What made me think this: in the install guide I did for
A1200/Blizzard, I overrode the ppc_model to basically put the Cyberstorm PPC processor into the Blizzard hardware configuration. The Cyberstorm is "limited" to 128MB of RAM, yet that processor configuration has no trouble running with the Blizzard 256MB of RAM. It would seem to be silly for a hard-coded RAM limit to be in the upper-layers, in the OS, especially since there are so many variations of possible amounts of RAM. This leads me to firmly believe that OS4.x just uses the configured RAM from the lower-level: the PPC expansion logic. And then in that expansion logic I think it is a reasonable assumption to make that the RAM "limits" come from the physical hardware layouts and not from address problems. The ROMs came from real hardware that could only hold so much RAM so they stopped at that amount of RAM. If the ROMs could be patched to configure more RAM at that lower-level then I think the higher-levels would just inherit it.