2.4.1 - How much, and what kind of, RAM can my Mac take?

There are several excellent resources for this. I recommend consulting at least the first two.

The first resource you should consult is Apple's very own Memory Guide, one of the few Service Source documents available to the general public. It has RAM and VRAM upgrade information for every Apple product ever produced that can be upgraded. One caveat: Apple's Memory Guide only lists supported RAM configurations. Many Macintosh models can take more than what Apple claims they can.

The second resource to check out is the excellent Guide to RAM Upgrades, or GURU, from NewerRAM (216 K download). GURU is pretty good about listing what will actually work, instead of only what Apple was able to test by the time they released the machine.

The third resource I recommend is Thomas Cook's Memory Guide, which is a nice online resource. The nice thing about Cook's guide is that you don't have to download anything.

The fourth resource is JAG's excellent RAM Guide, which also doesn't require downloading anything. It's based on the information from the first two sources here, so if you have those, you probably don't need to check this out, and vice versa.

The final resource I recommend is Apple's Hardware Tech Note #25, which deals with many early Macs.

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2.4.2 - Can I use PC RAM in my Mac?

As long as it's fast enough, you can use parity or non-parity RAM in any 68K Mac. Macs generally don't care if you put parity RAM in them; most will ignore the parity chip. There are a few special cases, however. The 128 through 512Ke are not upgradeable with standard SIMMs. The SE (all models) cannot use two-chip or three-chip SIMMs in some cases (though I have put three-chip SIMMs in SEs with no problem). Some very rare versions of the IIci, generally used only by the United States Department of Defense, required parity RAM, and no, no one has given a satisfactory answer as to why they used it.

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2.4.3 - How do I identify RAM chip sizes and parity/non-parity RAM?

Parity SIMMs have an odd number of chips on the SIMM. Non-parity SIMMs have an even number. The following is a rough and very general guide to identifying chip sizes. There are some numbers on the chips (the individual black chips, not the whole SIMM). There are usually some letters, some numbers, some letters, and then a couple more numbers. (The following is most accurate for 30-pin SIMMs.) For example:


comes from a (disregard the KM4 part) 256 kilobyte, (disregard the AP) 120 (add a zero to the numbers after the dash) nanosecond SIMM.

If they're parity chips, read the one farthest to the right with the pins down.

4256 or variants thereof: 256KB (these make great keychains but are virtually worthless in a Mac)
1100 or 1000: 1MB
2048, 2000, etc: 2MB
4400, 4000, etc.: 4MB
8096, 8000, etc.: 8MB

and so on. If this doesn't answer your question well enough, maybe the Chipmunk RAM Guide will.

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2.4.4 - Can I use EDO SIMMs?

While the jury is still out on this one, general consensus on the lists is that using EDO RAM in your Mac will probably not damage it (since several list members are running various Mac systems with EDO SIMMs in them). To be absolutely sure, though, make sure you get Fast Page Mode (FPM) SIMMs and forget about EDO.

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2.4.5 - I know my Mac has more than 8MB of RAM but the System says it's using almost all of it. What gives?

Your system isn't in 32-bit addressing mode. Without using 32-bit addressing, the Mac can only "see" 8MB of RAM for applications and the rest gets reserved by the System. If you have any Mac besides the SE/30, Mac II, Mac IIx, or Mac IIcx, you can simply go to the Memory Control Panel and turn 32-bit addressing on. If you have an SE/30, Mac II, Mac IIx, or Mac IIcx, you need to get Mode32, which will give your Mac the ability to turn on 32-bit addressing. Note that Mode32 is incompatible with System 6.

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2.4.6 - Can I use 64 MB (or larger) SIMMs in my Mac?

Low End Mac's Chris Lawson has done extensive research into using 64 MB, 72-pin SIMMs in various 68K Macs. The same advice should apply to 128 MB SIMMs. With his permission, I have reproduced the information here:

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:

EDO-A, as I'll call it, is a 4k refresh, single-banked, double-sided, 8-chip EDO module. EDO-B is a 4k refresh, single-banked, single-sided, 8-chip module that's slightly taller than EDO-A.

FPM-A is a 4k refresh, single-banked, double-sided, 8-chip FPM module. FPM-B is a 4k refresh, single-banked, double-sided, 12-chip FPM module. FPM-C2 is is a 4k refresh, double-banked, single-sided, 8-chip module that looks identical to EDO-B except for its ID sticker.

I have been told by the experts at Velocity Upgrades that a 2k refresh, 64 MB module would be impossible to build using today's available DRAM chips, so 4k refresh is as good as it gets.

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. If anyone can prove this wrong, I'd love to know, but I suspect it won't happen.

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 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, which, as I noted above, seems to be an impossibility.

The 660AV reads the single-banked modules (all of them) as 16MB 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 16MB 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 68MB 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 (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. The 63x/640/58x series should all be the same, although I haven't tried RAM in the 58x's second RAM slot, so I can't speak to that.

The C650 won't read any of them as 64 MB, similar to the C610. I suspect this is a RAM controller issue as well. I don't have a Quadra variant of either one, so I can't say if this problem carries over to the Quadras or not. I'm going to get in touch with Dan about this and see if I can check them out in his Q650. As in the case of the C610, the RAM controller may requires 2K refresh, which, as I noted above, seems to be an impossibility. The Q800 should be the same as its motherboard is identical to that in the Q650.

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2.4.7 - Why won't the Memory control panel allow me to create a RAM disk?

You may have a Mac that doesn't support a RAM disk via Apple's built-in RAM disk software. All 68040-based Macs support RAM disks via the Memory control panel, while no 68000- or 68020-based desktops allow them.

The 68030-based Macs known to support a RAM disk include the LC III/III+, Performa 45x/46x machines, and the 520 and 550/560-series.

All portable Macs should support the use of a RAM disk via the Memory control panel.

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