Direct Memory Access

Direct memory access (DMA) facilitates the maximum data transfer rate and microprocessor concurrence. Unlike programmed or interrupt controlled I/O, where data is transferred via the microprocessor and its internal registers, DMA (as its name implies) transfers data directly between an I/O device and memory (memory to memory DMA transfers are also possible). Whichever CPU is being used, it must have a DMA feature to determine when DMA is required, so that it can relinquish control of the address and data buses, as well as the control lines required to read and write to memory. In addition, the CPU must inform the I/O device that requires the DMA data transfer when it again requires control of the address and data buses and I/O control lines. Further to this, a separate DMA controller is required to actually perform the DMA I/O operations.

What is DMA?

The transfer of data between a fast storage device such as magnetic disk and memory is often limited by the speed of the CPU. Removing the CPU from the path and letting the peripheral device manage the memory buses directly would improve the speed of transfer .this transfer technique is called Direct memory access(DMA).During DMA transfer the CPU is idle and has no control of the memory buses. A DMA controller takes over the buses to manage the transfer directly between the IO device and memory.

The process of DMA is handled by a DMA chip and it can be considered as a primitive secondary processor whose job is to relieve the main processor of much of the burden of memory transfers between memory and I/O Devices. The DMA chip was originally an Intel 8237 device. There are three possible ways that the 8237 can transfer data - I/O to Memory, Memory to I/O and Memory to Memory.

Why we need DMA?

The assumption about I/O devices is that it's really, really slow, compared to the speed of the CPU. I/O devices include keyboard, mouse, printers, etc. While some devices may need quick response, usually when downloading or uploading data, I/O devices are general considered slow.

Rather than the CPU "polling" or asking the I/O device if it needs some action taken on its behalf, the I/O device interrupts the CPU by sending an interrupt signal (such as a positive edge). The CPU then temporarily stops what it is doing, and services the I/O device.

The code to do this is called the interrupt handler. The handler must already be loaded in memory before the interrupt has occurred .Occasionally, an I/O device needs to read from memory or write to memory. This can be very slow, as we shall see using Diagram of I/O devices.

What is actually DMA?

Direct memory access (DMA) is a system whereby samples are automatically stored in system memory while the processor does something else. The process of transferring data via DMA is given below:

1. When data is ready for transfer, the board directs the system DMA controller to put it into in system memory as soon as possible.

2. As soon as the CPU is able (which is usually very quickly), it stops interacting with the data acquisition hardware and the DMA controller moves the data directly into memory.

3. The DMA controller gets ready for the next sample by pointing to the next open memory location.

4. The previous steps are repeated indefinitely, with data going to each open memory location in a continuously circulating buffer. No interaction between the CPU and the board is needed.