Re: Larger memory option for VAXstations

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

>> The document I have makes it sound as though there are no "systems
>> which support both flavours" - [...]
> Well, since the 25-bit PTE changes the interpretation of the PTE,

Changes it?  Only in that there are some high bits which in the 25-bit
form are high bits of the PFN but in the 21-bit form are MBZ (well,
"reserved to DIGITAL", which amounts to much the same thing).  If
you're on hardware where everything you want to map is within 1G
(physical) of zero, it amounts to the same thing anyway. :-)

> it's something both the hardware and software have to agree on.

Of course.

> And yes, there are systems which supports both flavors.  Like I said,
> I have a VAX 7000-720 here, which is running with 1.25 Gig of
> memory...

That doesn't mean it supports both flavours.  It just means it supports
the 25-bit kind.

Indeed, I'm not sure what it would _mean_ for 25-bit-PFN-capable
hardware to "support" 21-bit PFNs, since the other four bits are
reserved anyway, meaning the interpretation of a PTE with those bits
zero is the same whether it's viewed as a 21-bit-PFN PTE with four MBZ
bits or a 25-bit-PFN PTE with those four bits happening to be zero.

...Or Am I Missing Something (tm)?

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