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Re: Rep:Re: [f-cpu] virtually or physically-addressed cache ?




----- Original Message -----
From: Michael Riepe <michael@stud.uni-hannover.de>
To: <f-cpu@seul.org>
Sent: Saturday, March 02, 2002 12:49 PM
Subject: Re: Rep:Re: [f-cpu] virtually or physically-addressed cache ?


> On Sat, Mar 02, 2002 at 02:07:32AM +0100, Yann Guidon wrote:
> > hi,
> >
> > Nicolas Boulay wrote:
> > > > - synonym problem (several different virtual addresses cannot span the same
> > > > physical addresses without being dupplicated in cache).
> > > Abgelehnt. This one causes severe problems.
> > > >>> Not really, it causes waste space, only !
> > i do not agree with "only".
>
> Neither do I (unless we're talking about the I-cache only).
>

Why might there be severe problems ? well, caches both contains a tag and a data associated to a virtual address. If there are two
different virtual addresses which span the same physical address and are present in the cache, it means that two entries in the
cache might also have their data to be different and so coherency would be broken : which data to keep and update into the external
memory ?

> > > > (2) physically-addressed caches (physical tags)
> > > > - do virtual-to-physical address translation on every access
> > > Not necessarily. The TLB lookup can be started as soon as loadaddr (or
> > > one of its variants) is called, and doesn't have to be repeated in all
> > > cases (e.g. with a postincremented pointer, you'll perform a range check
> > > first and only do a full lookup if the range check fails).
> > >
> > > >>> Consider that the LSU is a kind of virtually addressed caches.
> > that's one perspective.
>
> I'm not sure if I fully understand what exactly the LSU is supposed to do.
> In my mental picture, its job was to keep the data cache filled, not
> cache things itself.

I thought LSU is "Load Store Unit" and was like a functional unit to handle LOAD and STORE ?



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