Microsoft KB Archive/258757

= How to Enable Direct Memory Access (DMA) =

Article ID: 258757

Article Last Modified on 1/27/2007

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APPLIES TO


 * Microsoft Windows 98 Second Edition
 * Microsoft Windows 98 Standard Edition
 * Microsoft Windows 95
 * Microsoft Windows Millennium Edition

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This article was previously published under Q258757



SUMMARY
This article describes how to enable Direct Memory Access (DMA) on your Windows 98-based, Windows 95-based or Windows Millennium Edition-based computer. DMA (also referred to as bus mastering) is a technique that some components and devices use to transfer data directly to and from memory without passing through the Central Processing Unit (CPU). DMA reduces CPU overhead by providing a mechanism for data transfers that do not require monitoring by the CPU. The number associated with DMA indicates the direct memory access channel that the component or device uses to transfer data to and from memory. The following topics are included in this article:
 * Determine if DMA Is Enabled on Your Computer
 * How to Enable DMA for a Hard Disk
 * How to Enable DMA for a CD-ROM



Determine if DMA Is Enabled on Your Computer
By default, DMA is enabled for hard disks on Windows 98-based and Windows Millennium Edition-based computers, but it is enabled only with some specific versions of Windows 95 that are listed below. First, determine if DMA is enabled on your computer.

NOTE: If DMA is already enabled, you do not have to perform any of the following steps.

For additional information about how to determine if DMA is enabled on your computer, click the article number below to view the article in the Microsoft Knowledge Base:

141497 How to Use Device Manager to Check the Status of a Resource

DMA support for Windows 95 is available in Original Equipment Manufacturer (OEM) versions of Windows 95. If you purchased Windows 95 (the full or upgrade version) from a retail store, you have a retail version of Windows 95. Retail versions of Windows 95 cannot be upgraded to OEM versions, and there is no separate download to enable DMA support in retail versions of Windows 95. If you have an OEM version of Windows 95 that does not have DMA support, contact your OEM.

To determine the version of Windows 95 you are running:  Click Start, point to Settings, click Control Panel, and then double-click System. On the General tab, locate the version number in the System area, and then refer to the following table. 

If you are running an OEM version of Windows 95, the product ID number under the Registered To heading typically contains 20 digits. If digits 6, 7, and 8 contain the letters &quot;OEM,&quot; your version of Windows 95 is an OEM version. For example, the following sample product ID number indicates an OEM installation:

12345-OEM-6789098-76543

OEM Version Identification
Windows 95 may have been preinstalled on your computer. These installations are referred to as OEM installations. An OEM Service Release (for example, OSR2) is an updated version of a product for personal computer manufacturers (OEMs) to preinstall on new personal computers. This allows OEMs to install an integrated Windows 95 product that contains the latest available individual updates and supports recent advances in hardware that require core operating system support.

Additional OEM Information

 * In Windows 95 OEM Service Release 2 (OSR2), 2.1 (OSR2.1), and 2.5 (OSR2.5), not all files have the version stamp that is listed in the table. In OSR2.1 and OSR2.5, only files that were updated to provide support for the Win32 Driver Model (WDM) and Universal Serial Bus (USB) may have this version stamp. The other files have the same version stamps as the corresponding OSR2 files.

You can view file version information by right-clicking a file in Windows Explorer, clicking Properties, and then clicking the Version tab. If there is no Version tab, there is no version information available for that file.
 * Updates to Windows 95 OSR2 generally have a version number of 4.00.1112 or later.

How to Enable DMA for a Hard Disk
If you are using Windows 98, Windows Millennium Edition (Me) or a version of Windows 95 that includes DMA support, DMA is already enabled for your hard disk by default. If your version of Windows 95 does not include DMA support, use either of the following methods:

Method 1
Obtain and install the most current driver for your motherboard that provides DMA support. For information about how to do this, consult the motherboard manufacturer, or view the documentation included with your motherboard or computer.

Method 2
You can install any version of Windows 98 or Windows Me to obtain support for DMA.

How to Enable DMA for a CD-ROM
To enable DMA for a CD-ROM:
 * 1) If you are using Windows 98, Windows Me or a version of Windows 95 that includes DMA support, by default, DMA is available but is not enabled for your CD-ROM, and you should skip to step 2. If your version of Windows 95 does not include DMA support, use either of the following methods:
 * 2) * Obtain and install the most current driver for your motherboard that provides DMA support. For information about how to do this, consult the motherboard manufacturer, or view the documentation included with your motherboard or computer.
 * 3) * You can install any version of Windows 98 or Windows Me to obtain support for DMA.
 * 4) Click Start, point to Settings, click Control Panel, and then double-click System.
 * 5) On the Device Manager tab, double-click the CDROM branch to expand it, and then double-click your CD-ROM.
 * 6) On the Settings tab, click to select the DMA check box, and then click OK.
 * 7) Restart your computer, and then test to determine if your CD-ROM is working properly by using the CD-ROM drive. If you determine that your CD-ROM does not work properly with DMA enabled, disable DMA. To do so, repeat steps 2-4, but click to clear the DMA check box.

Keywords: kbhardware kbhowto KB258757

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