Path: utzoo!utgpu!watserv1!watmath!iuvax!mailrus!ncar!boulder!rieman
From: rieman@boulder.Colorado.EDU (John Rieman)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Apple is dumping 120ns SIMMS into SE/30s
Keywords: cheap chips
Message-ID: <16475@boulder.Colorado.EDU>
Date: 2 Feb 90 00:23:28 GMT
References: <15@hite386.UUCP>
Sender: news@boulder.Colorado.EDU
Reply-To: rieman@boulder.Colorado.EDU (John Rieman)
Organization: University of Colorado, Boulder
Lines: 29

In article <15@hite386.UUCP> steve@hite386.UUCP (Steve Hite) writes:
>
>   About 6 weeks ago I purchased a new SE/30 1/40 from my university.  
> ...
>1 meg of 256Ks that came with the machine.  I was very surprised to find
>that they were 120ns chips...nah, I was upset!  My question is this...
>
>   Why does Apple put low speed memory chips in their high speed SE/30s? 
>   Are they too cheap to use 80ns?  
>
Too smart would be a better explanation.  Why should they put in
more expensive chips that give the same performance?  The SE/30, like 
every other machine I'm familiar with, accesses memory at a fixed speed.  
120ns or faster chips will support this speed -- but faster chips don't
give you any better response.

Think of it this way:  The processor puts a request out to memory.
It waits 120ns.  Then it gets the data it requested off the bus.
As long as memory can respond in 120ns OR LESS, everything is fine.
If memory responds in 80ns, the data just sits there for another
40ns, until the processor grabs it.

(The processor isn't actually just sitting dead for 120ns -- it's
preparing to grab the data.  And yes, the story is more complex
than this, I'm sure.  But you get the idea.)

-john
rieman@boulder.colorado
U. of Colo.