Karbosguide.com. Software Tip 17

Enable DMA on your harddisk

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  • It is extremely important to activate the DMA function in Windows!

    A few readers have reported problems with harddisk after enabling DMA; however I still recommend it since it gives such better performance. If you experience unstability, which will happen in some cases, please disable DMA.

    Some of the combinations of motherboards and HDDs are unstable in this mode, and seems to be critical to the temperature of the HDD. Even 5 degrees celcius can have an impact on stability, and if the fan of your computer is dusty or underdimensioned, you might get this effect. Replacing a 40-wires HDD cable by 80-wires UDMA-66 cable will decrease the electrical noises in the IDE channel and make your computer more stable in DMA mode.

    For example press Windows+Break to open the System Properties dialog box. Select the Device Manager tab:

    Double click on Disk drives, then select your hard disk and click on Properties:

    Now select the Settings tab:

    Verify that you have a check mark in the DMA box:

    DMA transfer between hard disk and RAM is much more effective than when the DMA channel is not in use. With DMA a write or read operation can be executed in two to four clock ticks. Without DMA it will cost the CPU a minimum of 16 clock ticks per operation! That makes a big difference.

    Notice that this enabling of DMA for hard disks and CD-ROM is far from automatic. In some motherboards it is activated automatically when Windows is installed, but that is probably the exception. So check for yourself that it is activated!

    OBS: Some people report that enabling DMA to hard drives causes problems. On newer PCs with good cooling there should be no problems.

    A tip from a reader

    We never experienced problems ourselves from enabling DMA on any of our PCs. However here is a comment, that might prove a useful addendum:

    You forgot one very important part that is required to enable DMA in Windows 98. After you check the DMA box in the device mamager- disk drive window, do the following:

    To make sure that DMA doesn't cause any internal Windows conflicts (assuming the drive supports DMA), you are going to need to add these lines to the Mshdc.inf file at the bottom of the [ESDI_AddReg] tag (if they aren't already there):


    Only add the bottom two lines (the bolded ones) if you have more than two IDE drives (HD/CD devices) connected to your system.

    Within the DISKDRV.INF file, add the same lines under the [DiskReg] entry. If the lines are already there, you don't need to continue on, because you are done. Otherwise, copy the two modified files to another directory.

    I recommend creating a new directory named inf2 in your windows system directory. Then remove all of the items under the Hard Drive Controllers and all of the Hard Drives (only hard drives, not removable media) from under the Disk Drives heading.

    Then reboot your system.

    When the computer's Hardware Wizard pops up to reinstall the drives, set it so you can choose from the compatible drivers list and check the date on the drivers. They should be for the same day you modified them. If they are, great, use them, otherwise you will need to point them at the copies (the inf folder is invisible at this point, which is why you need the second copies) and use those. Thanks guys!

    David Gillespie

    --- We hope this will be useful, and thanks to David!

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