MCM66128L20

Of course. The thing is, a normal 64K DRAM chip -- a 4164, for example -- has 15 connections used. That's 8 address lines (multiplexed address, of course), data in, dara out, RAS/, CAS/, WE/, +5v, ground. That chip comes in a 16 pin DIL pacakge with one pin not used. It is my belief that in thses piggybacked modules, each package contains a 64K DRAM die, but the pinout of the 2 DIL packages in the piggyback is slightly differnt. One pin is not-used on one device and RAS/ (I think) on the other. And they're the opposite way round on the other package. So when they're piggybacked, you end up with effectively 2 64K chips, with common connections for everything but RAS/, which is separate for the 2 chips. That's hwo you can select one chip and not the other. I've seen an module with 2 surface-mount ceramic pacakges on top. It was used in one version of the PERQ memory board. Again, much the same idea as regards selectign the 2 devices. I've never seen half-good 256K DRAMs :-). There were some 32K DRAMs which were hald-good 64K chips, though. -tony