From Winn L. Rosch's book:
Direct Memory Access is a capability provided by some computer bus architectures that allows data to be sent directly from an attached device (such as a disk drive) to the memory on the computer's motherboard. The microprocessor is freed from involvement with the data transfer, thus speeding up overall computer operation.
Usually a specified portion of memory is designated as an area to be used for direct memory access. In the ISA bus standard, up to 16 megabytes of memory can be addressed for DMA. The EISA and MCA standards allow access to the full range of memory addresses (assuming they're addressable with 32 bits). PCI accomplishes DMA by using bus mastering (with the microprocessor "delegating" I/O control to the PCI controller).
An alternative to DMA is the Programmed Input/Output (PIO) interface in which all data transmitted between devices goes through the processor. A newer protocol for the ATA/IDE interface is Ultra DMA/33, which provides a burst data transfer rate up to 33 MB (megabytes) per second. Hard drives that come with Ultra DMA/33 also support PIO modes 1, 3, and 4, and multiword DMA mode 2 (at 16.6 megabytes per second).
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From CDEncyclopedia:
On PCs, there are eight DMA channels commonly used as follows. Most sound cards are set to use DMA channel 1.
DMA Used for
0 8-bit transfer
1 8-bit transfer
2 Floppy disk controller
3 8-bit transfer
4 Cascaded from 0-3
5 16-bit transfer
6 16-bit transfer
7 16-bit transfer
DMA Disk Transfers
There are verious modes of data transfer on IDE disk drives. The PIO modes use the CPU, and the DMA modes bypass the CPU.
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From whatis.com:
Direct Memory Access is a capability provided by some computer bus architectures that allows data to be sent directly from an attached device (such as a disk drive) to the memory on the computer's motherboard. The microprocessor is freed from involvement with the data transfer, thus speeding up overall computer operation.
Usually a specified portion of memory is designated as an area to be used for direct memory access. In the Industry Standard Architecture bus standard, up to 16 megabytes of memory can be addressed for DMA. The Extended Industry Standard Architecture and Micro Channel Architecture standards allow access to the full range of memory addresses (assuming they're addressable with 32 bits). Peripheral Component Interconnect accomplishes DMA by using bus master (with the microprocessor "delegating" I/O control to the PCI controller).
An alternative to DMA is the Programmed Input/Output (Programmed Input/Output) interface in which all data transmitted between devices goes through the processor. A newer protocol for the ATA/IDE interface is Ultra DMA/33, which provides a burst data transfer rate up to 33 MB (megabyte) per second. Hard drives that come with Ultra DMA/33 also support PIO modes 1, 3, and 4, and multiword DMA mode 2 (at 16.6 megabytes per second).
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Ultra DMA (UDMA, or, more accurately, Ultra DMA/33) is a protocol for transferring data between a hard disk drive through the computer's data paths (or bus) to the computer's random access memory (random access memory). The Ultra DMA/33 protocol transfers data in burst mode at a rate of 33.3 MBps (megabyte per second), twice as fast as the previous Direct Memory Access (Direct Memory Access) interface.
Ultra DMA was developed as a proposed industry standard by the Quantum Corporation, makers of hard disk drives, and Intel, makers of chipset that support computer bus technology.
Ultra DMA support in your computer means that it will boot (start) and open new applications more quickly. It will also help users of graphics-intensive and other applications that require large amounts of access to data on the hard drive. Ultra DMA uses Cyclical Redundancy Checking (cyclic redundancy checking), offering a new level of data protection.
Because the Ultra DMA protocol is designed to work with legacy application Programmed Input/Output and DMA protocols, it can be added to many existing computers by installing an Ultra DMA/33 Peripheral Component Interconnect adapter card. Ultra DMA uses the same 40-pin Integrated Drive Electronics interface cable as PIO and DMA.