[geeks] Sun-1 with Two CPUs?

[Jonathan Patschke]

Background: I've been listening to a lot of iTunes U lately.  Listening to
lectures keeps me fresh and excited about what I do.

In Lecture 14 (at 52:00) of the Fall 2009 CS 61C course at UC Berkeley,
Dr. Culler mentions that the implementation of the Motorola 68000's
hardware page fault handling mechanism was defective to such a degree that
a second "processor" was needed (running one instruction behind) in order
to stash the PC of the instruction that caused the page fault so that it
could be retrieved from the page fault handler.

Looking at photos of the early Sun CPU boards[0] shows only one 680x0 chip
(they're hard to miss).  Also, for what they cost, even in quantity, using
two of them in such a manner would be a bit silly (and difficult-to-
impossible, when you take into consideration interactions with external
state), so I can only assume that this "processor" was a bit of custom
hardware that noticed when the processor was in the instruction prefetch
state and stashed the address somewhere for retrieval later if the
processor faulted.

I can't find anything written about such a mechanism, though.  Has anyone
here heard of this?

I offer my apologies to Dr. Culler in case he happens upon this message
somehow.  One of the unfortunate side-effects of wonderful things like
iTunes U and OpenCourseWare is that the whole world is available to
critique your every word.


[0] http://www.solivant.com/sun100/
-- 
Jonathan Patschke
Elgin, TX
USA