Recently I described problems that a few Windows 95/Windows 98 and Windows NT users have been having with hard drives that use Direct Memory Access (DMA) or the faster Ultra DMA (UDMA) standard. If you're experiencing these problems, my last two columns provide work-arounds.
I hope my columns, however, haven't scared you away from using DMA if your system is capable of it. This technique can improve your hard disk performance.
To see whether your system is capable of hard disk DMA/UDMA, run the System Control Panel and click the Device Manager tab. Double-click the Disk Drives icon, then double-click a disk drive and click the Settings tab. If you see a DMA check box, drivers are present that are DMA-capable. If not, you do not have such drivers.
Windows 98 and Windows 95B automatically install DMA-capable drivers if they detect suitable hardware. It is also possible to install such drivers under Windows 95, Win 95A, and NT (using Service Pack 3).
If you see a DMA check box and it is not checked, you may be able to improve your hard disk performance by turning on this capability. Before you do this, however, you should do two other things.l Make an emergency boot diskette and verify that it works to start up your system, in case enabling DMA causes some problem. (If this happens, start Windows in Safe Mode, turn the DMA check box off, then reboot.)l Get a utility that reports on hard drive performance, including DMA performance, so you can see the difference you get. A good utility is HD Tach, a program available at tcdlabs.simplenet.com/hdtach.htm. The trial version of HD Tach tests your drives' read performance, and the registered version, priced at $49.95, also tests write performance.
I used HD Tach to test two hard drives with and without DMA enabled. One drive improved its read burst speed from 8.4MBps to 11.9MBps. The other improved from 8.3MBps to 11.2MBps. More important, the CPU utilisation dropped from 83 to 29 per cent on one drive and from 61 to 20 per cent on the other. Lowering the CPU utilisation of a drive allows a computer to process more data or serve more users at the same time as disk files are being transferred.
I asked Microsoft for a definitive answer on whether or not Windows 98 automatically enables DMA. It turns out that Windows' DMA drivers, when installed for the first time, do try to enable DMA but may disable it if your system fails certain tests. Specifically, the drivers (1) query the motherboard chip set, (2) query the drive itself, and then (3) test a short pattern of disk reads and writes to see if they are reliable at DMA speeds.
Microsoft representative David Alles put it this way: "On a machine that is upgraded to Windows 98, we retain the DMA settings (or lack thereof) of the previous state. If it was Windows 95 Gold, DMA will be off. But if a user had an OSR2 [Windows 95B] machine and had turned on DMA, it will remain on in Windows 98.
"When users check the DMA box in Device Manager, sometimes it appears unchecked after the system reboots. In such a case, we have determined that at least one of the three criteria mentioned above has not been met, so the system is not suitable for DMA," Alles continues.
To test whether a drive supports DMA, run \Tools\Reskit\Help\ Rk98book.chm on the Windows 98 CD-ROM.