS/360), Tape Operating System (TOS/360), or Disk Operating System/360 (DOS/360, which evolved into DOS/VS, DOS/VSE, VSE/AF, VSE/SP, VSE/ESA, and then z/VSE).

The larger S/360 models used Operating System/360
OS/360 and successors
OS/360, officially known as IBM System/360 Operating System, was a batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was heavily influenced by the earlier IBSYS/IBJOB and Input/Output Control System packages...

 (OS/360): Primary Control Program (PCP), Multiprogramming with a Fixed number of Tasks (MFT), which evolved into OS/VS1
OS/VS1
Operating System/Virtual Storage 1, or OS/VS1,was an IBM mainframe computer operating system designed to be run on IBM System/370 hardware....

, and Multiprogramming with a Variable number of Tasks (MVT), which evolved into MVS
MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers...

. MVT took a long time to develop into a usable system, and the less ambitious MFT was widely used. PCP was used on intermediate machines; the final releases of OS/360 included only MFT and MVT.

When it announced the S/360-67 in August 1965, IBM also announced TSS/360
TSS/360
The IBM Time Sharing System TSS/360 was an early time-sharing operating system designed exclusively for a special model of the System/360 line of mainframes, the Model 67. Made available on a trial basis to a limited set of customers in 1967, it was never officially released as a supported product...

 (Time-Sharing System) for delivery at the same time as the 67. TSS/360, a response to Multics
Multics
Multics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...

, was an ambitious project that included many advanced features. It never worked properly, was delayed, canceled, reinstated, and finally canceled again in 1971. It was replaced by CP-67
CP-67
CP-67 was the control program portion of CP/CMS, a virtual machine operating system developed for the IBM System/360-67 by IBM's Cambridge Scientific Center. It was a reimplementation of their earlier research system CP-40, which ran on a one-off customized S/360-40...

, MTS (Michigan Terminal System
Michigan Terminal System
The Michigan Terminal System is one of the first time-sharing computer operating systems. Initially developed in 1967 at the University of Michigan for use on IBM S/360-67, S/370 and compatible mainframe computers, it was developed and used by a consortium of eight universities in the United...

), TSO (Time Sharing Option
Time Sharing Option
In computing, Time Sharing Option is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 , MVS, OS/390, and z/OS.- Overview :TSO fulfills a similar purpose to Unix login sessions...

 for OS/360), or one of several other time-sharing
Time-sharing
Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...

 systems.

CP-67, the original virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...

 system, was also known as CP/CMS
CP/CMS
CP/CMS was a time-sharing operating system of the late 60s and early 70s, known for its excellent performance and advanced features...

. CP/67 was developed outside the IBM mainstream at IBM's Cambridge Scientific Center
Cambridge Scientific Center
The IBM Cambridge Scientific Center, established in February 1964 by Norm Rasmussen, was situated at 545 Technology Square , Cambridge, Massachusetts in the same building as MIT's Project MAC...

, in cooperation with MIT researchers. CP/CMS eventually won wide acceptance, and led to the development of VM/370 (aka VM/CMS) and today's z/VM
Z/VM
z/VM is the current version in IBM's VM family of virtual machine operating systems. z/VM was first released in October 2000 and remains in active use and development . It is directly based on technology and concepts dating back to the 1960s, with IBM's CP/CMS on the IBM System/360-67...

.

The S/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 12 K, which was pretty hefty for a Model 20. Many customers ran quite happily with 4 K 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
Job Control Language
Job Control Language is a scripting language used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem....

), 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.

A little known and little used suite of 80 column punched-card utility programs known as Basic Programming Support (BPS) (jocularly: Barely Programming Support) was available for the S/360-30. It was a precursor of TOS on the Model 30.

Component names


IBM created a new naming system for the new components created for System/360, although well-known old names, like IBM 1403
IBM 1403
The IBM 1403 line printer was introduced as part of the IBM 1401 computer in 1959 and had an especially long life in the IBM product line. The original model could print 600 lines of text per minute and could skip blank lines at up to 75 inches/second. The standard model had 120 print...

 and IBM 1052
IBM 1050
IBM 1050 Data Communications System is a computer terminal subsystem to send data to and receive data from another 1050 subsystem or IBM computer in the IBM 1400, IBM 7000 or System/360 series. It first became available in 1963 and was used widely during the 1960s.-General:IBM 1050 Data...

