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PC Hardware in a Nutshell, 3rd Edition by Barbara Fritchman Thompson, Robert Bruce Thompson

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Direct Memory Access (DMA) Modes

DMA is a data transfer mode that allows bidirectional transfer of data between drives and memory without intervention from the processor. If you use a multitasking operating system such as 32-bit Windows or Linux, using DMA mode increases performance by freeing the CPU to do other things while data is being transferred. DMA doesn’t improve performance with single-tasking operating systems such as DOS or 16-bit Windows because the processor must wait until the transfer is complete before doing anything else. Table 13-2 lists DMA modes, not all of which are supported by all drives and all interfaces. Note that ATA-3 introduced no new modes.

Table 13-2. ATA DMA modes

DMA mode

Cycle time

Transfer rate

revision

Single Word Mode 0 (SDMA-0)

960 ns

2.1 MB/s

ATA

Single Word Mode 1 (SDMA-1)

480 ns

4.2 MB/s

ATA

Single Word Mode 2 (SDMA-2)

240 ns

8.3 MB/s

ATA

Multiword Mode 0 (MDMA-0)

480 ns

4.2 MB/s

ATA

Multiword Mode 1 (MDMA-1)

150 ns

13.3 MB/s

ATA-2

Multiword Mode 2 (MDMA-2)

120 ns

16.6 MB/s

ATA-2

Ultra-DMA Mode 0 (UDMA-0)

240 ns

16.6 MB/s

ATA-4

Ultra-DMA Mode 1 (UDMA-1)

160 ns

25.0 MB/s

ATA-4

Ultra-DMA Mode 2 (UDMA-2)

120 ns

33.3 MB/s

ATA-4

Ultra-DMA Mode 3 (UDMA-3)

90 ns

44.4 MB/s

ATA-5

Ultra-DMA Mode 4 (UDMA-4)

60 ns

66.7 MB/s

ATA-5

Ultra-DMA Mode 5 (UDMA-5)

40 ns

100.0 MB/s

ATA-6

Ultra-DMA Mode 6 (UDMA-6)

30 ns

133.3 MB/s

ATA-6/7

The SDMA modes were obsoleted by ATA-3, and are useful only for older ...

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