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Quadra 800 RAM Quirk

Some early adopters of Apple's most powerful Macintosh, the Quadra 800, have run into a bit of a problem with RAM. The 800 can address up 136 MB of RAM if you pop four 32 MB SIMMs in alongside the 8 MB soldered on, although 16 MB SIMMs are a bit more common. The Quadra 800 is optimized for speed, and thus cares a great deal about what type of SIMMs it uses, but it's unclear quite where the problems lie. First, some background.

Since it's still prohibitively expensive to produce the 16 megabit DRAM chips needed for non-composite 16 MB SIMMs, most 16 MB SIMMs use 32 of the more common 4 megabit chips along with additional circuitry to fool the Mac appropriately. This method of using multiple smaller DRAM chips to create a large SIMM results in composite SIMMs, in contrast to non-composite SIMMs that use the same size DRAM chips (4 megabit DRAM chips making up a 4 MB SIMM gives you a non-composite SIMM; 4 megabit DRAM chips making up a 16 MB SIMM gives you a composite SIMM). Interestingly, the new 72-pin 8 MB SIMMs are non-composite - the 72-pin SIMMs have two banks, so the 8 MB SIMM is essentially two non-composite 4 MB SIMMs. So what's the problem? We're getting there.

The Quadra 800 requires 60 nanosecond RAM, and the composite SIMMs are indeed clocked at 60 nanoseconds, but remember that additional circuitry I mentioned? It appears to add a slight amount of overhead, which has the effect of slowing the SIMMs slightly, which is not good, since that means in essence that the Quadra 800 expects information from RAM at a certain speed, but receives it slower. That's when all hell breaks loose.

In addition, and sources at RAM vendor Technology Works wouldn't discuss this further other than to say they were working on it with Apple, there is a power problem. I don't understand this fully, not being a hardware guru, but I gather the Quadra 800 isn't designed to power 32 chips per SIMM, so there's some sort of power problem that occurs when you put two or more 16 MB composite SIMMs in the Quadra 800. Other vendors claim to be working on a solution as well, so I don't expect this to last for too long, but beware if you're buying these 16 MB SIMMs right now.

Errors range from an immediate sad Mac on startup to weird system enabler errors during startup to bus errors when launching or using applications. Sources have said that the errors vary with different SIMMs, different arrangements of SIMMs, and different SIMM vendors. Needless to say, this sort of problem makes the Quadra 800 owner unhappy. Sometimes the errors even go away entirely for a session, only to return after a cold boot.

Dale Adams, one of the designers of the Quadra 800 at Apple, has said that Apple has never claimed that the Quadra 800, or any Mac, would work with composite SIMMs. David Limp, Apple's Quadra Product Manager said that Apple has a tech note about not using composite SIMMs on Quadras, although I was unable to find it on the March Developer's CD or ftp.apple.com. David did say that Apple would reissue that note again soon, presumably with updated information. The matter is exacerbated by the fact that most, if not all, memory vendors sell only composite 16 MB SIMMs; non-composite 16 MB SIMMs with 16 megabit chips are uncommon, and when you can find them, expensive. Try $1,200 for a true 16 MB SIMM, versus $680 for a composite 16 MB SIMM. A true 32 MB SIMM is proportionally cheaper, at $2,000 per SIMM, not that it's much comfort. I've heard of even higher prices, and since few vendors have the 32 MB SIMMs yet, non-composite SIMMs are not financially feasible at this time.

So buying new SIMMs isn't really a reasonable financial solution. The 4 MB SIMMs all work fine, but if you need to use 16 MB SIMMs, what are you to do? We've heard of a strange solution that is by no means guaranteed to work (but tell us if it does). There's this desk accessory from 1987 called RAM-Zero, and when you run it, it clears memory and restarts the Mac. We have no idea why it was originally developed, but it appears to be freely distributable, and one person said that after he ran RAM-Zero, his Quadra 800 with 16 MB composite SIMMs worked fine until the next cold boot. Who am I to argue with success? RAM-Zero is available at <sumex-aim.stanford.edu> for anonymous FTP as:


Despite RAM-Zero, I can't currently recommend that you buy a Quadra 800 with the intention of immediately filling it with RAM. The price will come down on the true 16 MB SIMMs eventually; it always does, and as David Limp said, "Composite SIMMs (of any density) are not supported on ANY Macintosh computer." It's not a popular statement, and it's one you might want to keep in mind if memory vendors talk about their composite SIMMs being approved by Apple. There's no telling what's what in this case.

We haven't had any reports of problems with the Centris 650, which uses the same interleaved memory scheme as the Quadra 800 (the 610 doesn't do interleaved memory, according to Apple's Developer's Notes). Nor have we heard of the problem cropping up on the Quadra 700, 900, and 950, despite David Limp's warning. However, the Centris 650 can take 80 nanosecond SIMMs, so that might reduce any timing problems, and the earlier Quadras don't use the 72-pin SIMMs, which might make a difference. There's just something about the Quadra 800 and 16 MB composite SIMMs, and I suspect the same thing will apply to the Apple Workgroup Server 80, which is essentially the same machine.

As an aside, David Limp also mentioned that many of the composite SIMMs are made in form factors that are too tall to fit properly into the Quadras. This affects all Quadras, and judging from several reports from users, installing these tall SIMMs can be a major pain, if possible at all. Check that before buying.

-- Information from:
Dale Adams -- adams9@applelink.apple.com
David Limp -- limp@applelink.apple.com


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