Editing IBM 8100 (section)

==Architecture==
The 8100 was a 32-bit processor, but its instruction set
reveals its lineage as the culmination of a line of
so-called Universal Controller processors{{#tag:ref|{{cite journal
|title=SNA multiple-system networking
|author1=James P. Gray
|author2=T. B. McNeill
|journal=IBM Systems Journal
|issue=2
|volume=18
|year=1979
|doi=10.1147/sj.182.0263
|pages=263–297
}}}}<!--|accessdate=20 May 2009-->{{Failed verification|date=February 2021|reason=Nothing about Universal Controller in source, 8100 only mentioned in passing}} internally designated
UC0 (8-bit), UC.5 (16-bit) and UC1 (32-bit).
Each processor carried along the instruction set and architecture
of the smaller processors,
allowing programs written for a smaller processor to run on a larger one
without change.

The 8100 had another interesting distinction in being one of the first commercially available systems to have a network with characteristics of what we now call local area networks, in particular the mechanism of packet passing.  It was called the "8100 Loop" or "R-Loop"<ref>{{cite journal|url=http://www.research.ibm.com/journal/sj/343/christensen.pdf|title=Local area networks- Evolving from shared switched access|first1=K. J.|last1=Christensen|first2=L. C.|last2=Haas|first3=F. E.|last3=Noel|first4=N. C.|last4=Strole|journal=IBM Systems Journal|volume=34|issue=3|year=1995|doi=10.1147/sj.343.0347|pages=347–374|archive-url=https://web.archive.org/web/20030709203536/http://www.research.ibm.com/journal/sj/343/christensen.pdf|archive-date=July 9, 2003}}</ref> and it supported various attached terminals (such as the [[IBM 3101|3104]]), printers (such as the free-standing [[IBM 3270|3268-1]]) and other devices. Topologically this arranged terminals in a ring, with redundant sets of wires which allowed for a break in the wire to be tolerated simply by "turning back" the data on each side of the break.

The 8100 also supported an "intelligent" terminal called the 8775 (which shared the same case as the [[IBM 3270|3279]] colour display terminal for IBM's mainframes and, like the 3279 was designed at [[Hursley House|IBM's UK Development Lab]] at Hursley Park, England) which was the first to ship with the ability to download its functionality from the host computer to which it was attached.