Introduction

The processor core is capable of doing multiple operations in a single cycle, including calculations, data fetches, data stores and pointer increments/decrements. In addition, the core can orchestrate data transfer between internal and external memory spaces by moving data into and out of the register file.

All this sounds great, but in reality you can only achieve optimum performance in your application if data can move around without constantly bothering the core to perform the transfers.

This is where a direct memory access (DMA) controller comes into play. Processors need DMA capability to relieve the core from these transfers between internal/external memory and peripherals, or between memory spaces. There are two main types of DMA controllers. "Cycle-stealing" DMA uses spare (idle) core cycles to perform data transfers. This is not a workable solution for systems with heavy processing loads like multimedia flows. Instead, it is much more efficient to employ a DMA controller that operates independently from the core.

Why is this so important? Well, imagine if a processor's video port has a FIFO that needs to be read every time a data sample is available. In this case, the core has to be interrupted tens of millions of times each second. As if that's not disruptive enough, the core has to perform an equal amount of writes to some destination in memory. For every core processing cycle spent on this task, a corresponding cycle would be lost in the processing loop.

We know from experience that PC-based software...