From: kanefsky@halcyon.com (Steve Kanefsky)
Subject: Re: My $.02 about RAM Caches...
Organization: The 23:00 News and Mail Service
Date: Tue, 17 Nov 1992 17:44:08 GMT

In article <1992Nov16.215823.207@physc1.byu.edu> seth@physc1.byu.edu writes:
>Hi. I wanna share a couple of things I have found about RAM caches. This is
>particularly in regard to the posting by Chuck Chung entitled something like
>"68040 33 MHz -> 40 MHz...", where he asked about cache cards. I don't have a
>quadra, so I cannot comment specifically on that issue, BUT, I have messed
>about a bit with caches. Here goes: The 68030 chip has two caches (like the
>68040, but they don't work quite the same way), which are a 256 BYTE cache and
>a 256 BYTE instruction cache. Now, this doesn't seem like a lot, but IT IS. I
>found a control somewhere (I forgot where, but I could probably find it again
>if someone wants it. it is probably to found at sumex-aim). Anyways, I found
>that can turn off the caches in the 68030. I did some tests. I turned these
>256 BYTE caches off, and ran Speedometer. I found out that these caches just
>about DOUBLE the speed of my IIsi (still at 20 MHz...). I also have a 64 K 
>external cache from Logica Research. Now, I understand that an internal cache
>is faster and all, but this 64K cache still only brings my machine up maybe
>25% over the performance I get with JUST the 2 internal 256 BYTE caches. Now,
>if you were to draw a graph of performance enhancement versus size of cache,
>you would probably see that the graph rises VERY fast at first, as you go from
>having no cache at all to having ANYTHING, but then it quickly flattens out.
>We see that the first 512 bytes of cache memory doubled the computer's speed,
>and the next 64000 bytes could only bring it up another 25%. You can see that
>the difference between a 32 K cache and a 64 K cache is not going to be that 
>much, and the difference between a 64K cache and a 128 K cache even less. Now,
>the 8 K worth of cache memory of the 68040 is internal cache, which is faster
>(at least from what I have read; I could be wrong) than external cache. With
>8 K cache, you have probably gotten a sizeable chunk of the possible increase
>over 512 bytes, so I would guess that spending the $400 for the DayStar cache
>may not bring much in the way of extra speed. I don't know this as a fact, but
>I would definitely check out with somebody who did spend the money and has one.
>If you do, by all means run Speedometer and post the results for us all to see.
>I too have seen the DayStar ads, and I have to laugh when I see the line
>"Up to 40% speed increase." That's what the Logica Research ad said too. Sure,
>8-bit graphics performance went up 40%, and THAT IS IT. Everything else was
>down around 25%. 

I think the speed gains from a RAM cache are dependent on the difference
between the speed of the CPU and RAM access time.  For example, if you had  
a 50mhz CPU, everything else being equal, the CPU could process a lot more    
data in the same amount of time, and could thus benefit from a larger cache.
I actually do have a 50mhz 68030 CPU in my LCII (DayStar PowerCache), and
turning the cache on approximately doubles my performance.  Running the
50mhz 68030 without the cache on is only a slight (about 50%) improvement over
the 16mhz '030.  If you'd like some Speedometer results, I can send them
(but I don't have the non-cache results handy right now).

How well the DayStar FastCache Quadra performs will depend on how much of     
a bottleneck the rest of the system is relative to the CPU.  I must admit  
I have seen a couple people on CompuServe complain that they didn't get as
big of a speed boost as they expected.  Luckily, you can buy one with a   
30-day money-back guarantee from most Mac mail-order places, so you can
try one out and see if it makes a difference.  I *can* assure you that       
DayStar doesn't want to sell *anything* to anyone that isn't going to be  
happy with it.  They have an excellent reputation for customer service.   

One thing that just occured to me:  I wonder if the FastCache will work
with programs that can't handle the '040's internal caches.  The caches
in the PowerCaches are certainly compatable with everything under            
the sun.  In this case, you could run at nearly full speed with programs 
where you'd normally have to disable the '040's internal caches and slow your
machine down to a crawl.  I'll check with Daystar on this and find out. 
   
   
-- 
Steve Kanefsky