Re: Larger memory option for VAXstations

[Miod Vallat <miod%online.fr@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. :-)

This is not as trivial. In the 21-bit flavour, bit 21 to 24 are free to
use by the software, but bit 25 is reserved and must be zero. In the
25-bit flavour, bit 25 is free to use by the software.

So software fiddling with bit 25 on a processor supporting only 21-bit
flavour would hit undefined behaviour, and software fiddling with bits
21 to 24, expecting a 21-bit flavour, would misbehave when running with
the 25-bit flavour.

This is why, unless your software doesn't use any of the software bits,
it is necessary to know which flavour is in use before building your
page tables.