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Direct Memory Access

Direct memory access (DMA) is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory for reading and/or writing independently of the central processing unit. Computers that have DMA channels can transfer data to and from devices with much less CPU overhead than computers without a DMA channel.

Principle of DMA

DMA is an essential feature of all modern computers, as it allows devices to transfer data without subjecting the CPU to a heavy overhead. Otherwise, the CPU would have to copy each piece of data from the source to the destination. This is typically slower than copying normal blocks of memory since access to I/O devices over a peripheral bus is generally slower than normal system RAM. During this time the CPU would be unavailable for any other tasks involving CPU bus access, although it could continue doing any work which did not require bus access.

A DMA transfer essentially copies a block of memory from one device to another. While the CPU initiates the transfer, it does not execute it. For so-called "third party" DMA, as is normally used with the ISA bus, the transfer is performed by a DMA controller which is typically part of the motherboard chipset. More advanced bus designs such as PCI typically use bus mastering DMA, where the device takes control of the bus and performs the transfer itself.

A typical usage of DMA is copying a block of memory from system RAM to or from a buffer on the device. Such an operation does not stall the processor, which as a result can be scheduled to perform other tasks. DMA is 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, audio playback and streaming video.

DMA Controller

The processing unit which controls the DMA process is known as DMA controller. Typically the job of the DMA controller is to setup a connection between the memory unit and the IO device, with the permission from the microprocessor, so that the data can be transferred with much less processor overhead. The following figure shows a simple example of hardware interface of a DMA controller in a microprocessor based system.


Functioning (Follow the timing diagram for better understanding).
Whenever there is a IO request (IOREQ) for memory access from a IO device. The DMA controller sends a Halt signal to microprocessor. Generally halt signal (HALT) is active low. Microprocessor then acknowledges the DMA controller with a bus availability signal (BA). As soon as BA is available, then DMA controller sends an IO acknowledgment to IO device (IOACK) and chip enable (CE - active low) to the memory unit. The read/write control (R/W) signal will be give by the IO device to memory unit. Then the data transfer will begin. When the data transfer is finished, the IO device sends an end of transfer (EOT - active low) signal. Then the DMA controller will stop halting the microprocessor. ABUS and DBUS are address bus and data bus, respectively, they are included just for general information that microprocessor, IO devices, and memory units are connected to the buses, through which data will be transferred.

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