Depending on what level of DMA your hard disk and interface support, enabling DMA transfers may or may not increase disk performance noticeably, but enabling DMA is always worthwhile, because it greatly reduces the burden that PIO transfers place on the processor. If a computer has 75% CPU utilization using PIO transfers, that same computer using DMA transfers may provide the same or better disk performance at perhaps 1.5% CPU utilization. With multitasking operating systems, those extra free CPU ticks translate into faster system response.
To use DMA transfers, the hard drive, BIOS, and chipset must explicitly support DMA, and the operating system must have DMA drivers installed, loaded, and enabled. All recent versions of Windows support DMA transfers, but DMA is not always enabled by default, as follows:
A fresh Windows install automatically installs DMA-capable drivers and tests the BIOS, interface, and hard drive for DMA compatibility. If any of these tests fail, DMA is disabled. If all three succeed, DMA is enabled automatically at the fastest DMA mode common to the drive and interface.
Upgrading an existing system to Windows XP automatically enables DMA only if DMA was previously enabled. If DMA was previously disabled, you'll have to enable it manually.
When you install a second hard drive and restart the system, immediately check the current DMA status of that drive and enable DMA if it is not currently enabled. To do so, take the following ...