Programming Embedded Systems, 2nd Edition

Direct Memory Access

Since we are discussing memory, this is a good time to discuss a memory transfer technique called direct memory access (DMA). DMA is a technique for transferring blocks of data directly between two hardware devices with minimal CPU involvement. In the absence of DMA, the processor must read the data from one device and write it to the other, one byte or word at a time. For each byte or word transferred, the processor must fetch and execute a sequence of instructions. If the amount of data to be transferred is large, or the frequency of transfers is high, the rest of the software might never get a chance to run. However, if a DMA controller is present, it can perform the entire transfer, with little assistance from the processor.

Here’s how DMA works. When a block of data needs to be transferred, the processor provides the DMA controller with the source and destination addresses and the total number of bytes. The DMA controller then transfers the data from the source to the destination automatically. When the number of bytes remaining reaches zero, the block transfer ends.

In a typical DMA scenario, the block of data is transferred directly to or from memory. For example, a network controller might want to place an incoming network packet into memory as it arrives but notify the processor only once the entire packet has been received. By using DMA, the processor can spend more time processing the data once it arrives and less time transferring it between devices. ...