Sparse addressing info wanted

[Dave Berry]

[follow-ups will go to net.arch]

Some implementations of untyped trendy languages (LISP, PROLOG etc.) use a 
form of soft-tagging (aka bounds checking) whereby the top byte of the address
of an object depends on the type of that object.  This makes run-time type 
checking relatively painless.
As I understand things, some virtual memory manglement implementations require 
all memory allocated to a process to be contiguous.  Implementations which don't
force this are said to allow "sparse addressing", and are more efficient for
soft-tagging language subsystems since they don't have to allocate large numbers
of redundant page table entries (and swap space?).
I'd like to know
  a) if I'm talking bullshit, in which case I apologise for wasting your time
  b) which hardware allows sparse addressing (e.g. I think Pyramids do, Vaxes
	don't; I've no idea about Suns, IBMs etc).
  c) if software can get in the way - e.g. UNIX malloc seems to be a hindrance.
	If this is so, are there any OS which do a better job?

This info is for a report a friend is having to write for her boss.
As ever, most replies should be by mail.
-- 
	Dave Berry. CS postgrad, Univ. of Edinburgh		
					...mcvax!ukc!{hwcs,kcl-cs}!cstvax!db