DMA : Computer Hardware Buyers’ Glossary


DMA (Direct Memory Access). PCs (Personal Computers) have a pair of tiny computers in addition to the main CPU (Central Processing Unit). They are quite stupid. All they can do is arrange to send data from RAM (Random Access Memory) 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 (Dynamic RAM) memory
1 8 default low DMA for Sound Blaster card, used by a SDLC (Synchronous Data Link Communication) 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 (Peripheral Component Interconnect) card or ISA (Industry Standard Architecture) SCSI (Small Computer System Interface) 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 (Programmed Input/Output) is faster for hard disk access.

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