Editing IBM 1620 (section)

==1620 non-decimal arithmetic==
Since the ''Model I'' used in-memory lookup tables for addition/subtraction,<ref name=Swansea>{{cite web
   |url=https://www.swansea.ac.uk/library/archive-and-research-collections/hocc/computersandsoftware/earlycomputers/ibm1620
   |title=IBM 1620   |website=Swansea University{{snd}} swansea.ac.uk
   |access-date=2017-12-19
   |archive-url  = https://web.archive.org/web/20171222105457/http://www.swansea.ac.uk/library/archive-and-research-collections/hocc/computersandsoftware/earlycomputers/ibm1620
   |archive-date=2017-12-22  |url-status=dead}}</ref> limited bases (5 to 9) unsigned number arithmetic could be performed by changing the contents of these tables, but noting that the hardware included a ten's complementer for subtraction (and addition of oppositely signed numbers).

To do fully signed addition and subtraction in bases 2 to 4 required detailed understanding of the hardware to create a "folded" addition table that would fake out the complementer and carry logic.

Also the addition table would have to be reloaded for normal base 10 operation every time address calculations were required in the program, then reloaded again for the alternate base. This made the "trick" somewhat less than useful for any practical application.

Since the ''Model II'' had addition and subtraction fully implemented in hardware, changing the table in memory could not be used as a "trick" to change arithmetic bases. However an optional special feature in hardware for octal input/output, logical operations,
and base conversion to/from decimal was available.

Although bases other than 8 and 10 were not supported, this made the Model II very practical for applications that needed to manipulate data formatted in octal by other computers (e.g., the IBM&nbsp;7090).