Direct memory access
Categories: Computer memory | Computer architecture | Motherboard
Direct memory access (DMA) allows certain hardware subsystems within a computer to access system memory for reading and/or writing independently of the CPU. Many hardware systems use DMA including disk drive controllers, graphics cards, network cards, and sound cards.
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Principle
DMA is an essential feature of all modern computers, as it allows devices of different speeds to communicate without subjecting the CPU to a massive interrupt load. Otherwise, the CPU would have to copy each piece of data from the source to one of its registers, and then write it back again to the new location. During this time the CPU would be unavailable for other tasks.
A DMA transfer essentially copies a block of memory from one device to another. While the CPU initiates the transfer, the transfer itself is performed by the DMA Controller. A typical example is moving a block of memory from external memory to faster, internal (on-chip) memory. Such an operation does not stall the processor, which as a result can be scheduled to perform other tasks. DMA transfers are essential to high performance embedded systems. It is also essential in providing so-called zero-copy implementations of peripheral device drivers as well as functionalities such as network packet routing, file copy, streaming video over a network, etc.
Examples
ISA
For example, a PC's ISA DMA controller has 16 DMA channels of which 7 are available for use by the PC's CPU. Each DMA channel has associated with it a 16-bit address register and a 16-bit count register. To initiate a data transfer the device driver sets up the DMA channel's address and count registers together with the direction of the data transfer, read or write. It then instructs the DMA hardware to begin the transfer. When the transfer is complete the device then interrupts the CPU.
"Scatter-gather" DMA allows the transfer of data to multiple memory areas in a single DMA transaction. It is equivalent to the chaining together of multiple simple DMA requests. Again, the motivation is to off-load multiple I/O interrupt and data copy tasks from the CPU.
DRQ stands for DMA request; DACK for DMA acknowledge. These symbols are generally seen on hardware schematics of computer systems with DMA functionality. They represent electronic signalling lines between the CPU and DMA controller.
See also
External links
- mmap() and DMA, from Linux Device Drivers, 2nd Edition, Alessandro Rubini & Jonathan Corbet
- DMA and Interrupt Handlingde:Direct Memory Access
es:Acceso directo a memoria fr:Accès direct mémoire gl:Acceso directo á memoria hr:DMA it:DMA nl:Direct Memory Access ja:Direct Memory Access pl:DMA zh:直接記憶體存取