IOmapped

IO-mapped I/O, also known as port-mapped I/O or isolated I/O, is a method for communication between a CPU and peripheral devices that uses a separate address space dedicated to I/O devices rather than the main memory address space. In this approach, devices are accessed through a distinct set of I/O addresses rather than memory addresses.

In architectures that provide I/O-mapped I/O, the CPU uses special instructions to read from and write to I/O ports. On x86, the typical instructions are IN and OUT, which transfer data to and from devices via a 16-bit port address space. Data transfers are commonly 8 bits wide, though some CPUs support wider transfers for certain ports. The I/O address space is separate from main memory, and ports are mapped to device registers such as status, control, data, or command registers.

Advantages of IO-mapped I/O include keeping the memory address space free for actual memory, and providing a simple, dedicated path for peripheral control. It can simplify hardware design for small, register-based devices and can offer straightforward interrupt and polling mechanisms. Limitations include a relatively small port address space (in x86, 64K ports), which can constrain the number of devices or registers, and the requirement to use a separate instruction set with distinct programming models, which can complicate software and tooling. Performance is largely determined by the underlying bus and device design rather than by the I/O scheme alone.

Compared with memory-mapped I/O, which places device registers in the same address space as memory, IO-mapped I/O is less common in modern general-purpose systems but remains in use in certain embedded systems and legacy PC designs. Notable examples include the IBM PC keyboard controller and peripheral interface devices that utilize port addresses in the processor’s I/O space.