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Memory-mapped input/output (MMIO) lets a processor access a device by reading or writing addresses in a designated region of its memory address space. Those addresses reach device registers or device memory—not ordinary RAM.
How memory-mapped I/O works
Think of an address as selecting a destination. For ordinary memory, an address selects RAM. For MMIO, hardware recognizes an assigned address range and routes accesses to a device. The processor may use instructions that look like normal memory operations, but the device interprets those reads and writes according to its own register and hardware rules. Linux kernel documentation describes device I/O addresses as part of the CPU address space.
The operating system and platform establish the mapping. In Linux driver code, a physical device address should not simply be treated as an ordinary pointer. The documented pattern is to map the resource with ioremap(), access it using I/O-aware functions such as readl() and writel(), and unmap it when it is no longer needed.
MMIO versus port-mapped I/O
Both methods let software communicate with devices, but they use different address spaces and access mechanisms.
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| Aspect | Memory-mapped I/O | Port-mapped I/O |
|---|---|---|
| Address space | A device resource occupies a region of the CPU memory address space. | Device ports occupy a separate I/O address space. |
| Example access | Linux drivers can use accessors such as readl() and writel() after mapping. Memory-reference instructions are an example on x86. |
Linux drivers can use helpers such as inb() and outb(). On x86, dedicated IN and OUT instructions access ports. |
| Key concern | Map the resource correctly and follow its access-width, ordering, and completion rules. | Use the separate port-access mechanism supported by the platform. |
The Linux device-I/O documentation says port space may be smaller and often slower than memory-mapped addresses, and that its Linux port-space access does not require preparation like ioremap(). That is Linux documentation’s general comparison, not a performance guarantee for every computer. The Intel 80386 programmer reference illustrates the x86 distinction between memory-reference instructions and the dedicated port instructions; it is a historical example, not a description of every current processor.
Why MMIO is not ordinary RAM
A device register can trigger an action, return changing status, or have other side effects. Consequently, ordinary memory assumptions do not necessarily apply. Linux warns that generic memcpy() and memset() are not guaranteed to perform I/O-address operations in order; for applicable I/O memory operations, it provides functions such as memcpy_toio(), memcpy_fromio(), and memset_io().
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- Ordering: Use the platform’s documented I/O accessors and ordering rules rather than assuming the compiler or hardware will treat a device access exactly like a RAM access.
- Completion: A write can be posted and complete asynchronously. Linux documentation notes that PCI writes may be posted; when a driver must know that a write has reached the device, a read from that same device may be needed.
- Alignment and width: Requirements vary by platform and device. Misaligned accesses can cause exceptions or unpredictable results, so follow the resource and accessor requirements.
- Caching: Do not assume every device mapping has the same caching behavior as RAM. Use the mapping and access rules for the specific platform and device.
These are Linux driver guidance points, not a universal API prescription for every operating system. Consult the relevant platform and device documentation before writing hardware-facing code. See the Linux guide to bus-independent device accesses for its mapping and accessor details.
MMIO is different from memory-mapped files
Memory-mapped device I/O concerns addresses used to communicate with hardware. Memory-mapped file I/O instead associates file contents with part of a process’s virtual address space so software can work with the file through that view. The shared word “mapped” does not make the mechanisms equivalent.
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