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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 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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