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.<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, that 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|tab 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.<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>