The I/O devices are addressed at certain reserved address ranges on the main memory bus. These addresses cannot therefore be used for RAM. Motorola and Mostec architectures, among others, use memory mapped I/O.
Video cards and other cards with on-board memory might be accessed in this way though the term applies not just to devices containing memory but to any device connected to the memory bus. Accessing the devices usually consists of reading and writing certain built-in registers though sometimes the mere presence of a particular address can trigger the device.
See also: Port-mapped I/O
This article was originally based on material from FOLDOC, used with permission. Update as needed.