, were retained. In this new naming system, components were given four-digit numbers starting with 2. The second digit described the type of component, as follows:
  • 20xx: arithmetic processors, for example the IBM 2030
    IBM 2030
    The IBM System/360 Model 30 was a popular IBM mainframe announced in 1964 across the world as the least powerful of the System/360 range of four compatible computers – the first range in the world to allow programs to be written that could be used across a range of computers...

    , which was the CPU for the IBM System/360 Model 30;
  • 21xx: power supplies and other equipment intimately associated with processors, for example the IBM 2167 Configuration Unit;
  • 22xx: visual output devices, for example the IBM 2250
    IBM 2250
    The IBM 2250 Graphics Display Unit was announced as part of System/360 in 1964. Unlike most modern computer displays, which show images in raster format, the IBM 2250 used vector graphics. A display list of line segments on a 1024 by 1024 grid was stored in the computer's memory and repainted on...

     and IBM 2260
    IBM 2260
    The text-only 960-character monochrome IBM 2260 cathode ray tube video display terminal plus computer keyboard was a 1964 predecessor to the more-powerful color text-and-graphics IBM 3270. The 2260 screen image was normally configured with 12 lines of 80 characters each, which corresponded to IBM...

     CRT displays, and the IBM 2203 line printer for the System/360 model 20;
  • 23xx: direct-access and random-access storage devices, for example the IBM 2311 and IBM 2314 disk drives, the IBM 2321 Data Cell
    IBM 2321 Data Cell
    The IBM 2321 Data Cell announced in April 1964 was a direct access storage device for the IBM System/360. It could hold up to 400 million bytes of data, but its access time was approximately 450 milliseconds.-Characteristics:...

    , the IBM 2361 Large Capacity Storage
    IBM 2361 Large Capacity Storage
    The IBM 2361 Large Capacity Storage was a component of the IBM System/360 models 50, 65 , and 75 computers. Storage was implemented using magnetic cores, the cycle time was 8 microseconds and the access time was 3.6 microseconds...

     (Core Storage, Large Core Storage or LCS) and the IBM 2365 Processor Storage
    IBM 2365 Processor Storage
    The IBM 2365 Processor Storage was a component of the IBM System/360 models 65, 67, 75 and 85 computers. Storage was implemented using magnetic cores, and the storage width was 72 bits, which comprised 64 data bits plus 8 parity bits. The IBM 2365 model 1 contained bytes of memory; all other...

    .
  • 24xx: magnetic tape drives, for example the IBM 2401, IBM 2405 and IBM 2415;
  • 25xx: punched card handling equipment, for example the IBM 2501 (card reader
    Punched card
    A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...

    ), IBM 2520 (card punch); IBM 2540 (reader/punch) and IBM 2560 (Multi-Function Card Machine or MFCM);
  • 26xx: paper tape handling equipment, for example the IBM 2671 paper tape reader;
  • 27xx: communications equipment, for example the IBM 2701, IBM 2705, IBM 2741
    IBM 2741
    The IBM 2741 was a printing computer terminal introduced in 1965.It combined a ruggedized Selectric typewriter mechanism with IBM SLT electronics and an RS-232-C serial interface. It operated at about 14.1 characters per second with a data rate of 134.5 bits/second...

     interactive terminal and the IBM 2780 batch terminal;
  • 28xx: channels and controllers, for example the IBM 2821 Control Unit
    IBM 2821 Control Unit
    The IBM 2821 Control Unit was used to attach card readers, card punches andline printers to the IBM System/360 and IBM System/370 families of computers.The devices attached were as follows:* The IBM 2540 card reader and card punch;...

    , IBM 2841 and IBM 2844; and
  • 29xx: miscellaneous devices, for example the IBM 2914 Data Channel Switch and the IBM 2944 Data Channel Repeater.

Peripherals


IBM developed a new family of peripheral equipment for the S/360, carrying over a few from its older 1400 series. Interfaces were standardized, allowing greater flexibility to mix and match processors, controllers and peripherals than in the earlier product lines.

