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Chapter 4: Mastering the Hardware Environment > Direct Memory Access – DMA

Direct Memory Access – DMA

Direct Memory Access or DMA for short is a mechanism that moves data between an I/O device and memory without the intervention of the CPU. DMA comes in two modes: System DMA and the more modern, Bus Master DMA. DMA provides high performance data transfers. There are several DMA transfer types.

  • Common Buffer DMA - In common-buffer DMA, the driver and the system share the same area of memory for data transfer. The driver is responsible for synchronizing access to the buffer. The memory is not cached, making this synchronization easier for the driver. After setting up a common buffer, both the driver and the hardware can write directly to the addresses in the buffer.
  • Packet DMA - Performed when there is a single existing buffer that must be mapped for use by the hardware. For example, using packet DMA in the transfer of a file from memory to a disk. Using common-buffer DMA for this action would be wasteful, because the file would have to be transferred to the common buffer before the hardware could transfer it to the disk.

  

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