The author's later paper describing the Alto OS they built: http://bwlampson.site/38-AltoSoftware/WebPage.html http://bwlampson.site/38-AltoSoftware/WebPage.ht

The author's later paper describing the Alto OS they built: http://bwlampson.site/38-AltoSoftware/WebPage.html http://bwlampson.site/38-AltoSoftware/WebPage.html It says "The Alto system is programmed in a variety of languages: bcpl, Mesa, and Smalltalk. Each language has its own instruction set and its own operating sys­tem... The distinctive features of the Alto OS are Its open design, which allows any part of the system to be re­placed by client program ..."