This is a somewhat edited re-post of a message I originally sent to the Quadlist about two years ago. Information on models irrelevant to CCs has been left in for completeness.


There has been much debate and hair-pulling lately over the use of 64 MB SIMMs in various 68K Macs. Most of the RAM vendors claim it won't work. Apple doesn't officially support it but other than that, doesn't say one way or another if it works or not. To top it all off, I received anecdotal reports from a few people that their various 68K Macs are working just fine with 64 MB RAM chips.

Several 68K Macs from my collection were tested on 13 Aug 2001 with the following 64 MB, 72-pin SIMMs:

Here are the test results:

The 030-based Macs

The LC III does NOT work with 64 MB RAM chips of any description. It gave the chimes of death on occasion but not every time, and the times it didn't give the CoD it refused to boot (chimed and then no video).

Because of the above, I suspect the LC 520 and 550 will not work with 64 MB modules either. I also suspect the Performa variants of the LC III and 520/550 will similarly not work, and that the Colo(u)r Classic II will probably also fail to boot with SIMMs greater than 32 MB installed.

My general conclusion: the RAM controller on the 030-based Macs with 72-pin SIMM slots appears to be insufficient to recognise a 64 MB SIMM. I have some anecdotal evidence that this is actually incorrect — some people have reported SIMMs of greater than 32 MB capacity working in such Macs — but I have insufficient information to make an informed conclusion at this point.

Interestingly enough, MicroMac claims their BigSIMMs (64 MB and 128 MB) will work in any Mac with a 72-pin slot. That is, any Mac that will fit those things. From what I've heard, they're huge (about double the height of a normal SIMM). If anyone has a BigSIMM, get a hold of me. I have some questions for you :)

The 040-based Macs

The Q605/LC475 does NOT work with double-banked modules. It works fine with all single-banked modules. Single-sided modules fit easily into the slot; the double-sided modules have a slightly thicker PCB and don't fit as easily. You have to manually push the clips over onto the module in order to secure it in the slot. The bottom-side chips are not running into the motherboard; I'm not entirely sure what's causing the difficulty. At any rate, they work fine with a little effort.

The LC575 works with all the single-banked modules as well. If you're using a 575 board in a Colo(u)r Classic, note that EDO-B and FPM-C2 are ever so slightly too tall to fit under the screw housing on the left of the CC chassis. Since the others are an appropriate size, simply avoid the use of these two modules.

The C610 (the Quadra should be the same) will not recognise any 64 MB SIMMs as 64 MB. All modules are read as 32 MB, no matter what. As such, I suspect the 610's RAM controller simply can't deal with anything more than 32 MB on a chip or requires 2K refresh.

The 660AV reads the single-banked modules (all of them) as 16 MB each. The double-banked module is read as 32 MB, indicating that the refresh rate isn't a problem. The 660AV's RAM controller only recognises 16 MB per bank, based on what I've found here, and as such, I don't think there is any way at all to use more than 68 MB in a 660AV, since there are two slots and 4MB on-board. RAM modules are not available with more than two banks.

My guess is that the 660AV may lack an address line on its RAM controller that would allow it to read 32 MB per bank, and this address line may be added if you have a lot of knowledge of the 660AV architecture and soldering skills. See The Gabezing Room (Japanese) for information about the 575 in this respect; I think the information (though maybe not specific details) is applicable to the 660AV as well.

The LC630 I have here will read all but the FPM-C2 normally. The FPM-C2 is read as 32 MB, as I expected. Interestingly, the various 630-series board with a second RAM slot work fine with single-banked 64 MB SIMMs in that slot and up to 128 MB, double-banked SIMMs in the main slot, for a total of 196 MB RAM, making these the kings of CC upgrades in the RAM department.

The C650 won't read any of my SIMMs as 64 MB, similar to the C610. I suspect a refresh rate issue, as I have one reader who has reported 256 MB RAM (four 64 MB SIMMs) in a Quadra 650. The SIMMs (with the label SEC KMM53632000BK-6) were composed of Samsung chips with 12 chips per side for a total of 24 chips on each SIMM. Eight on each side are labelled KKM44C1600BK-6 and four on each side are labelled KM41C16000CK-6.

If you have a Mac not listed here, contact me with questions.

Last modified on 16 May 2003
by Chris Lawson
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