Is there a way to increase the 136MB RAM limit in the Power Mac 6100 and 7100

[greystash]

Is there a way to increase the 136MB RAM limit in the Power Mac 6100 and 7100? I have a bunch of 128MB EDO sticks that work in these machines but nothing above 136MB is recognised. Are there any hacks that can be done to change this or is it locked in with the default CPU specs?
I have a G3 upgrade card and the only thing holding these machines back is the RAM limit.

 

[trag]

Is there a way to increase the 136MB RAM limit in the Power Mac 6100 and 7100?

The 6100 can take a pair of 128 MB SIMMs and go to 264MB.

If your 128MB SIMMs are only being seen as 64MB in the 6100, then the RAS signals are probably wired wrong on the SIMM. I'm not actually certain that "wrong" is the correct word. I'm not sure there was a standard for how they should be managed. But wrong for the 6100, in any case.

There are four RAS signal lines on a 72 pin SIMM. In order to address two separate "banks" on the SIMM the RAS lines are used in pairs. So one pair of RAS lines is used to activate one bank of memory. The other pair activates the other bank of memory.

For example 32 MB SIMMs are almost always 2 banks of 16MB. Your 128 MB SIMMs are two banks of 64MB.

The issue is that the 6100 logic board ties the members in the pair together. If the SIMM ties them together in the opposite fashion, then it won't work.

For example, there are RAS lines A, B, C and D. These exist on the logic board, and connect to pins on the SIMM socket, from which they connect to the memory SIMM.

Let's say the logic board ties A & B together and ties C & D together to form two pairs.

If the SIMM ties A & C together and B and D together, then that isn't going to work on the aforementioned logic board.

Point being, if you can find the right 128 MB SIMM you can get 264MB in the 6100.

The 7100 is limited to 136 MB. It creates support for four SIMM sockets by doing some unnatural things to the RAM upper address pins, which, in practice, mean that the largest RAM bank supported on a SIMM is 16MB. It supports two banks per SIMM, so 32 MB SIMMs work great, but nothing larger. If you try to use a single-bank 64MB SIMM, the 7100 will only see 16MB.

Now the interesting thing is that the 6100 and the 7100 use the same chipset. But, the 7100 has this extra chip on board through which the RAM upper address pins run. In theory, one could pull off that chip, rewire the connections to the SIMM sockets and give the 7100 just two usable SIMM sockets, but with the ability to support 264MB as in the 6100.

But that would require physically reworking the board.

 

[greystash]

The 6100 can take a pair of 128 MB SIMMs and go to 264MB.

If your 128MB SIMMs are only being seen as 64MB in the 6100, then the RAS signals are probably wired wrong on the SIMM. I'm not actually certain that "wrong" is the correct word. I'm not sure there was a standard for how they should be managed. But wrong for the 6100, in any case.

There are four RAS signal lines on a 72 pin SIMM. In order to address two separate "banks" on the SIMM the RAS lines are used in pairs. So one pair of RAS lines is used to activate one bank of memory. The other pair activates the other bank of memory.

For example 32 MB SIMMs are almost always 2 banks of 16MB. Your 128 MB SIMMs are two banks of 64MB.

The issue is that the 6100 logic board ties the members in the pair together. If the SIMM ties them together in the opposite fashion, then it won't work.

For example, there are RAS lines A, B, C and D. These exist on the logic board, and connect to pins on the SIMM socket, from which they connect to the memory SIMM.

Let's say the logic board ties A & B together and ties C & D together to form two pairs.

If the SIMM ties A & C together and B and D together, then that isn't going to work on the aforementioned logic board.

Point being, if you can find the right 128 MB SIMM you can get 264MB in the 6100.

The 7100 is limited to 136 MB. It creates support for four SIMM sockets by doing some unnatural things to the RAM upper address pins, which, in practice, mean that the largest RAM bank supported on a SIMM is 16MB. It supports two banks per SIMM, so 32 MB SIMMs work great, but nothing larger. If you try to use a single-bank 64MB SIMM, the 7100 will only see 16MB.

Now the interesting thing is that the 6100 and the 7100 use the same chipset. But, the 7100 has this extra chip on board through which the RAM upper address pins run. In theory, one could pull off that chip, rewire the connections to the SIMM sockets and give the 7100 just two usable SIMM sockets, but with the ability to support 264MB as in the 6100.

But that would require physically reworking the board.

Thanks for your detailed reply!! I'm not sure why I thought I had 128MB SIMMs, I actually have 2x 64MB SIMMs. So the 136MB reading makes sense now. Interestingly the 6100 will only recognise the sticks if they are installed as pairs. When only only one SIMM is present the 6100 only recognises the 8MB onboard memory. I haven't come across this with lower capacity SIMMs.

That's a shame about the 7100, I'm not skilled enough to make any adjustments on my board. Thanks for the info I appreciate it. Now to find the correct 128MB SIMMs!

 

[MrFahrenheit]

Thanks for your detailed reply!! I'm not sure why I thought I had 128MB SIMMs, I actually have 2x 64MB SIMMs. So the 136MB reading makes sense now. Interestingly the 6100 will only recognise the sticks if they are installed as pairs. When only only one SIMM is present the 6100 only recognises the 8MB onboard memory. I haven't come across this with lower capacity SIMMs.

That's a shame about the 7100, I'm not skilled enough to make any adjustments on my board. Thanks for the info I appreciate it. Now to find the correct 128MB SIMMs!

I actually just did this today!

https://www.reddit.com/r/Vintagemacintosh/comments/qa6yd9

 

[greystash]

I actually just did this today!

https://www.reddit.com/r/Vintagemacintosh/comments/qa6yd9

That was me you were talking to in the r/VintageApple post, It was very well timed for me haha

 

[just.in.time]

I also saw that post over on reddit.

