How to Enable IDE DMA Bus-Mastering Using SP2 Atapi.sys

ID: Q158873

The information in this article applies to:
  • Microsoft Windows NT Workstation version 4.0
  • Microsoft Windows NT Server version 4.0

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SUMMARY

A DMA Bus-Mastering-capable version of the Atapi.sys device driver is available in Service Pack 2 or later for Microsoft Windows NT 4.0. By default, Atapi.sys will perform disk I/O through the Programmed Input/Output (PIO), which is the transfer method used in previous versions of Windows NT.

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

Q164378 WinNT 4.0 SP2 or SP3 Atapi Does Not Support Ultra DMA Devices


MORE INFORMATION

There are two levels of Disk I/O support in the Windows NT 4.0 Service Pack 2 version of Atapi.sys:

  • Level 0 (Programmed I/O) (PIO mode): (Normal Default setting)

    This mode conducts PIO transfers only, which is the only transfer method used in previous versions of Windows NT.


  • Level 1 (DMA-Enabled mode): (Requires manual registry changes to enable)

    This mode enables DMA Bus-Master transfers, provided the following Criteria are met:


    1. PCI command register bit 2 (Master Enable) is set.


    2. PCI device Class code is 1.


    3. PCI device Subclass code is 1.


    4. Programmer's interface bit 7 is set.


    5. Bus-Master status register bits 5 and 6 are set. (These two bits are enabled by the system BIOS and are only set if the device or devices exist -- bit 5 is for the master and bit 6 is for the slave.)


    NOTE: If ANY of the previous criteria are not met, the system will default to Level 0 (PIO mode) transfers and no performance will be gained.


NOTE: DMA is used on a per-channel basis with the new Atapi.sys driver. It will not perform DMA Bus-Master transfers to one device on the channel and PIO to another device on the same channel.

Manual Registry Changes Required to Enable DMA Bus-Mastering

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).


It is highly recommended that a user perform a full system backup (including the local registry files) and update the system Emergency Repair Disk before making the following registry changes. The changes required to implement DMA Bus-Master transfers affect Windows NT hardware detection/boot sequence at a very low level.

Should the system fail to boot following the specified registry changes, the user may boot the system by selecting Last Known Good. If the Last Known Good boot fails, the user must recover his or her system by restoring a tape backup or using the Emergency Repair process.

  1. Start Registry Editor (Regedt32) and go to the following subkey:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Atapi\Parameters


  2. From the Edit menu, click Add Key.


  3. Enter the following key name:
    DeviceX (where X represents the IDE channel, either 0 or 1)


  4. Select the DeviceX subkey and click Add Value from the Edit menu.


  5. Enter the following information
    Value Name: DriverParameter
    Data Type: REG_SZ
    Value: DMADetectionLevel = 0x0 or 0x1
    where the STRING equals 0x0 for PIO mode or 0x1 for DMA-Enabled.


  6. Shut down and restart Windows NT.


Determining DMA Status

Whether DMA Bus-Mastering is enabled can be determined by looking in the Windows NT registry under

HKEY_LOCAL_MACHINE\HARDWARE\DEVICEMAP\Scsi\ScsiPortX
where X represents the IDE channel, 0=primary or 1=secondary channel.

Look at the following value:
DMAEnabled:REG_DWORD:0x0 or 0x1 0x1=DMA enabled

These entries will only show if the full Windows NT Service Pack 2 or later was installed on the system, not just the Atapi.sys device driver.

Additional query words:

Keywords : ntdriver
Version : winnt:4.0
Platform : winnt
Issue type : kbhowto


Last Reviewed: January 28, 1999
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