Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!julius.cs.uiuc.edu!psuvax1!rutgers!ub.d.umn.edu!cs.umn.edu!ux.acs!vx.acs.umn.edu!dhoyt From: dhoyt@vx.acs.umn.edu (DAVID HOYT) Newsgroups: comp.sys.mac.hardware Subject: Memory speeds can be critical (was: SIMMs for IIsi - what do I need?) Message-ID: <2915@ux.acs.umn.edu> Date: 17 Dec 90 16:15:07 GMT References: <110992@convex.convex.com> Sender: news@ux.acs.umn.edu Reply-To: dhoyt@vx.acs.umn.edu Organization: University of Minnesota, Academic Computing Services Lines: 37 Although most macs can get by with a mix of memory simms at different speeds it can cause problems if you mix the chips within the same bank. Imagine you have a chip that is rated at 120ns with a 120ns driver. (Greatly simplifying) you will have a signal that looks like this +---+ +---+ | | | | ----+ +-----------------------+ +---------------------- 0 ns Time-> 120ns I.e. the amount of time that the signal takes to get through the chip is roughly 120ns. Now with a chip rated at 60ns running at 60ns the signal will take 60ns to pass through the chip. All well and good. But now take the 60ns chip and run it at 120ns. The chip is still designed so that the signal will take 60ns to clear the chip. You'll end up with at situation like +---+ +---+ | | | | ----+ +--------------+ +---------------------- 0 ns Time-> 120ns Now most memory speeds are not exact. A chip nominally rated at 120ns may actually be usable at 110ns. Because of this, when the engineers design a system they allow for some amount of skew. For this reason, you can often get by with mixing and matching chips. But you can also get bit if the skew is too much. Which is why apple suggests using the same speed chips. On the macs with two lines (banks) of memory there are two drivers, one for each bank. With different drivers, the skew between banks is much less important than skew within banks. This is why the rule of thumb is 'use the same speed within banks' rather than 'use the same speed for all chips.' Now this is all a gross generalization and the real world is much more complex, but hopefully this will give some idea why it's a bad idea to mix-and-match willy-nilly. david | dhoyt@vx.acs.umn.edu | dhoyt@umnacvx.bitnet