Yes, 6100 can definitely do over 136MB. As you found it requires the RAM to be installed in matched pairs.

 

[trag]

Interestingly the 6100 will only recognise the sticks if they are installed as pairs.

Yes, the 6100/7100 and 8100 require RAM installed in matched pairs. The data bus on the X100 machines is 64 bits wide. 72 pin SIMMs only provide 32 bits at a time. So it takes two of them at a time to provide a full 64 bits.

 

[MrFahrenheit]

Yes, the 6100/7100 and 8100 require RAM installed in matched pairs. The data bus on the X100 machines is 64 bits wide. 72 pin SIMMs only provide 32 bits at a time. So it takes two of them at a time to provide a full 64 bits.

And the on-board ram in them is simulating 2x 4MB SIMMs. There are 16x 4mbit chips on two rows.

 

[powermax]

Now the interesting thing is that the 6100 and the 7100 use the same chipset. But, the 7100 has this extra chip on board through which the RAM upper address pins run. In theory, one could pull off that chip, rewire the connections to the SIMM sockets and give the 7100 just two usable SIMM sockets, but with the ability to support 264MB as in the 6100.

I would love to learn more about that extra chip. Is it an off-the-shelf IC or an ASIC?

 

[Phipli]

I would love to learn more about that extra chip. Is it an off-the-shelf IC or an ASIC?

Apple ASIC. Apple didn't use much off the shelf stuff in that era.

Basically in the 7100 and 8100 the aim was to divide the maximum RAM between more slots so you could fill it with smaller SIMMs instead of at the time unavailable huge SIMMs, or expensive big SIMMs.

Retrospectively... Its absolutely hilarious how much RAM you can get in a 6100.

 

[Snial]

Is there a way to increase the 136MB RAM limit in the Power Mac 6100 and 7100? I have a bunch of 128MB EDO sticks..

Let's take a step back here. I didn't know the PowerMac x100 series supported EDO Ram! I thought it only supported FPM. Or is it that EDO is backward compatible, so those Macs treat the EDO as FPM? I know my old PM 4400 expected EDO Ram, which meant AFAIK, it didn't support FPM.

 

[Powerbase]

Let's take a step back here. I didn't know the PowerMac x100 series supported EDO Ram! I thought it only supported FPM. Or is it that EDO is backward compatible, so those Macs treat the EDO as FPM? I know my old PM 4400 expected EDO Ram, which meant AFAIK, it didn't support FPM.

I believe they'll run with EDO but won't get any benefit from it.

 

[Phipli]

Let's take a step back here. I didn't know the PowerMac x100 series supported EDO Ram! I thought it only supported FPM. Or is it that EDO is backward compatible, so those Macs treat the EDO as FPM? I know my old PM 4400 expected EDO Ram, which meant AFAIK, it didn't support FPM.

The 4400 is weird because it uses 3.3V RAM.

Another thing to watch for is apparently, putting EDO in a 7200 or 8200 can cause damage. Not sure of the specifics, but the warnings are unique to that logic board.

 

[Snial]

The 4400 is weird because it uses 3.3V RAM.

So, par-for-the-course! When it comes to the 4400 we can stop at 'weird', because pretty much everything about it is ;-) ! But I did like my 4400/160 so I'm allowed to say that I note that in the recent French haul topic there was a 4400/200!

Another thing to watch for is apparently, putting EDO in a 7200 or 8200 can cause damage. Not sure of the specifics, but the warnings are unique to that logic board.

Wow, so there are some situations when a 6100 is much faster than an 8200 ;-) !

 

[Phipli]

Wow, so there are some situations when a 6100 is much faster than an 8200 ;-) !

Many situations... It is a lot easier to find a G3 for a 6100 than for an 8200!

Perhaps I'll bring my 8200 next meetup. Nobody seemed to appreciate my 7200, even though it had loads of amazing software installed

Viva le Catalyst

 

[Snial]

Perhaps I'll bring my 8200 next meetup. Nobody seemed to appreciate my 7200, even though it had loads of amazing software installed

I think I just got distracted by the wacko Performa 630 (?) with the hardware MPEG decoder that can handle windowed output. That just leaves the frogs standing! It's a reasonable conclusion then that it's not speed that helps make a great demo, but (particularly unusual) levels of creativity. For example, we think a Performa 630 is hopelessly underpowered for anything video oriented, but if e.g. iPlayer was streamed in a compatible format over raw ethernet, that computer could give you VHS-quality streamed video, which is usable.

Something equivalently bizarre with a 7200 would be a winner too. What would you do?

Viva le Catalyst

View attachment 68370

Oooo-kay!?!

 

[Phipli]

Something equivalently bizarre with a 7200 would be a winner too. What would you do?

Well, the one I brought was booting from a 250GB SATA hard disk, using a G3 processor mounted on a PCI card with RAM I tried my best to make it interesting

Oooo-kay!?!

It's the 7200 teams' mascot I think. Referencing some manga I don't know anything about called Sailor Moon. Catalyst was the codename.

Back in the day when apple staff were openly silly.

 

[Phipli]

Performa 630

Yup, it was a Performa 630 - clocked at 40MHz with 136MB of RAM

 

[Renegade]

Referencing some manga I don't know anything about called Sailor Moon. Catalyst was the codename.

Back in the day when apple staff were openly silly.

It was a very popular manga/anime among the French-speaking population of Europe in the early 90s. Not as popular as Dragon Ball or Saint Seiya though.

 

[Phipli]

It was a very popular manga/anime among the French-speaking population of Europe in the early 90s. Not as popular as Dragon Ball or Saint Seiya though.

I know of it, but I've not read/watched it.

More of a Cowboy Bebop person, although known to watch Fruits Basket after too much cider