The Explorer: The Nearly Secret DMA Can Speed Up Your Drives - InformationWeek
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Fred Langa
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The Explorer: The Nearly Secret DMA Can Speed Up Your Drives

Why don't more people use this fast, free speed-enhancement?

Are you interested in some faster performance without spending one dime? There's a good chance you can speed up your hard drives, CDs, CDRs and DVDs -- for free -- with Windows' almost-hidden DMA setting Doing so can make your drives as much as 15 percent faster, and reduce the load on your CPU by as much as 40 percent. But despite this easy-to-obtain speed benefit, some new systems still ship with the older, slower, non-DMA disk and CD/CDR/DVD access enabled; and many readers who could manually enable DMA access haven't done so. Here's the scoop with DMA, including how to see if it's working on your system; and if not, what to do about it!

DMA is "direct memory access" (sometimes also called "bus mastering"); a way a part of your computer to bypass the CPU and take a short cut through the system that can significantly speed operations.

In Windows, you can see your drives' current DMA settings by right-clicking on My Computer, then Properties, then Device Manager. Next, click on Disk Drives, then on your hard drive(s) -- you may see a nonspecific name such as "Generic IDE Disk Type 01" -- then on Properties, and then click on the Settings Tab. See if the DMA box is checked.

Next, follow the same steps for the CD-ROM(s) listed in your Device Manager.

Even if you have a system of reasonably recent vintage, there's an excellent chance you'll see an *UN*checked DMA option in the dialog box in one or both places. That's because non-DMA drive operations avoid possible compatibility issues. By choosing slower, more-conservative settings, system vendors can save themselves some support calls.

Microsoft Waffles
Microsoft is schizoid about DMA. On the one hand, it steers users to the slower, non-DMA settings by means of a dire warning that appears when you check the DMA box. The warning states, "Changing this setting may have undesirable effects on your hardware..." That's enough to scare off most people. Who wants to risk trashing a drive? But Microsoft's KnowledgeBase also says (in part):

Many people are familiar with the gains to be had from using IDE hard drives and CD-ROM drives in DMA mode; a typical machine today will use 40 percent of the CPU doing hard drive transfers in PIO mode and use only 25 percent of the CPU doing hard drive transfers in DMA mode, on the same hardware...

Just to further illustrate its schizoid approach to DMA, Microsoft's KnowledgeBase also states that, "By default, DMA is enabled for hard disks on Windows 98-based and Windows Millennium Edition-based computers..." Sounds great, but I have never -- not once in the hundreds of times I've installed Win98 and ME on various systems -- not once have I ever found this to be the case with my hardware. Windows makes DMA available -- it has the drivers -- but I have never found DMA to be auto-enabled. Rather, I always have to enable it manually.

As for other versions of Windows, Microsoft says Win95 can use DMA if you have DMA-capable hardware and a Win95-specific DMA driver from your hardware vendor. Oddly, the KnowledgeBase has almost nothing on DMA in Windows 2000.

Most major drive manufacturers have abundant DMA/Bus Mastering information (and drivers) on their sites; this is to be expected because today's fast ATA drives need DMA enabled to reach their full potential. See the Maxtor site or the Seagate site for example; both have good information on the subject.

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