Hardware & Infrastructure
Commentary
3/10/2004
10:30 AM
Fred Langa
Fred Langa
Commentary
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The Explorer: Soup Up Your Hard Drive with DMA

Holy smokes! Several weeks ago in my newsletter (see http://www.langa.com/newsletter.htm), I suggested that readers check to see if their hard drives were set up to use DMA -- direct memory access -- because using DMA can reduce the load on your CPU by up to 40 percent during disk operations: For the price of one simple mouse click -- that is, for free -- you can get a significantly faster, more responsive system.

In fact, when I activated DMA disk access on my system, I saw an immediate 15 percent increase in hard drive speed with no ill effects whatsoever. Many, many readers reported similar or even greater increases. Not too bad for a one-minute tweak, eh?

Today's column actually picks up where my newsletter item leaves off, but that column is too long to reprint here. If all this DMA business sounds foreign to you, please take a minute to read http://www.langa.com/newsletters/nov-26-99.htm and http://www.langa.com/newsletters/nov-29-99.htm; those links explain what's going on and tell you the basics of how to enable DMA on any standard PC of reasonably recent vintage.

But -- and there's always a "but" -- there are many potential glitches, and the more I looked into this, the more potential glitches I found. I still recommend that you use DMA if it's at all possible to do so because the improvements are so impressive. But you need to have your eyes open. Let me explain:

Hard drives normally use one of five ways or "modes" of shuffling data. The older way is called "programmed input/output" (PIO) and it comes in three major flavors with speeds ranging from 5.2MB/s to 16.6MB/s. The primary strength of PIO mode is that it's nearly universally supported -- it'll work on just about all systems, all the time.

The other two common modes are Multi-Word DMA (which also operates at 16.6MB/s) and UDMA, where the "U" stands for "ultra;" it can zip along at 33.3MB/s.

And just so you'll know, there's a new 66MB/s version of UDMA that's shipping in some drives today. These "UDMA/66" drives are not yet common, but they're out there.

So, UDMA and UDMA/66 are inherently much faster than any of the PIO modes. But even standard DMA at 16.6MB/s will net out faster than 16.6MB/s PIO because DMA reduces the load on the CPU, letting the system do something besides just waiting on the hard drive. If you can use DMA or UDMA, it almost always makes sense to do so.

That's almost always: There are exceptions. For example, most PCs come with two IDE "channels" to connect hard drives, ZIP-type drives, and CD ROMs (including CDRs and CDRWs) to the system; each channel supports up to two devices. (That's why typical hard drive cables have two places to attach things.) DMA can only be turned on or off for each channel/cable as a whole, so changing the DMA setting for a channel affects all the devices on that channel.

Thus, if you have a hard drive and a CD on the same cable and if you set the hard drive to use DMA, you're also automatically setting the CD to use DMA -- and some CDs work very poorly with DMA turned on. In fact, some makers of CDRs and CDRWs specifically recommend against using DMA modes. For cases like these, you have to segregate your storage devices so those that can use DMA share one cable and those that don't are on the other.

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