PC Hardware in a Nutshell, 3rd Edition

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 ...