A firsthand account from a Unix pioneer explaining the origins of two famous pieces of Unix lore: the cryptic error message 'values of β will give rise to dom!' found in mv.c of Sixth Edition Unix, and the notorious comment 'You are not expected to understand this' in the kernel's process-switching code. The author reveals the beta message originated from a typist accidentally printing a fragment of a patent application on a Model 37 Teletype in the early 1970s. The 'you are not expected to understand this' comment is clarified as a humble admission — the authors genuinely didn't understand the broken savu/retu process-exchange mechanism, which failed when porting to the Interdata due to differing compiler register-save conventions. Additional anecdotes cover an undocumented PDP-11 division routine bug and a hardware wiring error in the KS-11 memory mapping unit discovered via circuit drawings.
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Odd Comments and Strange Doings in Unix/* You are not expected to understand this */Comments I do feel guilty aboutA hardware storyQuestions this post answers
What is the origin of the Unix comment 'You are not expected to understand this'?
The comment was written as a humble admission, not a challenge — the authors genuinely did not understand the savu/retu process-exchange mechanism they were using. The mechanism was fundamentally broken because it depended on switching to a previous stack frame and executing function return code in a different procedure. It worked on the PDP-11 due to consistent compiler behavior but failed when porting to the Interdata, where procedure return code differed by register usage. Developers curious about Unix internals and systems history find deep dives like this through daily.dev.
Where did the Unix error message 'values of beta will give rise to dom' come from?
The phrase originated when a typist at Bell Labs accidentally interrupted a printout of a patent application on a Model 37 Teletype in the early 1970s, leaving a fragment visible — likely from something like 'varying values of beta will give rise to domain wall movement.' The striking, context-free phrase was later immortalized in the v6 Unix source, most likely by Bob Morris or Ken Thompson. Unix history threads like this surface regularly for systems programmers on daily.dev.