According to Apple documentation, the first 1G of address space in the Mac II family is allocated to RAM. The most any of the machines can physically accept is 128 MB. But, in theory, it should be possible to insert a card that decodes the other 872 M of address space and provides read and write access to some fast modern memory there.
Even better would be to only install 512K or 1MB in the host machine and provide 1023MB of fast RAM in that address space.
The first problem I see is that there's a memory controller on the logic board which is presumably decoding those addresses and if there are not enough SIMMs to provide memory in those addresses, then it might be issuing a /BERR if anything tries to address that space, regardless of what kind of card might be installed in the machine trying to respond to those addresses.
Obviously, the (built-in) memory controller must be able to decode the first 128M because there could be physical memory there. Anyone know if the memory controller decodes the rest of the RAM address space, or is that just left swinging in the breeze with nothing decoding it (IOW, available for an expansion card)?
So, anyone know what addresses the memory controller chip is actively decoding/responding to? The first 128M? The whole 1G? Only the amount of address space for which physical RAM is installed?
Also, how does the Mac determine how much memory is installed at boot time? Does the CPU perform a test? Or does the memory controller have a state machine that tests it and stores the result in a register for the CPU to read?
I know the ROM gets echoed throughout it's available address space, but the book I was reading last night did not say that is done with the RAM. I was reading "Designing Cards and Drivers...". I guess I should also check "Guide to the Macintosh Family Hardware".