> > actually, aside from IIRC the VAX {7,10}000, all VAXen have a 512M
> > physical limit.  it's because of the page format, they cant support
> > more than that without some hacks.
> 
> That doesn't sound right.  The VARM, at least the version I've got (rev
> 6.1, 20 May 1982) specifies 21-bit physical page numbers, which makes
> for a 1G physical limit.  Now, of course, I/O space has to come out of
> that, but that isn't more than several megabytes, at least not on the
> buses I'm familiar with - are there really machines that demand half a
> gig for I/O space?
> 
Yes, at least from the hardware PoV: The upper addressing bit tells that
the transaction is headed to I/O space (and therefore bypasses caches etc).

-- Ragge