In addition, the S/360 computers could use certain peripherals that were originally developed for earlier computers. These earlier peripherals used a different numbering system, such as the IBM 1403
IBM 1403
The IBM 1403 line printer was introduced as part of the IBM 1401 computer in 1959 and had an especially long life in the IBM product line. The original model could print 600 lines of text per minute and could skip blank lines at up to 75 inches/second. The standard model had 120 print...

 chain printer. The 1403, an extremely reliable device which had already earned a reputation as a workhorse, was sold as the 1403-N1 when adapted for the System/360.

Also available were optical character recognition
Optical character recognition
Optical character recognition, usually abbreviated to OCR, is the mechanical or electronic translation of scanned images of handwritten, typewritten or printed text into machine-encoded text. It is widely used to convert books and documents into electronic files, to computerize a record-keeping...

 (OCR) readers 1287 and 1288.

Most small systems were sold with an IBM 1052-7 as the console typewriter. This was tightly integrated into the CPU — the keyboard would physically lock under program control. Certain high-end machines could optionally be purchased with a 2250
IBM 2250
The IBM 2250 Graphics Display Unit was announced as part of System/360 in 1964. Unlike most modern computer displays, which show images in raster format, the IBM 2250 used vector graphics. A display list of line segments on a 1024 by 1024 grid was stored in the computer's memory and repainted on...

 graphical display, costing upwards of US $100,000. The 360/85 used a 5450 display console that was not compatible with anything else in the line; the later 3066 console for the 370/165 and 370/168 used the same basic display design as the 360/85.

Direct access storage devices (DASD)


The first disk drives for the 360 were IBM 2302s and IBM 2311s. The 156 kB/second 2302 was based on the earlier 1302 and was available as a model 3 with two 112.79 MB modules or as a model 4 with four such modules.

The 2311, with a removable 1316 disk pack
Disk pack
A Disk pack is a layered grouping of hard disk platters . A disk pack is the core component of a hard disk drive. In modern hard disks, the disk pack is permanently sealed inside the drive...

, was based on the IBM 1311 and had a theoretical capacity of 7.2 MB, although actual capacity varied with record design. (When used with a 360/20, the 1316 pack was formatted into fixed-length sectors
Disk sector
In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector...

, giving a maximum capacity of 5.4MB.)

In 1966, the first 2314s shipped. This device had up to eight usable disk drives with an integral control unit; there were nine drives, but one had to be reserved as a spare. Each drive used a removable 2316 disk pack with a capacity of nearly 28 MB. The disk packs for the 2311 and 2314 were physically large by today's standards — e.g., the 1316 disk pack was about 14 in (35.6 cm) in diameter and had six platters stacked on a central spindle. The top and bottom outside platters did not store data. Data were recorded on the inner sides of the top and bottom platters and both sides of the inner platters, providing 10 recording surfaces. The 10 read/write heads moved together across the surfaces of the platters which were formatted with 203 concentric tracks. To reduce the amount of head movement (seeking), data was written in a virtual cylinder from inside top platter down to inside bottom platter. These disks were not usually formatted with fixed-sized sectors as are today's hard drives (though this was done with CP/CMS
CP/CMS
CP/CMS was a time-sharing operating system of the late 60s and early 70s, known for its excellent performance and advanced features...

). Rather, most S/360 I/O software could customize the length of the data record (variable-length records), as was the case with magnetic tapes.
Some of the most powerful early S/360s used high-speed head-per-track drum storage devices. The 3,500 RPM 2301, which replaced the 7320, was part of the original S/360 announcement, with a capacity of 4Mb. The 303.8 kB/second IBM 2303 was announced on January 31, 1966, with a capacity of 3.913 MB. These were the only drums announced for the S/360 and S/370, and their niche was later filled by fixed-head disks.

The 6,000 RPM 2305 appeared in 1970, with capacities of 5 Mb (2305-1) or 11 Mb (2305-2) per module. Although these devices did not have large capacity, their speed and transfer rates made them attractive for high-performance needs. A typical use was overlay linkage (e.g. for OS and application subroutines) for program sections written to alternate in the same memory regions. Fixed head disks and drums were particularly effective as paging devices on the early virtual memory systems. The 2305, although often called a "drum" was actually a head-per-track disk device, with 12 recording surfaces and a data transfer rate up to 3 megabytes per second.

