Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!agate!sprite.Berkeley.EDU!chiueh
From: chiueh@sprite.Berkeley.EDU (Tzi-cker Chiueh)
Newsgroups: comp.unix.cray
Subject: Re: Protection in Cray
Message-ID: <1990Dec19.181343.10365@agate.berkeley.edu>
Date: 19 Dec 90 18:13:43 GMT
References: <1990Dec18.214228.25858@agate.berkeley.edu> <1249@sunc.osc.edu>
Sender: usenet@agate.berkeley.edu (USENET Administrator)
Reply-To: chiueh@sprite.Berkeley.EDU (Tzi-cker Chiueh)
Organization: U.C. Berkeley Sprite Project
Lines: 13

The kind of protection I have in mind is access right control (e.g., read-only)
"Normal virtual memory systems" perform this kind of protection check while 
doing logical-physical address mapping. The protection bits are either in page 
tables or TLB.  Now, since Cray doesn't have virtual memory, the question is 
does it provide access control, if so, where does it put this check ?
From the previous responses, it seemed that Cray only provides out-of-bound
protection check. Furthermore, this check is done for EVERY reference. 
If this is indeed the case, this protection check process should be as 
expensive as address mapping in machines that have VM. 
So why does Cray get rid of virtual memory altogether ?  Or does anybody 
know how much performance improvement can we gain from getting rid of VM ?

tzi-cker