But that's not the only issue because not all PCs support DMA or UDMA in the first place. For any flavor of DMA to work, the drive, the motherboard, the BIOS and the OS all must be DMA or UDMA capable. For UDMA/66 to work, you also need a special 80-pin cable as well. Just having a DMA or UDMA drive isn't enough -- the entire system has to support the mode you want to use, or the drive will revert to a slower mode, possibly all the way back to PIO.
There's not a lot I can tell you about your own hardware -- the potential number of motherboard/BIOS/cable/disk combinations is staggering. If you're unsure about your systems' DMA or UDMA capabilities, visit your system vendor or hard drive maker's Web site, and see what they have to say.
But I can tell you about Windows: Windows 95 and NT4 both need a patch or upgrade to support DMA/UDMA. Windows 95 SR2 supports DMA natively, but needs an upgrade to support UDMA. Windows 98 and the current Release Candidates for Windows 2000 natively support both DMA and UDMA. (See the links at the end of the article for Microsoft pages that can help you track down any patches you need; try your hard drive vendor's Web site, as well.)
There's not a lot of information on UDMA/66 yet, but chances are you'll need a special driver to get full speed from this type of hard drive: The special driver should either ship with the drive, or be available on the hard drive vendor's site. As mentioned earlier, UDMA/66 also needs a special cable; without both the special cable and the proper driver, the best your UDMA/66 drive will do is UDMA/33, or even standard DMA.
OK, this is sounding very complicated, so before your eyes glaze over, let me stop here and reiterate: With the most common types of hard drives on reasonably recent PCs, you probably will be able to select DMA and immediately enjoy must faster, problem-free drive throughput.
And in most cases, if you select the DMA option and it doesn't work for any reason, the system simply reverts back to the slower but ultra-reliable PIO mode, and that's that: no harm done. You then can try tracking down whatever it is -- drive, cable, motherboard, BIOS, or OS -- that's preventing DMA access from working. Thus, in most cases, you can try DMA with little risk. (Although, as I said in the original article, it's always good to have a current backup.)
But sometimes, things can go awry: On some small number of systems, a DMA problem does not result in a graceful return to PIO mode. On these mostly-older systems, things can get rather nasty:
For example, you may find your system has become unstable; or you may find that your system is now actually slower than before. In extreme cases, you may be able to boot only to DOS (which always uses PIO mode); you won't be able enter Windows. While none of these outcomes is acceptable, the last case is the worse: If you can't even get into Windows, how can you fix the problem? (No, Safe Mode won't help; you can't modify fouled-up DMA settings from within Safe Mode.)
So, if you're one of the unlucky ones who have a major problem in trying to enable DMA, what can you do?
If you can get into Windows, the simplest fix may be to right-click My Computer, select Properties, then Device Manager. Open Disk Drives, highlight your hard drive entries, and the click "Remove." Next, open Hard Disk Controllers, and Remove your controller entries. Reboot; when the hardware is automatically redetected and reinstalled, the problem may resolve itself.
If not, run Regedit, search for and entries referencing "IDEDMADRIVE." There'll typically be two: IDEDMADRIVE0 and IDEDMADRIVE1, although your system may be different. In any case, set the values of those entries to zero, save the edited Registry, exit, and reboot.
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