mmio vs port i/o

Memory-mapped I/O (MMIO) and port I/O (also called port-mapped I/O or PMIO) are two complementary methods of performing between the and peripheral devices in a . Another method, not discussed in this article, is using dedicated I/O processors—commonly known as on —that execute their own .

Memory-mapped I/O (not to be confused with I/O) uses the same to address both memory and I/O devices, and the CPU instructions used to access the memory are also used for accessing devices. In order to accommodate the I/O devices, areas of CPU's addressable space must be reserved for I/O. The reservation might be temporary—the could between its I/O devices and regular memory—or permanent. Each I/O device monitors the CPU's address bus and responds to any CPU's access of device-assigned address space, connecting the to a desirable device's .

Port-mapped I/O uses a special class of CPU instructions specifically for performing I/O. This is generally found on , specifically the IN and OUT instructions which can read and write a single byte to an I/O device. I/O devices have a separate address space from general memory, either accomplished by an extra "I/O" pin on the CPU's physical interface, or an entire dedicated to I/O.

A device's (DMA) is not affected by those CPU-to-device communication methods, especially it is not affected by memory mapping. This is because by definition, DMA is a memory-to-device communication method that bypasses the CPU.

is yet another communication method between CPU and peripheral devices. However, it is always treated separately for a number of reasons. It is device-initiated, as opposed to the methods mentioned above, which are CPU-initiated. It is also unidirectional, as information flows only from device to CPU. Lastly, each interrupt line carries only one of information with a fixed meaning, namely "an event that requires attention has occurred in a device on this interrupt line".

The main advantage of using port-mapped I/O is on CPUs with a limited addressing capability. Because port-mapped I/O separates I/O access from memory access, the full address space can be used for memory. It is also obvious to a person reading an program listing (or even, in rare instances, analyzing machine language) when I/O is being performed, due to the special instructions that can only be used for that purpose.

I/O operations can slow the memory access, if the address and data buses are shared. This is because the peripheral device is usually much slower than main memory. In some architectures, port-mapped I/O operates via a dedicated I/O bus, alleviating the problem.

There are two major advantages of using memory-mapped I/O. One of them is that, by discarding the extra complexity that port I/O brings, a CPU requires less internal logic and is thus cheaper, faster, easier to build, consumes less power and can be physically smaller; this follows the basic tenets of , and is also advantageous in . The other advantage is that, because regular memory instructions are used to address devices, all of the CPU's addressing modes are available for the I/O as well as the memory, and instructions that perform an operation directly on a memory operand--loading an operand from a memory location, storing the result to a memory location, or both--can be used with I/O device registers as well. In contrast, port-mapped I/O instructions are often very limited, often providing only for plain load and store operations between CPU registers and I/O ports, so that, for example, to add a constant to a port-mapped device register would require three instructions: read the port to a CPU register, add the constant to the CPU register, and write the result back to the port.

As processors have become obsolete and replaced with and in general use, reserving ranges of memory address space for I/O is less of a problem, as the memory address space of the processor is usually much larger than the required space for all memory and I/O devices in a system. Therefore, it has become more frequently practical to take advantage of the benefits of memory-mapped I/O. However, even with address space being no longer a major concern, neither I/O mapping method is universally superior to the other, and there will be cases where using port-mapped I/O is still preferable.