Rarely seen was the IBM 2321 Data Cell
IBM 2321 Data Cell
The IBM 2321 Data Cell announced in April 1964 was a direct access storage device for the IBM System/360. It could hold up to 400 million bytes of data, but its access time was approximately 450 milliseconds.-Characteristics:...

, a bizarre (and mechanically dramatic) device that contained multiple magnetic strips to hold data; strips could be randomly accessed, placed upon a cylinder-shaped drum for read/write operations; then returned to an internal storage cartridge. The IBM Data Cell [noodle picker] was among several IBM trademarked "speedy" mass online direct-access storage peripherals (reincarnated in recent years as "virtual tape" and automated tape librarian peripherals). The 2321 file had a capacity of 400 MB, at the time when the 2311 disk drive only had 7.2 MB. The IBM Data Cell was proposed to fill cost/capacity/speed gap between magnetic tapes—which had high capacity with relatively low cost per stored byte—and disks, which had higher expense per byte. Some installations also found the electromechanical operation less dependable and opted for less mechanical forms of direct-access storage.

Tape drives



The 2400 tape drives consisted of a combined drive and control unit, plus individual 1/2" tape drives attached. With the 360, IBM switched from IBM 7 track
IBM 7 Track
IBM's first magnetic tape data storage devices, introduced in 1952, use what is now generally known as 7 track tape. The magnetic tape is 1/2" wide and there are 6 data tracks plus 1 parity track for a total of 7 parallel tracks that span the length of the tape...

 to 9 track tape format. 2400 drives could be purchased that read and wrote 7 track tapes for compatibility with the older IBM 729
IBM 729
The IBM 729 Magnetic Tape Unit was IBM's iconic tape mass storage system from the late 1950s through the mid 1960s. Part of the IBM 7 track family of tape units, it was used on late 700, most 7000 and many 1400 series computers...

 tape drives. In 1967, a slower and cheaper pair of tape drives with integrated control unit was introduced: the 2415. In 1968, the IBM 2420 tape system was released, offering much higher data rates, self-threading tape operation and 1600bpi packing density. It remained in the product line until 1979.

Unit record devices



  • Punched card devices included the 2501 card reader and the 2540 card reader punch. Virtually every S/360 had a 2540. The 2560 MFCM ("Multi-Function Card Machine") reader/sorter/punch, listed above, was for the Model 20 only. It had notorious reliability problems (earning humorous acroymns often involving "...Card Muncher" or "Mal-Function Card Machine).
  • Line printer
    Line printer
    The line printer is a form of high speed impact printer in which one line of type is printed at a time. They are mostly associated with the early days of computing, but the technology is still in use...

    s were the IBM 1403
    IBM 1403
    The IBM 1403 line printer was introduced as part of the IBM 1401 computer in 1959 and had an especially long life in the IBM product line. The original model could print 600 lines of text per minute and could skip blank lines at up to 75 inches/second. The standard model had 120 print...

     and the slower IBM 1443.
  • A paper tape reader, the IBM 2671, was introduced in 1964. It had a rated speed of 1,000 cps. There were also a paper tape reader and paper tape punch from an earlier era, available only as RPQs (Request Price Quotation). The 1054 (reader) and 1055 (punch), which were carried forward (like the 1052 console typewriter) from the IBM 1050 Teleprocessing System. All these devices operated at a maximum of 15.5 characters per second. The paper tape punch from the IBM 1080 System was also available by RPQ, but at a prohibitively expensive price.
  • Optical Character Recognition (OCR) devices 1287 and latter the 1288 were available on the 360's. The 1287 could read handwritten numerals, some OCR fonts, and cash register OCR paper tape reels. The 1288 'page reader' could handle up to legal size OCR font typewritten pages, as well as handwritten numerals. Both of these OCR devices employed a 'flying spot' scanning principle, with the raster scan provided by a large CRT, and the reflected light density changes were picked up by a high gain Photo Multiplier tube.
  • MICR (Magnetic Ink Character Recognition) was provided by the IBM 1412 and 1419 Cheque Sorters, with Magnetic Ink Printing (for cheque books) on 1445 Printers (a modified 1443 that used an MICR ribbon). 1412/1419 and 1445 were mainly used by Banking Institutions.

