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Memory-mapped I/O (MMIO) gives hardware registers or device memory addresses in the processor’s memory address space. When software accesses one of those addresses, the system routes the operation to the device rather than to ordinary RAM. Drivers typically map device addresses into kernel address space and use I/O-specific accessors to interact with them.
What memory-mapped I/O means
With MMIO, a device exposes registers or another region of device memory at a range of addresses. The processor uses an address in that range to read from or write to the device. The address belongs to the system’s memory address space, but the target is hardware—not a location in ordinary RAM.
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MMIO is the most widely supported form of I/O in Linux. The specific address ranges and how they are discovered depend on the architecture and bus; for example, PCI devices can be discovered through the bus mechanism rather than relying on one universal address assignment.
How a driver accesses an MMIO device
- Find the device address range. The range may be specified by the platform or discovered through a bus such as PCI.
- Map the device address. A kernel driver generally uses an operating-system facility such as Linux
ioremap()to map the physical device address into kernel virtual address space. - Use I/O accessors. The driver reads or writes the mapped registers with the operating system’s I/O-specific accessors, following the device and architecture’s access rules.
This is not simply a matter of converting the device address into an ordinary C pointer and dereferencing it. The mapping and accessor APIs carry semantics that matter for hardware access.
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MMIO versus port-mapped I/O on x86
On x86, MMIO is one of two ways to address device I/O described in the cited 80386 reference. Port-mapped I/O uses a separate I/O address space and dedicated I/O instructions. MMIO instead places device addresses in the memory address space and uses memory-style addressing.
| Aspect | MMIO | Port-mapped I/O (x86 example) |
|---|---|---|
| Address space | Device addresses occupy a range in the memory address space. | A separate I/O port address space; the 80386 reference describes 16-bit ports, or 216 possible port addresses. |
| How software selects an address | Memory-style addressing selects a location in the device range. | Dedicated port-I/O instructions select an I/O port. |
| Driver access | Drivers map device addresses and use I/O accessors with appropriate semantics. | Uses the architecture’s port-I/O mechanism rather than an MMIO mapping. |
This comparison is specifically about the x86 model described by the 80386 reference; it should not be read as a claim that every processor architecture supports both approaches. Nor does the distinction establish that one approach is universally faster.
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Why MMIO is not ordinary memory
A device register can trigger an action or expose changing hardware state when accessed. Reads and writes may also have ordering requirements. Linux therefore warns drivers not to use general-purpose memset() or memcpy() on I/O addresses: those routines do not replace the documented I/O accessors and their required semantics.
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- Do not assume a mapped device address behaves like a normal RAM pointer.
- Use the platform’s mapping and I/O-accessor APIs.
- Follow the device, operating system, and architecture documentation for ordering and access rules.
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