> der Mouse wrote
> 
	>>> That doesn't sound right.  The VARM [...] 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, [...]
	>> IO space starts at 512 megs, as ragge has posted.
	>
	>But that's not a VAX architecture limit, it's an implementation
	>decision, right?  Not "because of the page format", but rather
because
	>the designers of the implementation in question decided to give up
RAM
	>space for something else, probably simpler decoding logic in the
	>physical memory interpretation circuits....

	No, I think the "I/O space starts at 512MB" was a hard and fast part
	of the archtiecture from day one. Many days later (in the late 80s)
	the architecture was modified to permit an alternative
implementation
	that allowed a 4GB address space: 3.5GB for RAM and still 0.5GB
	for address space.
	 
	Antonio