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Re: How to configure the cache size in r4000

To: Ralf Baechle <>
Subject: Re: How to configure the cache size in r4000
From: "Maciej W. Rozycki" <>
Date: Tue, 13 Jan 2004 17:48:17 +0100 (CET)
Cc: karthikeyan natarajan <>,
In-reply-to: <>
Organization: Technical University of Gdansk
Original-recipient: rfc822;
References: <> <> <>
On Tue, 13 Jan 2004, Ralf Baechle wrote:

> >  You cannot modify the size of the primary caches -- the values are
> > hardwired to the amount of cache available in the processor (8kB+8kB for
> > the original R4000).  However, if you take appropriate precautions, you
> > can alter the line sizes of the caches by modifying appropriate bits of
> > cp0.config.
> On some systems that's a dangerous and won't work due to some issue with
> the memory controller.  That's why Linux supports all possible combinations
> instead of reconfiguring caches.  Of course there's also the hope that
> developers of a system did configure the cache for the optimal performance.

 Plus there are processor errata related to certain values of line sizes.

> If reconfiguring is possible 32-byte D-cache and I-Cache lines are probably
> the optimum for non-tiny systems.  For the L2 cache I'd guess 64 or 128
> byte lines.

 Well, reconfiguring the line size of the L2 cache is system-specific and 
the size is most likely hardwired.

 BTW, the DECstation uses 16-byte lines for the D-cache and the I-Cache
and 32-byte lines for the S-cache.  With the S-cache size at 1MB and up to
480MB of RAM does it qualify as a tiny system? ;-)


+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

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