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To get the best performance of your IDE hard disk, then one should make sure that DMA transfer mode is enabled. Windows NT4 SP4 will automatically enable DMA transfer mode, but it might cause problems with drives, which doesn't support DMA transfers like old HDD and CD-ROM drives.

When DMA Transfer is enabled the following DWORD registry key is set:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \ATAPI \Parameters \DeviceX]
DriveParameter=0x1

Note the possible values are 0x1 (on - default), 0x0 (off), 0x2 (Forced on)

More Info MS KB Q191774 (Dmacheck.exe)