DMA : Computer Hardware Buyers’ Glossary

by Roedy Green ©1997-2008 Canadian Mind Products
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DMA

Direct Memory Access. PCs have a pair of tiny computers in addition to the main CPU. They are quite stupid. All they can do is arrange to send data from RAM to an i/o device or vice versa. The advantage is, they can do this without requiring much help from the main CPU. This frees the main CPU to do more useful work. Most of the time you can forget about DMA. However if you install a sound card, you may have to assign the card some DMA channels that don’t conflict with any other device in your machine. In theory, Plug & Play sound cards will automatically select unused DMA cards, but does not always work. Each device that wants to use DMA needs its own private channel. Each controller can keep four "conversations" going at once. Each conversation gets assiged to a channel number. Here is how the DMA channels are assigned:

Channel	width in bits	Use
0	8	used to refresh DRAM memory
1	8	default low DMA for Sound Blaster card, used by a SDLC card
2	8	diskette drive
3	8	free, often used by a sound card.
4	8	cascade, a kludge to hook the two DMA controllers together.
5	16	default high DMA used by a Sound Blaster card.
6	16	free, often used by a sound card.
7	16	free

The ordinary slow style DMA is called 3rd party DMA because the DMA controller acts as a third party to mediate between two other devices (usually RAM and a peripheral). Because these built-in DMA controllers are so slow and so ineptly designed, sometimes a PCI card or ISA SCSI card will provide its own replacement high speed DMA controller that runs at full bus speed. This is called first party DMA. The standard DMA controller is so incredibly slow that, PIO is faster for hard disk access.

PIO

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