[geeks] Sun-1 with Two CPUs?

[nate at portents.com]

On Thu, 24 Feb 2011 17:07:31 -0600 (CST), Jonathan Patschke
<jp at celestrion.net> wrote:
> 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?

The second processor I'm guessing was the Sun-1 MMU:

"By using the Motorola 68000 processor tightly coupled with the Sun-1 MMU
the Sun 68000 CPU board was able to support a multi-tasking operating
system such as UNIX. It included an advanced Sun designed multi-process
two-level memory management unit with facilities for memory protection,
code sharing and demand paging of memory." [0]

Note that not many Sun-1 were ever made, and partway through they switched
to the 68010:

"According to some sources, fewer than 200 Sun-1's were ever produced;
they are certainly rare. The switch from Motorola 68000's to 68010's
occurred during the Sun-1's reign." [1]

Also note that even though the Motorola 68010 fixed the MMU bug in the
68000 (among other design issues) and Motorola introduced their own MMU to
be paired with the 68010, Sun still continued to use their own MMU:

"The 68010 could be used with the 68451 MMU. However, problems with the
design, such as a 1 clock memory access penalty, made this configuration
unpopular. Some vendors such as Sun Microsystems used their own MMU
design." [2]

- Nate

[0] http://en.wikipedia.org/wiki/Sun1
[1] http://www.obsolyte.com/sunFAQ/faq_hardware/hwref1.html
[2] http://en.wikipedia.org/wiki/Motorola_68010