Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!bagate!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.misc
Subject: Re: Why is my 3000 slow? (Or is it?)
Message-ID: <19987@cbmvax.commodore.com>
Date: 20 Mar 91 04:51:51 GMT
References: <16647@chopin.udel.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Distribution: na
Organization: Commodore, West Chester, PA
Lines: 44

In article <16647@chopin.udel.edu> don@chopin.udel.edu (Donald R Lloyd) writes:

>	I've run seceral benchmark programs which tell me that the speed of
>the CHIP ram in my A3000/25/50 is running at only 85% or so of what it
>should be.  

How do you know what it should be?

>I popped open the case to take a look at the chips in there, and found that 
>the ones soldered on were 100ns chips!  (The ones I moved over from the fast 
>RAM section were 80ns, though).  Seems to me that 100ns CHIP RAM on a 3000 
>would cause a bit of a bottleneck... 

I guess you don't understand how computer memory works.

The memory rating of any chip is a _potential_.  It has nothing to do with
how fast a memory system work, only whether or not it will work.  The memory
controller, in this case the Agnus chip, is responsible for driving the DRAM
chip at an appropriate speed.   

An analogy would be found in your stereo system.  You have speakers rated
for a certain wattage.  This is your DRAM.  And you have an amplifier.  This
is your DRAM controller.  If you crank more watts out of the amp than the
speakers are rated for, they may blow.  If your DRAM controller tries to run
DRAM faster than its rated for, you may get memory failures.  On the other
hand, you may have a 50W speaker and only a 40W amp -- that works fine.
Similarly, you may have a 100ns DRAM in a system that only need 120ns parts;
that generally works fine too, and so would an 80ns part, but the memory
doesn't go any faster than a 120ns part in that system.

In the case of your Amiga, all Agnus chips run their memories at the same
speed, which is a 280ns cycle time.  Typically, a DRAM's minimum cycle time is
a little less than twice its access time (the number stamped on it).  So an
Agnus should work fine with a 100ns part, which can cycle in 200ns or less,
though it should also work OK with a 120ns part, which cycles in 240ns or
less.  That's a guideline, not a hard rule -- other features of a memory 
system may cause it to require faster parts than you'd expect.  But you can
rarely succeed using slower parts.  You would know if your chip memory wasn't
fast enough, since the computer would not work in such a case.  Microprocessors
may tolerate wait states, Agnus does not.
-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	"What works for me might work for you"	-Jimmy Buffett