>Hmm i don't know simply not having traces for the second memory controller would effectively do it
>wouldn't it. i don't see where it would be any different than simply leaving dimm slots for one of the
>channels unused. That in effect disables one of the controllers.
It doesn't work if the channel wired to the mobo happens to be the defective one. In addition, if a channel is defective, maybe the defect will prevent it from automatically disabling itself.
>seems more economical than taping out another chip sans i memory controller and fabbing it separately.
I don't know the costs involved. Maybe it is different with chips - I doubt it- but in mass producition, generally developement costs are dwarfed by production costs. They will spend whatever it takes to reduce production costs, because it always pays. They have guys looking how to save 1/4 cent by using a cheaper screw in an automobile that costs $20,000. If he finds one, his salary is in effect paid for.
Chips are always undergoing revisions. People often use the jargon for one kind: stepping. Therefore I don't think small revisions, updates, and fixes to chips are so cost prohibitive as people make it out to be. They just don't do it if it doesn't pay.
With foresight they could put one memory controller in a rectangular section that is omitted in the second chip design, making a complete revision unnecessary. If you look at the layout of chips, they generally do confine certain functions to certain areas. That way only one section needs to be revised, substituted or removed.
Balanced against revision costs and separate production:
Yes, saving defective chips from the junk pile is cost effective. But for objects that represent MORE sales than complete version, such as a cheaper version, it seems unlikely to me that you will be saving anything. Probably 90% or more of the chips will not be defective, so almost all of the chips that have a disabled section will in reality not be defective. You will be creating more problems, because defects go up in direct proportion to chips size. And you will be spending more on each chip because the number of chips per wafer will be reduced.
Defects go up in direct proportion to chips size, so anything you can do to reduce chip siize reduces cost per chip. You don't just save by salvaging the inevitable, but small number of, defective versions, you save on every single chip.
Things other than CPUs and GPUs do not use bleeding edge and scarce processes. CHipsets for instance. They use routine processes that are readily available in large quantity. Separate production therefore is not necessarily cost prohibitive. The foundaries of chip companies are always making a variety of chips and looking for more contracts, because making chips, not designing, is their business.