[TUHS] Unix use of VAX protection modes

[Jonathan Gray via TUHS]

On Wed, Jan 21, 2026 at 07:47:40AM -0800, Larry McVoy via TUHS wrote:
> 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.

Clem mentioned a paper from Forest in
https://www.tuhs.org/pipermail/tuhs/2020-January/020130.html

Forest Baskett
Pascal and Virtual Memory in a Z8000 or MC68000 based Design Station
COMPCON 80, Digest of Papers, pp 456-459,
IEEE Computer Society, Feb. 25, 1980

Also referenced in
https://bitsavers.org/pdf/stanford/sun/The_SUN_Workstation_Draft_Mar80.pdf