From: trimble@alleg.edu (Erik Trimble)
Subject: Re: (Q) Daystar PowerCache cards
Organization: Allegheny College
Date: Fri, 10 Apr 1992 00:50:45 GMT

In article <D88-JWA.92Apr9211256@hemul.nada.kth.se> d88-jwa@hemul.nada.kth.se  
(Jon W{tte) writes:
> > joungwoo@mensa.usc.edu (John Kim) writes:
> 
>  "share" the load with the original? If it indeed put the original CPU out of
>  job anyway, then wouldn't it be cheaper to by Mac IIsi or IIci and install
>  the Daystar 50 MHz PowerCache rather than buying a Quadra (unless you really
>    need the extra features of a Quadra)?
> 
>A 50 MHz 68030 is slower than a 25 MHz 68040.

Actually, a 25Mhz 68040 really blows away a 50Mhz 68030 on floating point  
calculations, but otherwise, the speed is roughly comparable (within 10%)
The result of this disparity is that theoretically, the machines should have  
roughly comparable speeds. HOWEVER....

> 
>Not only that, but the REAL bottleneck in most computers these days
>is the memory. A 68040 executes most instructions in one or two
>cycles; since it runs at 25 MHz memory should really be 40 ns to keep
>up... (well, you could actually design memory so that it's one cache
>line wide, and get by with 80 ns DRAM - why didn't Apple do that in
>the 900 ?) that's what caches are for.
> 
>So even if the processor on the PowerCache is fast; it'll run at
>wait states, since the memory is still the same speed.

True, the on-chip caches of an 040 really speed things up, but the PowerCache  
has a SRAM cache on the board; the 030 DOES NOT run at wait states. (that is,
it runs at 0 wait states, like the 040)  

> 
> >Last Question: Does the 68882 FPU installable on these cache card have to
> >operate at the same frequency? For example, can a 20 MHz 68882 (a slower
> >FPU) used with a 50 MHz PowerCache?
> 
>Not advicable. In theory, possible, but I don't think the PowerCaches
>are built for it. And you'd get that much worde FP performance anyway...

Daystar did a nice job here with the PowerCache. You do NOT have to have the  
same speed FPU as the processor. If the 'Cache does not detect a FPU present,  
it sends all FPU calls to the FPU on the IIci's motherboard, which then does  
the math, and send the results back to the 'Cache. Adding a 20Mhz 68882 would  
be kinda senseless, since the IIci's FPU runs at 25Mhz. SO...  Either you  
spring for the cash (<- pun! :) and get a 50Mhz FPU, or just use the 25Mhz one  
on the motherboard. 

The main argument for upgrading to a Q700 instead of a PowerCache is that you  
get a whole new computer. The Quadra700 comes with ethernet, great on-board  
video, and most importantly, WAS DESIGNED TO ELIMINATE POTENTIAL BOTTLENECKS  
THAT SLOW PERFORMANCE. By upgrading to the PowerCache, you still are stuck with  
some of the prime limitations of the IIci which merely adding a faster  
processor cannot solve:  a slow video sub-system, slower SCSI speeds, and a  
system architecture that is not optimized to take advantage of the speedy  
PowerCache.   The PowerCache is good, however, if you don't need ethernet, have  
a video card already, and don't want to spring for the extra $500 to $1000 more  
that Apple wants for the Quadra upgrade.

-Erik
trimble@alleg.edu