Translations:Kernel/Gentoo Kernel Configuration Guide/39/en

Unless you're dealing with a pre-1995 system, your IDE controller will also support an alternative transfer mode, known as Direct Memory Access (DMA). DMA is much much faster, and CPU utilisation is barely affected while data transfers are taking place. If you are suffering from really poor general system performance and you are using an IDE disk, chances are that DMA is not being used.