The YMCA is the memory controller in the 840av and 660av. There was some interest in seeing if these machines can support more than 32MB SIMMs, possibly with some hacking similar to the hack for the djmemc memory controller in the 610/650/800.
There's two main registers that are important for bank sizing: one that tells the memory controller where to map the bank, so when you have different size SIMMs installed, there is just one contiguous chunk of available ram. The other tells the memory controller what size the bank should be, or where to stop making ram available.
On the djmemc, the bank location register can set address bits A29-A22, which would allow the RAM banks to be placed anywhere within 1GB. It looks like the YMCA controller can only set address bits A26-A20, which only allows placement of 128MB.
Ultimately, this seems to mean the YMCA can't support more than 128MB of total memory.
I'm left wondering if this was a reaction to the djmemc's smallest bank size being 4MB which, AFAICT meant those machines could only support 4MB or larger SIMMs, while the YMCA can support bank sizes of 1-32MB.