Speeding up my CD/DVD-ROM: what are PIO and DMA, and how do I change the setting?

One of the problems with this setting is that it’s very possible that it’s
in slightly different places on different computers. Another confusion is that
while we talk about DMA and PIO as being a setting of the device – in your case
the DVD player – it’s actually a setting of the controller that it’s connected
to. Which means looking in a slightly different place in Device Manager.

I’ll show you mine, and perhaps that’ll help you find yours.

And then if you’re still around, I’ll briefly summarize what PIO and DMA
are, and why they might make such a huge difference in speed.

]]>
<![CDATA[

DMA mode in Device Manager

In the start menu, or in Windows Explorer, right click on Computer and click on Manage. In the resulting dialog click on Device Manager:

We’ll start by looking at the CD/DVD devices installed on my system for at least one clue. Expand the DVD/CD-ROM drives section by clicking on the triangle in front of it:

As you can see, I have something called an “Optiarc” something-or-other. Yours is likely different, unless you happen to have a similar Dell laptop.

Right click on that device (the Optiarc in my case) and click on properties to get more information:

There’s actually nothing here about DMA/PIO – sorry. But what I wanted to highlight here was the channel that’s displayed. That’ll potentially help confirm a setting we’re about to check elsewhere, so just note it for now – in my case it’s “1”.

Click on Cancel to close this dialog, and now expand IDE ATA/ATAPI controllers:

Look at all those … channels.

“IDE ATA/ATAPI controllers” are the controllers to which your hard disks, CD-ROM and DVD disk drives (as well as occasionally other devices) are attached.

Right click on the ATA channel that corresponds to the channel we saw above – ATA channel 1 in my case – and click on properties:

Now click on the Advanced Settings tab:

First, note that the “Device Type” column correctly reflects that this is a ATAPI Cdrom (it may say Cdrom even if you have a DVD drive – it’s a generic term). That helps confirm we’re looking at the right thing.

And below that? The “Enable DMA” setting.

Make sure that’s checked, and click OK to exit. (You may need to reboot before it’ll take effect.)

DMA? PIO? WTH?

Let’s at least clear up the acronym soup first:

• DMA: Direct Memory Access

• PIO: Programmed Input/Output

• WTH: What The Heck?

Oversimplifying, as usual, but there are basically two ways for hardware such as disk drives and DVD players to transfer data to and from the computer itself. PIO has the computer itself involved in the transfer of the actual data: every byte, perhaps in groups of 2, 4 or 8, is transferred to or from the device by specific instruction of your computer’s CPU. DMA bypasses the CPU completely and allows the device controller to write directly to your computer’s RAM when the CPU’s not looking.

DMA is much faster. Much, much faster. And it doesn’t impact your system nearly as much since it leaves the CPU alone for the bulk of the data transfer.

So, why have PIO at all? Mostly for compatibility. Not all hardware supports DMA, and (apparently) not all hardware that does support it supports it well. Thus we need PIO to fall back on.

Things were fast, why’d it change?

That whole “fall back on” thing I just mentioned.

For the most part, everything defaults the way we might want and expect: DMA mode. However, depending on things such as the specific device and the device-driver software, your drive may elect to fall back to PIO mode if it has trouble.

And then forget to revert to DMA mode. (I say “forget”, but there are actually reasons that the device driver might decide to stay in PIO mode.)

The scenario that I’ve heard of, but not experienced myself, is a difficult to read CD or DVD. In attempting its darnedest to read the media the device might well say “DMA’s not working, let’s try PIO”, and when that then works for whatever reasons it decides to stay in PIO mode from then on.

Without telling you.

And with all the speed impact that implies.

So if you find your DVD, CD, or even your internal hard disks not performing quite as well as you might imagine, the DMA/PIO setting is worth a check.