Use Raspberry Pi Gpio Header as Memory Mapped Interface
The Raspberry Pi exposes 17 GPIO pins on P1, and the Rev. 2 board exposes 4 more on P5. These pins cannot be used directly as a memory-mapped I/O bus, such as you might have found on an old-time microprocessor-based system.

I suppose you could bit-bang such a bus, but the limited number of pins would be a problem. You could, for example, define an 8-bit data bus, a read strobe, a write strobe, and use the remaining 7 (or 11) bits as an address bus. However, since the available GPIO bits are nonsequential, this would be slow to operate; the driver on the RPi would have to set and clear bits individually.

A better approach would be to use the I2C and/or SPI (serial) busses that are available on the P1 header to implement external memory and/or memory-mapped I/O.