Designing Embedded Hardware

DMA

Direct Memory Access is a way of streamlining transfers of large blocks of data between two sections of memory or between memory and an I/O device. Let’s say you want to read in 100 MB from disk and store it in memory. You have two options.

The processor can read each byte at a time from the disk controller into a register, then store the contents of the register to the appropriate memory location. For each byte transferred, the processor must read an instruction, decode the instruction, read the data, read the next instruction, decode the instruction, and then store the data. Then the process starts over again for the next byte.

The second option in moving large amounts of data around the system is DMA. A special device, called a DMA controller (DMAC), performs high-speed transfers between memory and I/O devices. Using DMA bypasses the processor by setting up a channel between the I/O device and the memory. Thus, data is read from the I/O device and written into memory without the need to execute code to perform the transfer on a byte-by-byte (or word-by-word) basis.

In order for a DMA transfer to occur, the DMAC must have use of the address and data buses. There are several ways in which this could be implemented by the system designer. The most common approach (and probably the simplest) is to suspend the operation of the processor and for the processor to release its buses (the buses are tristate). This allows the DMAC to take over the buses for the short period required ...