A
digital system architecture, where there are two
memory buses: one for
devices attached to the system
like
hard disk controllers,
parallel and
serial UARTs,
keyboard handlers and
graphics chips, and another separate bus for
RAM. This is convenient in some ways, since devices tend to be slower than
RAM and hence you can actually access both simultaneously without too much loss of performance. This also means you have separate load and store instructions for I/O devices other than RAM.
The name comes from the fact that I/O devices are "mapped" (in other words, where they live, or their address is marked as being somewhere in) to the I/O memory space, rather than the "real" memory space.