Re: New Vax - future directions :-)

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2021-07-07 17:22, Hans Rosenfeld wrote:
> On Wed, Jul 07, 2021 at 04:46:36PM +0200, Johnny Billquist wrote:
> > I think you are underestimating the limitations of the PDP-11 compatibility
> > mode of the VAX...
>
> I certainly am. :)
>
> > No split I/D space, no FP, any kind of trap exits out to VAX mode, anything
> > that depends how the memory management of the PDP-11 works will have
> > issues... Even some instructions you might play with are missing (but I
> > don't think most people would miss them, especially not since we don't have
> > split I/D space anyway).
> >
> > Very few 2.11BSD programs would work in this environment... The most serious
> > limitation is the lack of split I/D space.
>
> So I guess we have another few feature requests, then... :)

This might be extremely hard, since it's a feature that comes from the 
design of the PDP-11 MMU. The VAX MMU is very different, and don't allow 
this. (Allow might be the wrong word here - it's just not possible.)
(Anyone talked about execute-only protection of memory... The PDP-11 
have it. :-) )

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol