Mystery re: MCM66128L20

Chuck Guzis cclist at
Tue Apr 10 19:12:58 CDT 2007

On 10 Apr 2007 at 22:57, Tony Duell wrote:

> Have you tried just a new chip in place of a piggybacked module? I wonder 
> if the new chips are actually 128K bits -- that is, they're really the 2 
> old chips of the pigyback on one die, in one package. If you piggybacked 
> a new chip onto half an old pair, you'd end up with 2 chips 'in parallel' 
> for half of the memory space, that might not matter (if you never read an 
> un-wirtten-to location, there will be no contention becasue the same 
> value will have been written to both chips)

This reminds me of the early revs of the PC AT, which used (IIRC) 
stacked 64K DRAMs (the bottom used a different select than the top 
one, so that they could be just soldered pin-to-pin) to get a 128K 


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