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Memory-mapped I/O (MMIO) gives device registers or device memory addresses within the processor’s memory address space. Software can use those addresses to communicate with hardware, but the accesses are device operations—not ordinary reads and writes to RAM.
How memory-mapped I/O works
A device makes registers or other device memory available in an assigned address range. When the processor accesses an address in that range, the system routes the operation to the device rather than to RAM.
A driver generally cannot use the device’s physical address as if it were an ordinary pointer. In Linux, for example, a driver maps a device address into kernel virtual address space with a facility such as ioremap(). It then uses the documented I/O accessors to read or write device registers.
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How MMIO differs from port-mapped I/O
The distinction is especially clear on x86, which supports both approaches. The 80386 reference describes port I/O as using a separate 16-bit I/O address space with 216 ports. MMIO instead places device locations in the memory address space.
| Aspect | Memory-mapped I/O | Port-mapped I/O on x86 |
|---|---|---|
| Address space | Shares the processor’s memory address space with system memory. | Uses a separate 16-bit I/O port address space, as described in the 80386 reference. |
| How software selects a location | Uses memory-style addresses and the processor’s normal addressing modes. | Uses dedicated port-I/O instructions. |
| Driver access | Maps device addresses as needed and uses I/O-specific accessors with the required semantics. | Uses the architecture’s port-I/O mechanism. |
This comparison is specific to the cited x86 design; it should not be read as a claim that every processor architecture supports both models.
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Why device addresses are not ordinary RAM
Device registers can have access rules and ordering requirements that ordinary memory does not. Linux warns drivers not to use general-purpose memset() or memcpy() on I/O addresses. Use the operating system’s documented I/O accessors instead; a direct C pointer dereference is not a portable or universally valid substitute. The Linux device-I/O documentation describes the mapping, accessors, and relevant cautions.
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What MMIO does not mean
- It does not mean the device address range is physically backed by RAM. The address is routed to device hardware.
- It is not the same thing as a memory-mapped file, which maps file contents into a process’s address space.
- It is not inherently faster or better than port-mapped I/O. The sources establish no universal performance advantage; that depends on the architecture and workload.
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