PC Hardware in a Nutshell, 3rd Edition

Enabling PATA DMA Mode Transfers

Depending on what level UDMA 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, your drive, BIOS, and chipset must explicitly support DMA, and your operating system must have DMA drivers installed, loaded, and enabled. All versions of Windows 95, Windows 98, and Windows NT/2000/XP support DMA transfers, but DMA is disabled by default in some environments, as follows:

Windows 95B, Windows 98, and Windows 2000/XP

A fresh install automatically installs DMA-capable drivers and tests the system for DMA support. Setup queries the chipset to determine if it supports DMA. If it does, Setup queries the drive itself to determine what DMA level, if any, it supports. If the drive is also DMA-capable, Setup does a series of reads and writes to determine if the system reliably supports DMA. 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 95B, ...