Input–output memory management unit

In computing, an input–output memory management unit (IOMMU) is a memory management unit (MMU) connecting a direct-memory-access–capable (DMA-capable) I/O bus to the main memory. Like a traditional MMU, which translates CPU-visible virtual addresses to physical addresses, the IOMMU maps device-visible virtual addresses (also called device addresses or memory mapped I/O addresses in this context) to physical addresses. Some units also provide memory protection from faulty or malicious devices. An example IOMMU is the graphics address remapping table (GART) used by AGP and PCI Express graphics cards on Intel Architecture and AMD computers. On the x86 architecture, prior to splitting the functionality of northbridge and southbridge between the CPU and Platform Controller Hub (PCH), I/O virtualization was not performed by the CPU but instead by the chipset. The advantages of having an IOMMU, compared to direct physical addressing of the memory (DMA), include: Large regions of memory can be allocated without the need to be contiguous in physical memory - the IOMMU maps contiguous virtual addresses to the underlying fragmented physical addresses. Thus, the use of vectored I/O (scatter-gather lists) can sometimes be avoided. Devices that do not support memory addresses long enough to address the entire physical memory can still address the entire memory through the IOMMU, avoiding overheads associated with copying buffers to and from the peripheral's addressable memory space. For example, x86 computers can address more than 4 gigabytes of memory with the Physical Address Extension (PAE) feature in an x86 processor. Still, an ordinary 32-bit PCI device simply cannot address the memory above the 4 GiB boundary, and thus it cannot directly access it. Without an IOMMU, the operating system would have to implement time-consuming bounce buffers (also known as double buffers). Memory is protected from malicious devices that are attempting DMA attacks and faulty devices that are attempting errant memory transfers because a device cannot read or write to memory that has not been explicitly allocated (mapped) for it.

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