MmMapIoSpace

MmMapIoSpace is a Windows kernel-mode routine that maps a physical address range into the system's virtual address space. It is used by device drivers to access memory-mapped I/O (MMIO) regions or other hardware memory directly from the kernel.

Signature and behavior: PVOID MmMapIoSpace(PHYSICAL_ADDRESS PhysicalAddress, SIZE_T NumberOfBytes, MEMORY_CACHING_TYPE CacheType); The function returns a pointer to the mapped virtual address on success, or NULL on failure. The CacheType parameter selects the caching policy for the mapped memory, with common options including noncached, cached, or write-combined, chosen based on device requirements and performance considerations.

Usage: A driver calls MmMapIoSpace with the device’s physical address and the size of the MMIO region to obtain a kernel virtual address that can be used to read or write device registers. When the region is no longer needed, the driver must call MmUnmapIoSpace with the same virtual address and size to release resources. The mapped region resides in the kernel’s nonpaged address space and is not accessible from user-mode code.

Notes and considerations: The provided physical address should correspond to a documented I/O memory region for the hardware. Mappings are typically performed during device initialization and unmapped during driver unload or error handling. The chosen caching type affects semantics and performance; noncached or write-combined options are common for MMIO to avoid stale data or ordering issues. If the physical range is invalid or overlaps protected memory, MmMapIoSpace may fail and return NULL.

Related: MmUnmapIoSpace, MmMapLockedPagesSpecifyCache, MmMapIoSpaceEx.