System / 360

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A system / 360 for electronic data processing in the VW plant in Wolfsburg (1973)
An IBM system 360/20 in the Deutsches Museum, Munich
IBM 2314 disk drives and IBM 2540 card reader / punch for the System / 360

System / 360 or S / 360 for short is the name of a mainframe architecture made by IBM in 1964. Before that, IBM built the 700/7000 series . The System / 360 was followed by the System / 370 announced in 1970 .

At the beginning of the 1960s, IBM dominated the computer market, but the systems were mostly incompatible with one another and not from a uniform line, so that a great deal of effort was required to support the different product families. This was to be remedied with the System / 360, with which IBM presented a single, uniform and easily expandable system on the mainframe market in 1964. With the total development costs of $ 5 billion for the project (twice the annual income of IBM in 1962), IBM boss Thomas J. Watson Jr. risked the existence of the company, but soon after its introduction the system dominated the mainframe market. In the 1970s, over 70 percent of mainframes were from IBM, descendants of the 360 ​​family of products. It was mainly because of them that IBM sales rose from $ 3.6 billion in 1965 to $ 8.3 billion in 1971, and in 1982 they made up over half of the company's sales. It was one of the most successful computer systems of all time. The specifications published by IBM also allowed other vendors to offer peripheral devices for the System / 360, often at a lower cost. Conversely, other companies such as Amdahl and Univac began to develop computers compatible with the 360 ​​system. Just like the hardware, the software has also been standardized and made compatible with the System / 360.

The main architects were Frederick P. Brooks , Gerrit Blaauw , Gene Amdahl . The overall direction was Bob O. Evans . Was also involved u. a. Erich Bloch . Brooks, Bloch and Evans received the National Medal of Technology and Innovation for this in 1985 .

Design criteria

The main design criteria were:

  • Versatile use. Before the S / 360, computers were optimized for either commercial or scientific-technical purposes.
  • 8 bit character size. In contrast to contemporary architectures, which worked with 6-bit units, bytes of 8 bits were used.
  • 32- or 64-bit floating point words with a hexadecimal base.
  • Sign for integer values ​​is 1 bit
  • Possibility of packed decimal numbers: Each digit occupies half a byte (4 bits, binary coded decimal numbers ), the sign occupies the last 4 bits in the last byte.
  • Character strings of variable length have a length field and are not terminated with a special character.
  • Dispensing with a stack . This makes linkage conventions necessary since the status must be saved when subroutine calls are made.
  • Basically indexed addressing using a base register . Programs are basically independent of physical addresses.
  • Binary addressing
  • All 16 registers are universal registers that can be used both as accumulators and, with the exception of register 0, for addressing. However, it is advisable to use only registers from no. 3 (up to a maximum of 8) as the base register (addressing). Register 1 and 2 are forcibly used by certain commands (e.g. TRT). Registers 12 to 15 are used to call subroutines and some are also required for other purposes. One register is sufficient to address a memory area of ​​4096 bytes (4k bytes). In the machine instructions, only 2 bytes are available for a memory address, 4 bits for the register number and 12 bits for the displacement .
  • The universal registers are 32 bits wide, the right 24 bits are used for addressing, which enables an address space of 16 megabytes. In the case of the BAL and BALR (Branch and Link) commands, the return address is stored in a register; the condition code is saved in the left 4 bits of the register. Due to these special features, it was not so easy to expand the address space beyond the 16 MB with the successor systems.

Starting with version 67, essential elements of virtual memory were also implemented, after this had previously been removed from the design of the initial versions . This was also found in the successor model System / 370.

The S / 360 architecture has been continuously developed over the past forty years ( System / 370 , System / 390 ) and is currently culminating in the System z architecture.

Operating systems

Punch card for the IBM mainframe computer with EBCDIC character set

With the System / 360 three operating systems appeared , TOS / 360 for installations without hard disks , DOS / 360 for smaller and OS / 360 for larger installations with hard disks. OS / 360 is the forerunner of the current z / OS .

The 360/20 system was a special feature. It was originally designed as a pure punch card system to replace tabulating machines and only had a limited set of instructions (machine commands). The register width was 16 bits with 8 instead of 16 registers.

Some of the operating systems for System / 360 allow multiprogramming : MFT (Multiprogramming with a fixed number of tasks) and MVT (Multiprogramming with a variable number of tasks). OS / VS1 in System / 370 evolved from MFT and MVS in System / 370. With the Model 67, IBM also offered the first time sharing systems.


In addition to the original System / 360 hardware and the operating system OS / 360 can now on Windows and Linux systems with the free Hercules - emulator to run.

Name meaning

The system claimed to be comprehensive. Therefore, the number 360 was chosen in the sense of 360 °. However, the claim could not be fulfilled, so that the number was defined differently in retrospect: 3 was rated as an IBM standard and 60 for a product developed in the 1960s . Hence the successors were called S / 370 and S / 390 .

System / 360 Series (source: IBM archive)
model Available from RAM in kB
20th 1966 4th
22nd 1971 24-32
25th 1968 16-48
30th 1965 16-64
40 1965 32-256
44 1966 32-256
50 1965 128-8192
65 1965 256-8192
67 1966 256-1024
75 1966 256-8192
85 1969 512-4096
91 1967 2048-6144
95 1968 1024-6144
195 1971 1024-4096


In the Eastern bloc three series were developed by computer systems. Series I ESERs were largely identical to the IBM System / 360. These included the systems R40 / EC 1040 ( VEB Kombinat Robotron DDR ) or EC 1022 (EC EWM / Soviet Union).


  • System / 360 section in Gordon Bell , Daniel Siewiorek , Allen Newell (Eds.) Computer structures. Principles and Examples , McGraw Hill 1982, online at Microsoft Research , are reprinted Blaaw, Brooks The structure of System / 360 , Part 1 (Outline of the logical structure), IBM Systems J., Volume 3, 1964, No. 2, p 119-135, and Part 2 (System implementations) by WY Stevens, ibid. Pp. 136–143 (chapters 43 and 44 of the book). There is also Part 3 (Processing unit design considerations) by Amdahl, p. 144, Part 4 (Channel design considerations) by A. Padegs, p. 165, Part 5 by Blaaw (Multisystem Organization), p. 181
  • SG Tucker Microprogram control for SYSTEM / 360 , IBM Systems Journal, Vol. 6, No. 4, 1967, pp. 222-241
  • Clarence B. Germain The programming manual of the IBM / 360 , Munich 1969

Web links

Commons : System / 360  - collection of images, videos and audio files

Individual evidence

  1. ^ Computer Museum for the IBM / 360
  2. [The mainframe turns 50, or, why the IBM System / 360 launch was the dawn of enterprise IT Joab Jackson, The mainframe turns 50, or, why the IBM System / 360 launch was the dawn of enterprise IT, PC World 2014]
  3. Amdahl, Blaauw, Brooks Architecture of the IBM System / 360 , IBM Journal of Research and Development, Volume 8, April 1964, pp. 87-101