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.