IBM System/360 Model 20: Difference between revisions

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The '''IBM System/360 Model 20''' is the smallest member of the [[IBM System/360]] family announced in November 1964. The Model 20 supports only a subset of the System/360 instruction set, with binary numbers limited to 16 bits and no [[floating point arithmetic]].<ref name=FuncChar>{{cite book|publisher=IBM|id=A26-5847-3|title=IBM System/360 Model 20 Functional Characteristics|date=1967|url=http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/A26-5847-3_360-20_funChar_Apr67.pdf}}</ref> In later years it would have been classified as a 16-bit [[minicomputer]] rather than a [[Mainframe computer|mainframe]], but the term "minicomputer" was not current, and in any case IBM wanted to emphasize the compatibility of the Model 20 rather than its differences from the rest of the System/360 line. It does, however, have the full System/360 decimal instruction set, thatwhich allows for addition, subtraction, product, and dividend of up to 31 decimal digits.

Developed by IBM in [[Böblingen]], Germany,<ref name=Pugh>{{cite book|last1=Pugh|first1=Emerson W.|last2=Johnson|first2=Lyle R.|last3=Palmer|first3=John H.|title=IBM's 360 and Early 370 Systems|year=1991|publisher=MIT Press|isbn=0-262-16123-0|url=https://archive.org/details/ibms360early370s0000pugh|url-access=registration}}</ref>{{rp|pp.217–218,352}} the system was intended for [[Computer data processing|data processing]] and as a replacement for [[Unit record equipment|tabtabulating equipment]]. An incompatible small computer, the [[IBM 1130]] introduced the following year, was designed for scientific and engineering computing.

The design of the Model 20 was the result of a tug of war within IBM. [[John Haanstra]] had wanted a low-end machine that could execute [[IBM 1401]] instructions. [[Fred Brooks]] wanted the machine to have System/360 architecture. The final result was a compromise where cost concerns predominated: the system leased for under $2000 per month ({{inflation|US|2000|1964|fmt=eq}}) including [[#IBM 2560 Multi-Function Card Machine|Multi-Function Card Machine]] and line printer, compared to around $3000 ({{inflation|US|3000|1964|fmt=eq}}) for a comparable 1401 system.<ref name=Pugh />{{rp|p.446}}

Based on number of systems sold, the Model 20 was the most successful model of System/360. According to Pugh "The number of Model 20 processors installed by the end of 1970 in the United States exceeded 7,400."<ref name=Pugh />{{rp|p.639}} Other models, however, brought in greater revenue. Despite their popularity there are relatively few Model 20s in existence in working condition in 2020.<ref>{{cite web |last1=Bradley |first1=Adam |last2=Blackburn |first2=Christopher |last3=Vaughan |first3=Peter |title=IBM 360 Model 20 Rescue and Restoration |url=https://ibms360.co.uk/ |accessdate=Mar 2, 2020}}</ref>

[[File:IBM system 360.JPG |thumb|Another IBM System/360 Model 20 CPU.]]

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==Architecture==

The Model 20 was available with six memory sizes: 4, 8, 12, 16, 24 and 32&nbsp;KB. As in other models of System/360, memory is byte-addressable.<ref name=FuncChar />{{rp|p.1}} It has eight 16-bit [[Processor register|general purpose registers]] numbered R8 through R15 which can be used in computations as base [[Addressing mode#Base plus offset, and variations|register]]s. All of memory is also directly addressable through a feature, called direct addressing, that combines the twelve-bit displacement and the low-order bits of what would normally be the base register field of the instruction (R0-R7) to form a combined fifteen bit address.<ref name=FuncChar />{{rp|p.4}} No storage protection is provided, except for the low 144 bytes of "protected area".

The instruction set is a subset of System/360 consisting of 37 instructions instead of 143,<ref name=FuncChar />{{rp|pp.7–25}}<ref>{{cite book|last=Pugh|first=Emerson W.|title=Building IBM: Shaping an Industry and Its Technology|year=1995|publisher=MIT Press|isbn=0-262-16147-8|url=http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=7339|access-date=2012-10-15|archive-date=2012-05-05|archive-url=https://web.archive.org/web/20120505165349/http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=7339|url-status=dead}}</ref>{{rp|p.384}} with some incompatible instructions, such as a BASR (Branch And Store Register) rather than BALR (Branch And Link Register).

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==Remaining machines==

Despite having been sold or leased in very large numbers for a [[mainframe computer|mainframe]] system of its era, only a few of System/360 Model 20 computers remain. These are primarily the property of museums or collectors. Examples of existing systems include:

* Two Model 20 processors along with numerous peripherals (forming at least one complete system) located in Nürnberg, Germany were purchased on [[eBay]] in April/May 2019 for €3710 by two UK enthusiasts who, over the course of some months, moved the machine to [[Creslow Park]] in [[Buckinghamshire]], United Kingdom. The system was in a small, abandoned building left untouched for decades, and apparently had been used in that building since all peripherals were still fully wired and interconnected.<ref name="ibms360">{{cite web|url=https://ibms360.co.uk/?page_id=22|title=Project History|website=IBM 360 Model 20 Rescue & Restoration|access-date=2019-05-20|year=2019}}</ref> As of September 2024 the systems have been moved on a long-term loan basis to the [[System Source Computer Museum]] in [[Hunt Valley, Maryland]], USA for display and restoration.<ref>{{cite web|title=The 360’s have a new home!|work=IBM 360 Model 20 Rescue & Restoration Blog|date=September 11, 2024|url = https://www.ibm360.co.uk/?p=916}}</ref>