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Another Windows 95 secret: DMA for hard drives
technofile  by al fasoldt
Columns and commentaries in a life-long dance with technology 
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Another Windows 95 secret: DMA for hard drives 

Warning: This could cause problems on some systems. For a full explanation of this procedure, go to this page at Axcel's tips site. For immediate help, see one reader's troubleshooting techniques at the end of this article. 

Technofile for Feb. 22, 1998

By Al Fasoldt

Copyright © 1998, The Syracuse Newspapers

I've lamented the poorly chosen defaults in Windows PCs for years. But this time I've got a doozy.

The latest Dumb Move from Microsoft turns your speedy hard drive into a marshmallow, as slow as a February thaw. The sad part about this is that everyone who buys a new PC these days gets a fast drive that has been chortled down to a slowpoke.

Here's what the problem is. The current version of Windows 95 (called Windows 95B) has a checkbox that activates DMA transfers to and from your hard drive. DMA means direct memory access. With DMA in use, the hard drive "talks" to the PC's memory directly, bypassing the central processing unit (CPU) chip. This not only speeds up the hard drive; it speeds up everything—every operation—because the CPU is free to do other things.

Microsoft decided to leave that setting turned off for its own reasons. (Probably, it was afraid users with an odd combination of parts in their PCs would sue when DMA drive access fried their data.) The result: Yet another case of Windows PC performance dropping to the lowest common denominator.

Feel free to give Microsoft a Bronx cheer for this one. And don't hesitate to turn on the DMA setting if you have a fairly new PC. Here's how to do it.

First, check to see if you have Windows 95B. You must have 95B to do this, so don't write to ask if what I'm describing works with the older versions of Windows 95. Right click My Computer and choose Properties. Under System you'll see a number starting with "400." If it has a "B" at the end, you have 95B.

With the Properties window still open, click Device Manager. Click the "Disk drives" tab. Double click any hard drive icons (you may have more than one). Click the "Settings" tab. If you see a DMA checkbox, the drive supports DMA transfers. Click the checkbox and close the window.

Do the same for your CD-ROM drive (or drives, if you have more than one). When you're through, Windows will tell you to reboot.

After you start up Windows again, go back to the Device Manager and see if the checkboxes are still checked. If any of them are not, Windows is telling you that the drive in question does not actually support DMA properly. (If that's the case, you're out of luck.)

What if you have 95B and a new, supposedly fast hard drive but don't see any DMA checkbox? This may mean you have what is called a bus-mastering driver that handles the DMA operations outside the Device Manager's grasp. To check that out, double click "Hard disk controllers" and look for "bus master" or a similar phrase. That will indicate your PC is using Intel's newest DMA drivers, which operate independently of Windows' Device Manager.

Activating DMA access can boost performance in many ways. The most noticeable is a general smoothness in the way Windows handles normal tasks (word processing or games, for example) while it is accessing data on the hard drive.

If you run a test program, you'll see a huge increase in drive speed. Without bus-mastering DMA, the drives on my 233 MHz Pentium-class PC show about 1 megabyte per second in disk throughput. With DMA enabled, their speed rises to about 8 megabytes per second. This is the uncached speed. With the Windows Vcache enabled, throughput rises to about 30 megabytes per second—a phenomenal rate.

Be sure to check all normal operations after you turn on DMA access. If things start to go wrong, turn it off.

After this article appeared, a few readers said they had problems with their CD-ROMs after checking the DMA box. Here is a solution that worked for Jeff Baum, as he told it to me:


I don't deserve the medal [you offered me]; Rob from CyberMax deserved and received a thousand thanks from me. This is the information I got from CyberMax after 1 and 1/2 hours together (Another 1 and 1/2 trying to use the knowledge I have.)

1. Click on Start--Run-- type in Sysedit

2. The system editor will come up. The first window is autoexec.bat. Look for a line that has mscdex in it and type an rem at the beginning of the line. Save settings and exit. Then reboot.

If it still doesn't show up...

1. Restart the computer, but tap on the F8 key until it comes up with a menu screen. Choose Safe Mode.

2. In safe mode, right click with the mouse once on my computer and choose properties. Then choose device manager.

3. If cd-rom is listed in device manager, highlight it and click remove. Then close device manager and restart the computer from the shutdown menu.

If still doesn't show up....

1. click on start--Run-- type in Regedit

2. This will bring up the registry editor screen. Take the plus out of HKEY LOCAL MACHINE, plus out of system, plus out of current control set, plus out of services, plus out of VxD, double click on IOS.

3. On the right hand side of the screen now you should have some data such as default, static, etc. Look for something that says NOIDE. If it is there, highlight it and press the delete key on the keyboard. Say yes to deleting and then close the registry editor. Restart the computer.

If it still doesn't show up...

1. Look in device manager under hard disk controls and see if any have a yellow circle with an ! in the middle of it. If there is, highlight them and click remove. Close the device manager and restart the computer.

2. It will redetect hardware as it reboots, just let it run the wizard.

Al--these were what we believe to be the successful steps in getting my CD ROM back.

Just a funny note. I went to work today and first thing I said to my friend (who also ALWAYS read your articles) "Do not try the DMA setting that was written in the paper!" He told me he already did and had no problems. As a matter of fact, he said his machine does run a littler faster. And he has a 300 Pentium II. Go figure!!

Suggestion: Maybe you should write an article "WARNING: Don't believe everything you read!!" With the experience I gained from this at least I tried, I lost, but--I survived!!

Take Care

Jeff Baum

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