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From: mash@mips.COM (John Mashey)
Newsgroups: comp.arch
Subject: Re: RISC as a "technology window"?
Message-ID: <15964@winchester.mips.COM>
Date: 26 Mar 89 00:14:16 GMT
References: <1552@vicom.COM> <15690@cup.portal.com> <1562@vicom.COM> <15702@clover.ICO.ISC.COM> <27681@apple.Apple.COM> <15695@winchester.mips.COM> <22974@ames.arc.nasa.gov> <51@microsoft.UUCP> <11402@pasteur.Berkeley.EDU>
Reply-To: mash@mips.COM (John Mashey)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 32

In article <11402@pasteur.Berkeley.EDU> carlton@betelgeuse (Mike Carlton) writes:

>I agree that the MIPS scheme is nice, but it does have its drawbacks.  In
>particular, they've fixed the cache control.  If they got it right (for your
>application) then no problem.  Otherwise you're out of luck.  It would be
>possible to make some of the details configurable, but I believe that MIPS 
>doesn't allow this.
Actually, R3000s allow a fair amount of flexibility.
1) I & D-caches can have different sizes.
2) The number of words refilled into the cache upon cache miss is settable
from 1 to 32 words.
3) You can use instruction-streaming, or not.
4) You can cause partial-word writes to invalidate the corresponding cache
word, or cause it to do a read-modify-write.

>If I remember right (somebody borrowed my MIPS book so I can't verify), the 
>MIPS cache control requires a write-through cache.  Personally, I don't want 
>a write-through cache.  Another aspect is the write latency (i.e. how many 
>cycles does your cache take to handle a write-hit?).  I think the MIPS 
>controller assumes a single cycle.  This implies you've got to build a cache
>to handle this, and this will get trickier when you can buy a 40 or 50MHz MIPS.

The first-level cache is a write-thru cache.  People often build 2nd-level
caches to be write-back for multi-processors.  It does expect single-cycle
caches;  it will get trickier.  Of course, the higher clock rates, sooner or
later, require everybody doing CMOS/BiCMOS micros to build integrated
"superchips" anyway if they want to be competitive.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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