Re: Does the KA630 have a TB?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> Yes, it has a translation buffer.  I don't know offhand the size of
> it.  But you can see about the TBIA and TBIS in 
> https://bitsavers.org/pdf/dec/vax/630/EK-KA630-UG-001_Feb86.pdf,
> chapter 4.8

That's more or less the same as what EL-00032-00-decStd32_Jan90 says
about TBIA/TBIS.

> But I seriously doubt the translation buffer in the uVAX II covers
> anything close to 8M.

I would doubt it too.  It was a first cut (and I may keep it, at least
optionally, for other reasons anyway).

> But I also fail to see that it would matter exactly how big it is.

_Exactly_ how big, that probably doesn't matter.  I suspect its size
matters because that NetBSD version actually does run on the KA630,
without the TBIA/TBIS usage, whereas when provided with an
unlimited-size TB (8M entries is enough to cache entries for an entire
4G if VM) it fails - and I've added enough debugging code to be
confident that it's failing because the TB holds stale entries.  The
only explanations I've come up with are (1) that the KA630 doesn't have
a TB (which Antonio Carlini refuted) and (2) that it is small enough,
and/or its replacement policy is such, that the stale entries get
flushed from the real-hardware version before they can cause trouble.

> The population of the TB is done completely hidden from the OS.

Its population, yes.  But, if the OS doesn't flush it correctly - which
that version of NetBSD doesn't - then its _de_population may not be.

> And I have a hard time thinking that NetBSD would depend on any
> specific size of the TB.

I don't think it does, not on any specific size.  I think it, that
version, just depends on its size and replacement policy being such
that that stale entries get replaced before they cause trouble.

> I would rather suspect that there is something wrong in your
> implementation.

Well, yes, since my emulation doesn't behave like the real thing, it
obviously is incorrect in some respect(s).  I was trying to find out
what the real thing is like in enough detail to bring my emulation in
line with it.

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