Hardware Reference
In-Depth Information
There are two distinct types of direct memory access: singleword (8-bit) and multiword (16-bit).
Singleword DMA modes were removed from the ATA-3 and later specifications and are obsolete.
DMA modes are also sometimes called busmaster ATA modes because they use a host adapter that
supports busmastering. Ordinary DMA relies on the legacy DMA controller on the motherboard to
perform the complex task of arbitration, grabbing the system bus and transferring the data. In the case
of busmastering DMA, all this is done by a higher-speed logic chip in the host adapter interface
(which is also on the motherboard).
Systems using the Intel PIIX (PCI IDE ISA eXcelerator) and later South Bridge chips (or equivalent)
can support busmaster ATA. The singleword and multiword busmaster ATA modes and transfer rates
are shown in Tables 7.6 and 7.7 , respectively.
Table 7.6. Singleword (8-Bit) DMA Modes and Transfer Rates
Table 7.7. Multiword (16-Bit) DMA Modes and Transfer Rates
Note that multiword DMA modes are also called busmaster DMA modes by some manufacturers.
Unfortunately, even the fastest multiword DMA Mode 2 results in the same 16.67MBps transfer speed
as PIO Mode 4. However, even though the transfer speed is the same as PIO, because DMA offloads
much of the work from the processor, overall system performance is higher. Even so, multiword
DMA modes were never very popular and have been superseded by the newer Ultra-DMA modes
supported in devices that are compatible with ATA-4 through ATA-7.
Table 7.8 shows the Ultra-DMA modes now supported in the ATA-4 through ATA-7 specifications.
Note that you need to install the correct drivers for your host adapter and version of Windows to use
this feature.
Table 7.8. Ultra-DMA Support in ATA-4 Through ATA-7
 
 
 
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