From: sl@wimsey.bc.ca (Stuart Lynne)
Subject: Re: 486/66 .. Into a 486/50 motherboard?
Organization: Wimsey Information Services
Date: Fri, 18 Dec 1992 02:23:14 GMT

In article <92351.46019.J056600@LMSC5.IS.LMSC.LOCKHEED.COM> J056600@LMSC5.IS.LMSC.LOCKHEED.COM writes:
>In <1992Dec16.155510.11854@cs.odu.edu>:
>
>
>>Is it possible to replace the 50MHz 486DX chip on a 486/50 mommyboard with a
>>66 MHz DX/2??  I am thinking of replacing my CPU, but I don't want to replace
>>my entire motherboard if I don't have to.  Thanks.
>
>Sure, you probably could, but would it be worthwhile?  You'd have to change
>the clock crystal and the CPU.  In fact, the clock crystal would have to be
>*slowed down* from 50 MHz to 33 MHz.  The addition of the DX2 would make the
>CPU behave like a 66 MHz machine (instead of 50), but the rest of the machine
>would act like a 33 MHz machine (again instead of 50).  If you are using
>*very* CPU-intensive applications, this might help.  If you're doing things
>which requires mostly disk, video or memory, a DX2/66 might be almost a
>downgrade from a DX/50.  Most people I know who have seen both (I've never
>seen a DX2/66) say there's virtually no difference in performance.
>
>Another requirement for a DX2/66 is *lots* of external cache--without at
>least 256K, the DX/50 would probably be faster because the DX2/66 would
>encounter a lot of wait states without it.  Many people even advocate a 512K
>cache.  Again, I haven't seen it, so I'm relying on opinions of people who
>know more about the DX2/66 than I do.

With a properly designed board there will be no difference in bandwidth due
to changing between 33 & 50 Mhz. This assumes that the board is designed to 
accomodate both speeds and the BIOS knows how to setup the RAM access wait
states properly for the oscillator speed and RAM speed.

Once it does this it makes no difference whether you are running 33 or 50. You
are accessing the RAM as fast as you can. The *only* way that there would be
a difference would be if your RAM was fast enough to run without wait states
with a 50Mhz clock. 

Since 20 nanosecond SRAM still needs wait states to run with 33 Mhz clock I 
suspect it will be a while before DRAM gets fast enough to run with no 
wait states at 50 Mhz :-)

Does anyone out there know how fast memory would have to be to be used without
wait states by a 50Mhz 486?

The EISA board I'm using right now is by ASUS and has a jumper that tell's the
board whether you have a 50Mhz crystal or 33Mhz or slower crystal.

I can't see that there would be any difference is cache requirements between the
DX2-66 or DX-50. The more the merrier. And yes the faster the processor gets the
more important it is to increase the cache size. But it won't decrease performance
if you don't have more. Just won't be as fast as it could be. Some of the newer 
EISA boards are setup to allow 512KB of cache. But you need to get larger SRAM's
that are probably fairly pricey still (32kb SRAM's instead of 8kb SRAM's).

And yes I do see better performance from the DX2-66 compared to the DX-50.

-- 
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