Re: NetBSD in SIMH vax with 512 MB RAM

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

On 2023-03-23 19:20, Mouse wrote:
> > Or else you could just say that as soon as anything is set to an
> > undefined behavior, the whole machine is just undefined everywhere,
> > and you can start interpreting instructions as 6502 opcodes.
>
> As far as the formal spec goes, I think that's pretty much what it
> says.  Note that there is text that specifically says that if the
> scalar processor's operation goes UNDEFINED, so does the vector
> processor's, if any - even if the source of the UNDEFINED scalar
> processor opration has nothing obvious to do with the vector processor.
> (This text is in chapter 1, page 1-2, where it talks about UNDEFINED
> and UNPREDICTABLE behaviour.)

Right. But as I said, I think that is carrying it a bit far.

> > I guess in a sense, that would be within the specs, but hardly a
> > useful interpretation.
>
> That particular example, yes.  But, for example, if it saves some
> gates, I could see a hardware designer simply merging all of
> 80000000-ffffffff into a single space, with SLR as its length register.
> Provided SLR is in range, this is operationally identical, and if SLR
> is out of range, then anything goes anyway.

That would be in violation of the VARM.
Because a virtual address of C000 0000 should unconditionally generate a 
fault, independent of what the SLR contains, according to the VARM.

> > > Yeah.  I wonder what's behind it.  [...]
> > You could also ask why P0LR<26:24> are ignored.  Same story.
>
> !!  I missed that.  Thank you for pointing it out.
>
> If anything, that one is even stranger.  The sign bit, that at least
> could make a tiny, if twisted, bit of sense.  But the 0x07000000 bits?
> That's just bizarre.
>
> The only other place I see bits 26:24 of anything mentioned is that
> they're the ASTLVL bits in the PSL.  But I have trouble imagining DEC
> caring about software that confuses PSL with P0LR, quite aside from all
> the other reasons I have trouble seeing that as relevant.
>
> > I have no clue about that, and find it incredibly strange.
>
> Agreed!  (And my VAX emulator has a couple of bugs for me to fix.)

:-D
If I ever come up with any answer about those bits, I'll let you know. 
But for now, they are just really strange. But somehow DEC thought it 
important enough to define them in the VARM.

  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