It is perfectly valid to speculate that the 2GB limit is not directly a memory controller limitation, but nothing is proven by the fact that OS 9 and OS X have different limits.
With any sort of complicated digital device, there are often multiple limiting factors. It stands to reason that the memory controller itself can not go higher than 2GB by design due to limited address space on a 32-bit system. Mac OS X may be able to take full advantage of this 2GB while Mac OS 9 has a separate, additional limitation that holds it back to even less than 2GB.
If it isn't a memory controller limitation, then it's a Mac OS X limitation. You could try installing >2GB RAM and then running linux to see if more RAM appears through the OS. You will never get a full 4 GB of RAM on any 32-bit computer though.
With any sort of complicated digital device, there are often multiple limiting factors. It stands to reason that the memory controller itself can not go higher than 2GB by design due to limited address space on a 32-bit system. Mac OS X may be able to take full advantage of this 2GB while Mac OS 9 has a separate, additional limitation that holds it back to even less than 2GB.
If it isn't a memory controller limitation, then it's a Mac OS X limitation. You could try installing >2GB RAM and then running linux to see if more RAM appears through the OS. You will never get a full 4 GB of RAM on any 32-bit computer though.