Sanity Check (putting 4MB SIMMs in a Mac II)

[pathw]

I decided to try putting more memory into my Mac II (currently at 8MB). I purchasd 4 4MB PAL SIMMs from MemoryTen and dropped them into Bank B. The first time I tried, the system started up, but only 4MB was visible. I realized that I was in  24 bit mode, so changed to 32 bit mode and restarted. I immediately got the error chimes, before any video (and no video came on). I tried reseating the new memory a few times, but nothing changed. (I put back in the original four 1MB SIMMs and everything is working as expected.) Note that  I am running System 7.1 with MODE32 and have a PMMU installed.

Any suggestions on other things to try before I contact the memory vendor about a return?

As an aside, I tried to make sure that I understood which was Bank A and Bank B, and noticed that the SIMM slots were labelled:

(Bank A)     (Bank B)

SIM4 SIM3 SIM2 SIM1

The fact that the numbering is in "reverse" order seems a little strange.

Thanks.

 

[Unknown_K]

Try just the 4 x 4mb SIMMs in a single bank.

 

[pathw]

Thanks for the suggestion, but the information I have is that larger than 1mb SIMMs only work in Bank B, and Bank A has to be fully populated before putting anything into Bank B. Is this incorrect? (Worth a try in any case. Maybe I'll get a more detailed error message.)

 

[dougg3]

This technote has a bunch of useful Mac memory configuration info:

http://tech-insider.org/mac/research/acrobat/9204.pdf

On page 7, it indicates you're right (assuming you don't have IIx ROMs installed). Bank A can only take 1 MB SIMMs, and it has to be populated before you can put anything in Bank B.

 

[unity]

I think 20MB is max without the FDHD kit.  if you have an FDHD kit then I think 68MB is max. Also you said you have a PMMU, you are sure its not a HMMU, right? Its such a messes up machine when it comes to RAM. I installed an 030 to get over all the issues.

 

[pathw]

> I think 20MB is max without the FDHD kit.

That's what I was shooting for.

> Also you said you have a PMMU, you are sure its not a HMMU, right?

Yep. (I have both, but the PMMU is installed.)

> Its such a messes up machine when it comes to RAM.

Until I started playing with it again, I didn't realize how limiting the 8MB is. The processor is powerful enough to run a number of games I wanted to check out, but the memory is often a bottleneck.

 

[pathw]

And thanks dougg3:

> On page 7, it indicates you're right (assuming you don't have IIx ROMs installed). Bank A can only take 1 MB SIMMs, and it has to be populated before you can put anything in Bank B.

Good to have the confirmation that I am reading this correctly.

 

[Unknown_K]

News to me about the RAM limit.

 

[pathw]

In filling out the RMA information, I realized that the new memory was 70ns while this old memory was 80ns. Are there problems with mixing 70ns and 80ns memory?

 

[dougg3]

II memory has to be at least 120 ns. Both 70 and 80 are faster than that, so they should both be fine mixed together. That's my understanding of how DRAM speeds work, anyway...

 

[pathw]

Update ... Ends up that they sent me the wrong memory the first time (and put a sticker on it labelling it as the correct memory, so they didn't realize it until I had sent it back). The next set of memory also didn't work. They are out of parts, so are just refunding me my money.

So, that experiment didn't pan out. Any suggestions on where to find 4MB PAL SIMMs for a Mac II? I think that I am still interested in trying this.

 

[yuhong]

The custom PCBs should not be difficult to make, hopefully with a modern GAL. All that is needed is to force WE high during a CAS before RAS refresh cycle.

BTW, what is happening is that normally the larger SIMMs are put in bank A because of the way the Mac II memory controller is designed. Unfortunately the original Mac II ROM did not support anything bigger than 1MB SIMMs, the newer FDHD/IIx ROMs fix this. MODE32 supports larger SIMMs being put in bank B as a workaround by using the PMMU to remap it into contiguous memory.

 

[yuhong]

Actually, I think what is happening is that the Mac II ROMs had bugs with more than 8MB of RAM that made such configs unbootable. The Mac II memory controller normally required the address space for Bank A to be larger than Bank B. MODE32 worked around by using PMMU to remap it as mentioned above (after the Mac II booted thinking there is only 8MB of RAM).