>> The address space between 1.5 and 2.0 GiB is used to map and access the memory
>> of on-board and expansion hardware. That's why the OS can't use it as system RAM
>> and it's also why 32-bit addressing would give us access to 3.0 to 3.5 GiB
>> system RAM instead of the entire 4 GiB.

> True, but I'm not even considering moving mapped I/O (which could be placed anywhere
> in the 32bit map).

Huh? Where would you place the mapped I/O on a 32-bit system if not in the upper address space between 3.0/3.5 GiB and 4.0 GiB? I don't think there are options.