Say you have 5MB memory installed and you access memory location 5MB+1 (linear, read or write doesn't matter). How does the 80386+ respond to this? Does it wrap arround memory? Does it fire the invalid segment descriptor interrupt? Or does it just ignore it and give 0 for that/those memory location(s)?
From what I have learned is that accessing that memory is uncharted territory, ie it depends largely on the system, but the best way to find out is to write data there and see if it is readable. (obviously testing in a VM would be safer, but might not give the best accuracy)
Are you sure you don't mean unreal mode? Protected mode lets you use the entire 32-bit address space as virtual memory and map some subset (or all) of that to physical memory. If a particular address is accessed when it's not mapped, it will throw an interrupt. Most OSs use that to do a page-in from swap space or to throw an error (segmentation fault).