[TUHS] Unix use of VAX protection modes

[Larry McVoy via TUHS]

On Wed, Jan 21, 2026 at 10:38:27AM -0500, Rich Salz wrote:
> On Wed, Jan 21, 2026 at 9:51???AM Larry McVoy via TUHS <tuhs at tuhs.org> wrote:
> 
> > I think I sort of know what they did. They ran 2 68k cpus in lock step,
> > with one some number of instructions behind.  When the leading CPU
> > faulted, it stopped the other one before it ran the instruction that
> > faulted, the stopped one somehow was redirected to handle the fault.
> 
> 
>  Apollo did something similar.  If the first CPU faulted, the second would
> halt it, page in the memory and restart the first one.

Yeah, I think it was a pretty standard trick, I just can't remember who came
up with it first.  My aging memory thinks it was Forest but I may have made
that up.
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Larry McVoy           Retired to fishing          http://www.mcvoy.com/lm/boat