Understanding DMA

In the world of computers, the central processing unit (CPU) is–well–central. Your first computer course probably explained it like the brain of the computer. However, sometimes you can overload that brain and CPU designers are always trying to improve both speed and throughput using a variety of techniques. One of those methods is DMA or direct memory access.

As the name implies, DMA is the ability for an I/O device to transfer data directly to or from memory. In some cases, it might actually transfer data to another device, but not all DMA systems support that. Sounds simple, but the devil is in the details. There’s a lot of information in this introduction to DMA by [Andrei Chichak]. It covers different types of DMA and the tradeoffs involved in each one.

DMA is especially useful for transferring blocks of data (for example, data from a disk drive, audio, or video data) at high speeds. It is also useful for slow data (like UARTs) so that the CPU doesn’t have to block itself waiting for a slow I/O device. In the old days, sometimes the processor wasn’t fast enough to read a fast stream, but today it is likely that the processor is super fast. You just don’t want to tie it up with a slow I/O device. But that has changed how DMA architectures work over time. Usually, when a block transfer completes, the CPU gets a single interrupt so it can process the incoming data or queue up more data to send to the device.

The primary way to differentiate DMA schemes is what happens to the processor while the memory is in use by another device. An older processor is likely to use block mode where the processor simply stalls while the memory is in use. That makes sense because the I/O device is probably faster than the CPU anyway so the loss in terms of executed instructions will be small.

With faster processors, burst mode DMA is popular because it will limit how long the CPU is paused. In fact, many modern burst controllers will try to wait until the CPU is not using memory anyway and only stall if the CPU tries to use memory during the brief transfer.

Some processors and DMA systems can figure out when the CPU will not be using memory for a bit and do transfers totally during that time. This is usually known as transparent mode.

You might think that DMA is for “big computers,” and certainly, [Andrei’s] article centers on PC’s, SPARC, and Atmel SAM devices. However, block mode DMA on an 8085 (with an external controller) is also mentioned. We also remember that the RCA 1802 had DMA on the CPU which was amazing in its day. That DMA is what made a simple front panel possible for the ELF computers and a cool (for its day) video graphics chip. Admittedly, if you are writing with a modern operating system and you aren’t writing device drivers, you probably don’t need to use DMA. But for real-time systems you can easily analyze, DMA can be both a great simplification and a boost to overall system throughput.

While this intro has a lot of background, it doesn’t show any real concrete examples. If you want to see [Mike Harrison’s] practical results of using DMA for SPI on a PIC32, be sure you read our post on that from last year.

Filed under: ARM, Microcontrollers