Turn on Direct Memory Access (DMA) for ATAPI devices to lower CPU usage

Many new hard drives come with ability to use DMA (Direct Memory Access), making the input/output of the drive run at a higher speed. This will lower the CPU usage of the system when writing to the drive from 90 percent to less than 30 percent.


To verify that DMA is enabled on your machine, use your favorite Registry editor to edit the Registry key.

HKEY_LOCAL_MACHINEHARDWAREDEVICEMAPScsiScsiPortX
where X is 0 for the primary channel and 1 for the secondary channel.

If the setting of the DMAEnabled value is 1, then DMA is turned on.

ATAPI can also turn on DMA on a channel-by-channel basis, provided the channels are capable of using DMA. To turn DMA on for a channel, apply the following registry change.

HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServices
atapiParametersDeviceX
"DriverParameter" = "DMADetectionLevel = 0x1."

To disable DMA, set the string to
"DMADetectionLevel = 0x0."

NOTE: Please remember that editing your registry can be risky, so always have a verified backup before you begin.

This was first published in May 2001

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