I/O-mapped and Memory-mapped

The first address space is called the memory space and is intended mainly for memory devices;
the second is reserved exclusively for peripherals and is called the I/O space.

However, peripherals can also be located within the memory space,
at the discretion of the hardware designer.
When that happens, we say that those peripherals are memory-mapped and
that system has memory-mapped I/O.
Some processors support only a memory space,
in which case all peripherals are memory-mapped.