>>>> 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.

> Actually it can be done anywhere Andreas. [...] there is no restraint on where to
> map I/O. At the top of memory is a common spot, but hardly a requirement.

Yes, of course it can be done. But what would be the point? You'd still have only 3.0 to 3.5 GiB available RAM on a 32-bit system, as I said, not 4 GiB, even if you place the mapped I/O in the lower address space or in the middle or anywhere else. You started talking about "moving mapped I/O" and I really don't get the point of that in the context of what we discussed.