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Mac II (and IIx) RAM limits


Well-known member
Hi all!

After researching a bit, I'm still a bit confused about the subject... it's clear that both machines will work fine with 8 MB (eight 1 MB SIMMs) and that MODE32 is required in order to recognize larger amounts (32 bit addressing). But:

•My II is pretty stock -- no PMMU (has the Apple kludge instead), standard ROMs (no FDHD). Does it really need the 68851 PMMU for anything over 8 MB? Is the Bank A limited to smaller SIMMs than Bank B?

•My IIx booted fine with four 2 MB sticks, but were detected as 1 MB only :( Those very same SIMMs make Death Chimes on the SE/30... Will regular 4 (or 16!) MB SIMMs do?

What's a PAL SIMM anyway? My 2 MB SIMMs do have a (PAL?)16L8 chip and four 514400 DRAM chips. They work fine on the Colour Classic, though.

Thanks a lot in advance!



Well-known member
I don't want to give the wrong advice, but wouldn't a 32-bit clean ROM help with the max RAM issue? If it does, you could always order a ROM SIMM from dougg3 on famous ROM SIMM page. You could then use that to install a 32-bit clean ROM.



Well-known member

I read the desciption somewhere once, but I can't remember where. Maybe in one of Apple's reference manuals, such as GTTMFH. So I'm not very helpful, but from what little I recall, the PAL SIMMs massage one or more of the control lines in some way.

Regarding your 2 MB SIMMs with PALs on board:

PAL simply stands for Programmable Array Logic. PALs are a type of PLD and are very versatile.

So, while Apple called SIMMs with a PAL on board to provide the specific function which the IIx needed, "PAL SIMMs", not every SIMM with a PAL on board, has that particular special logic for the IIx implemented.

In the case of your 2 MB SIMMs, they're really like a pair of two-chip 1 MB SIMMs on the same board. The PAL decodes the address and directs the lower half of the addresses to one 1 MB SIMM and directs the upper half of the addresses to the other 1 MB SIMM. But there's no reason to believe that the PAL decoding the addresses for the 2 MB SIMM also implements the logic that the IIx needs.

I wish I could remember what that was... Maybe something to do with refresh and RAS and CAS?



Well-known member
Many thanks to all for your replies and interesting links!

Things are getting clear now... in order to put over 8 MB on the Macintosh II, I need:

1) The 68851 PMU (which I don't have) :(

2) Specifically for Bank A, update the ROMs to those of the IIx (or newer...)


Issue 1 should make me forget about the others and having over 8 MB on my Mac II ever... anyway, 68851s don't seem to be that hard to get -- who knows! ;)

The II has no ROM SIMM slot, but DIP sockets instead -- even easier to update for me! Although it's a minor issue, I could go up to a reasonable 20 MB (or even 68!!!) without it. Plus, I don't know what the upgrade ROM would think about the installed IWM (instead of the expected SWIM)

Of course, "clean" ROMs are needed for the OS in order to access more than 8 MB, but anyway the software workaround (MODE32) seems fine, so it's a minor issue, I think.

Finally, issue 3 is shared with the IIx, and the question remains: 30-pin SIMMs are relatively easy to buy, but how can I be sure that they'll work on the II/IIx?

All the best,



Well-known member
The issue with large SIMMs (over 1 MB) in Mac II and Mac IIx is the video cards. 4 Mb chips added a new test mode, and it happens that the TFB video card would cause signals on the buss which would lock RAM into this test mode. Therefore, 4 MB and larger SIMMs for Mac IIs need a PAL or other device to lock out the problematic RAS / CAS sequence. You DO need a PMMU memory controller to use more than 8 MB, and you need either 32-bit clean ROMs from the FDHD kit or MODE 32. The 32-bit ROMs are compatible with both IWM and SWIM disk controllers. IMHO, unless you've got some of the special RAM on hand, it's not worth the effort and cost to expand a II or IIx beyond 8 MB. The software which ran on those Macs will run quite happily in 8 or less.

It's possible that a later model SuperMac or Radius video card would be compatible with large RAM, but I never heard of anyone testing such a setup. If you actually want to USE a NuBuss 68K Mac with lots of slots and beaucoup RAM, look for a Quadra 950. Those old beasties can be found here and there for cheap, and they'll accept up to 1/4 GB of RAM.



Well-known member
A Mac II is perfect with 8MB of RAM anyway. Years ago I stripped out some RAM from some IIx machines (should have snagged them but they were very ugly looking) and I think they were only 1MB. Plenty of old Mac II/IIx probably shipped with 4 x 256Kb SIMMs (1MB).