Path: utzoo!attcan!uunet!husc6!bloom-beacon!gatech!ncar!unmvax!pprg.unm.edu!hc!ames!oliveb!pyramid!ctnews!mitisft!dold
From: dold@mitisft.Convergent.COM (Clarence Dold)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Brown Bag Memory Test--worthless?
Message-ID: <512@mitisft.Convergent.COM>
Date: 22 Nov 88 01:32:10 GMT
References: <301@bilver.UUCP>
Organization: Convergent Technologies, San Jose, CA
Lines: 36

in article <301@bilver.UUCP>, bill@bilver.UUCP (Bill Vermillion) says:
> appeared that a low bit was stuck. I found ONE 64k chip where a 256 should be.
> System memory test and the board mfrs memory test both passed
> 
> Talking with a local repair tech he mentioned the memory test supplied for his
> machines by the machine mfr would pass memory when one of the memory chips
> legs was bent out from the socket.
> 
A memory test that allows a 64k chip to pass, in lieu of a 256k chip, is 
missing the same test that the 'bent leg' diagnostic is missing. 
The 64k chip looks like a 256k with one pin missing.
If a chip has a floating address bit, it will respond to both the proper
address, and an address at the inverse of the floating bit.  
The important point is that it succeeds in read/write at both addresses.  It 
doesn't fail either of them.  Some other chip probably is also reading and 
writing the same data, but it works.
A proper memory test will write a unique pattern to each location in RAM,
and then read those locations.  If two locations have the same data, then 
somebody isn't addressing properly!  Of course if you have >64k you can't
uniquely test each word, so patterns of floating bit possibilities are 
tested.

On a separate thought:
A Memory diagnostic that checks refresh will write a pattern to all of 
memory, and then wait some arbitrary length of time (14 seconds), before
reading.  It then waits another 14 seconds, and reads again.
If refresh isn't working, or if a read causes a bit toggle due to bad
parity circuits, this will catch it.

A memory test that only takes 5 seconds to run isn't testing a whole lot.

-- 
---
Clarence A Dold - cdold@starfish.Convergent.COM		(408) 435-5274
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