Using DMA

As we stated earlier, DMA stands for direct memory access. DMA enables a device to transfer data directly to RAM without using the attention of the processor for the entire transfer period. The result is a faster and more direct method of data transfer. This method was especially useful in older PCs, enabling the DMA channel to transfer data in the background, thus freeing the processor to tend to other duties. DMA is typically used by devices such as floppy disks, hard disks, tape devices, and network cards.

This graphic shows the CPU intervening to transfer data.

This graphic shows the use of DMA for direct transfer of data.

Note 

The term channel is often used in describing DMA. In older PCs, a DMA controller chip handled DMA activities. The chip contained four DMA channels that were numbered 0 to 3. Technology enhancements allowed the inclusion of up to eight channels. Each type of bus standard allows a different number of DMA channels.

The following table shows the standard DMA channels that most systems use. When two devices are configured to the same DMA channel, neither device can transfer the data to memory correctly. The only available DMA channel on older PCs is DMA channel 3.

System Device	DMA Channel
Available	0
Available	1
Floppy disk controller	2
Available	3
Not available (used for internal purposes or second DMA controller)	4
Available	5
Available	6
Available	7

Tip 

If your device does not operate correctly using various combinations of DMA channel assignments, try the Disable DMA option.

Tip 

If you install a piece of hardware and your computer does not work, remove the piece of hardware that you installed and restart your computer. Then, if the computer works, you know that the new piece of hardware conflicts with an existing piece of hardware.