04:55 PM
Fred Langa
Fred Langa

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.

CDs, DVDs, and CDRs, Too!
Curiously, although most newer CDs (and CD-Rs and DVDs) support DMA operation, there's not a lot of information on its use -- except for the usual dire warnings. And the warnings are amplified with CD-R: If you take things at face value, you might assume that, with DMA enabled, you'll never burn another CD again.

But I've been using DMA on all my hard drives and CD-type devices -- including CDRs and DVDs -- for some time now. With most hard drives, I've benchmarked the before and after speeds, and found an immediate 5 to 15 percent speed increase with DMA enabled. And some things (loading large apps, for example) feel much, much more than just 5 to 15 percent faster. I've had no trouble whatsoever using DMA.

Because DMA is a way to bypass the CPU, you might suspect that DMA's benefit is greatest on slower systems. And while there's some truth to that -- the more CPU-bound your system is, the more speed-increase you may see from DMA -- even the fastest system can benefit. On my newest system, for example, a 1.2GHz Athlon box with 256MB of RAM and an Ultra-ATA hard drive, manually enabling DMA speeded my hard drive read operations by almost 10MB/sec, and speeded writes by 13MB/sec.

But due to the vagaries of OS, hardware, and driver support, not every system sees DMA speed improvements all the time: The only way to be sure is to do your own before and after tests, using something like the free drive-throughput tests at WinTune to benchmark your before and after results. (You also can use other reputable hard-drive tests, such as those in Norton Utilities, or on the free PC Pitstop site.)

Want more info? We originally covered DMA access in this space more than a year ago. That article will tell you about the five different "modes" of drivewoperation, master/slave issues (when you have more than one device on an IDE cable) and lots more.

Ready To Try?
If you want to try DMA mode, visit the vendor site for your system and/or hard drive brand and search for information and advice on whether or not to use the DMA option. Or try this: Your system's BIOS information may show whether or not your have a DMA-capable drive.

Related resources:

Soup Up Your Hard Drive

Maxtor On DMA

Seagate on DMA


If the answer is clearly yes or no, then stick with what the manufacturer says. But if the answer is unclear, and if you have a good backup, you might want to give it a try. Microsoft says that under Windows, if your drive doesn't support DMA transfers, nothing bad will happen, and the drive will simply revert to non-DMA mode. But I never, ever counsel you to trust your data to what's "supposed" to happen: Always make a backup of your essential files first, just in case.

Chances are, you probably can enable DMA on some or all of the drives and CDs that currently do not have it enabled, and pick up a nontrivial amount of speed that you've paid for -- but haven't been getting!

Give DMA a try, and then join in the discussion area to tell us what your results were!

To discuss this column with other readers, please visit Fred Langa's forum on the Listening Post.

To find out more about Fred Langa, please visit his page on the Listening Post.

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