Re: NetBSD in SIMH vax with 512 MB RAM

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

>>> [...] - I noticed that when SIMH is configured with 512MB of
>>> memory, the calculated size of the system page table ends up being
>>> a bit larger than the 2^21 entries needed to map all of system
>>> space.  [...SLR gets a value >0x200000...]
>> I [] think that's a theoretical bug; EL-00032-00-decStd32_Jan90 says
>> [...SLR values outside 0..200000 are reserved...]
> Let's first of all agree that a page table size larger than 2^21
> entries is pointless.

Well, worse than pointless; it could completely break operation.

But...

> The MMU will pick which page table to use based on the top two bits
> of the virtual address.  [...]

If the P0LR, P1LR, and SLR values are within range, yes.  If not, it
might not[%]; as I read the VARM, an implementation could, for example,
treat the system region (80000000-bfffffff) and reserved region
(c0000000-ffffffff) as a single range, with the SLR specifying how much
of it is accessible.  That's well within the latitude "UNDEFINED
operation" permits, as I read it.

Not that we should depend on such a thing, of course, possibly
excepting very-model-dependent code.

[%] I see similar text specifying that P0LR and P1LR have similar
    restrictions to the 0..200000 range, with values outside that
    producing UNDEFINED operation, except for an odd note that "Writing
    P1LR<31> has no effect.  The bit always reads as 0.".

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