Remaining machines


Few of these machines remain. Despite being sold or leased in very large numbers for a mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 system of its era, only a few System/360 computers are known to exist today, none of which are in working condition. Most machines were scrapped when they could no longer profitably be leased, partly for the gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 and other precious metal content of their circuits, but mainly to keep these machines from competing with IBM's newer computers, such as the System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

. As with all classic mainframe systems, complete System/360 computers were prohibitively large to be held in storage, and too expensive to maintain. The Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...

 owns a System/360 Model 65, although it is no longer on public display. The Computer History Museum
Computer History Museum
The Computer History Museum is a museum established in 1996 in Mountain View, California, USA. The Museum is dedicated to preserving and presenting the stories and artifacts of the information age, and exploring the computing revolution and its impact on our lives.-History:The museum's origins...

 in Mountain View, CA has a non-working System/360 Model 30 on display, as does the Museum of Transport and Technology
Museum of Transport and Technology
The Museum of Transport and Technology is a museum located in Western Springs, Auckland, New Zealand. It is located close to the Western Springs Stadium, Auckland Zoo and the Western Springs Park. The museum has large collections of civilian and military aircraft and other land transport vehicles...

 (Motat) in Auckland, New Zealand and the Vienna University of Technology
Vienna University of Technology
Vienna University of Technology is one of the major universities in Vienna, the capital of Austria. Founded in 1815 as the "Imperial-Royal Polytechnic Institute" , it currently has about 26,200 students , 8 faculties and about 4,000 staff members...

 in Austria. The University of Western Australia
University of Western Australia
The University of Western Australia was established by an Act of the Western Australian Parliament in February 1911, and began teaching students for the first time in 1913. It is the oldest university in the state of Western Australia and the only university in the state to be a member of the...

 has a complete System/360 in storage at its Shenton Park warehouse. The IBM museum in Sindelfingen
Sindelfingen
Sindelfingen is a German town near Stuttgart at the headwaters of the Schwippe that is the site of a Mercedes-Benz assembly plant.-History:* 1155 First documented mention of Sindelfingen...

 has two S/360s (a Model 20 and a Model 91 floating point machine). The control panel of the most complex System/360 model type built, the FAA
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

 360 - 9020, comprising 3 System/360 model 65s and 3 System/360 model 50s is on display in the Computer Science department of Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...

 as IBM 360 display and Stanford Big Iron. It was manufactured in 1971 and decommissioned in 1993. The IBM Endicott History and Heritage Center in Endicott, NY has a non-working System/360 and an associated 2401 magnetic tape drive on display.

See also

  • History of IBM
    History of IBM
    International Business Machines, abbreviated IBM and nicknamed "Big Blue", is a multinational computer technology and IT consulting corporation headquartered in Armonk, New York, United States. The company is one of the few information technology companies with a continuous history dating back to...

  • List of IBM products
  • Dr. Gene Amdahl
    Gene Amdahl
    Gene Myron Amdahl is a Norwegian-American computer architect and high-tech entrepreneur, chiefly known for his work on mainframe computers at IBM and later his own companies, especially Amdahl Corporation...

     (architect)
  • Dr. Gerrit Blaauw
    Gerrit Blaauw
    Gerrit Anne Blaauw is one of the principal designers of the IBM System/360 line of computers, together with Fred Brooks, Gene Amdahl, and others....

     (architect)
  • Dr. Fred Brooks
    Fred Brooks
    Frederick Phillips Brooks, Jr. is a software engineer and computer scientist, best known for managing the development of IBM's System/360 family of computers and the OS/360 software support package, then later writing candidly about the process in his seminal book The Mythical Man-Month...

     (System/360 project manager)
  • Bob Evans (computer scientist)

From the IBM Journal of Research and Development

  • Architecture of the IBM System/360 — By S/360 architects Gene Amdahl (HW
    Computer hardware
    Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

    ), Fred Brooks (OS
    Operating system
    An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

    ), and G. A. Blaauw (HW)
  • Solid Logic Technology — By E. M. Davis, W. E. Harding, R. S. Schwartz and J. J. Corning

From IBM Systems Journal


General