Microsoft KB Archive/182532

= How To DMA-Capable ATAPI Device Driver for Windows NT =

Article ID: 182532

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 4.0 Developer Edition

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This article was previously published under Q182532



SUMMARY
Windows NT Service Pack 2 includes a new Windows NT device driver for direct memory access (DMA)-capable ATAPI devices. Using this new driver in Windows NT improves CPU utilization over standard PIO, increasing system usability while IDE operations are in progress.



MORE INFORMATION
Using PCI bus master DMA, the CPU has to service only a single interrupt that is generated when the IDE command completes. This is compared to standard PIO, where the CPU has to service an interrupt from the device, every time a block of data becomes available to the host. Performance figures on a checked build showed approximately 20 to 30 percent CPU utilization using DMA, versus approximately 90 to 100 percent utilization during PIO transfers. Performance figures for DMA on a free build are even better, showing single-digit percentages in some cases.

DMA Requirements
Registry keys must be added to enable direct memory access (DMA). The levels of DMA support include DMA detection level zero (disabled; this is the default) and DMA detection level one. The following criteria must be met before the driver will allow support for DMA detection level one:


 * PCI command register bit two (Master Enable) is set.
 * PCI device Class code is one.
 * PCI device Subclass code is one.
 * Programmer's interface bit seven is set.
 * Busmaster status register bits five and six are set. Bit 5 is set for the master and bit 6 is set for the slave if the device exists. These bits should be set by the BIOS, but many system BIOSs currently do not do this.

Determining DMA Status
Whether DMA is enabled can be determined by looking in the registry under:

(where x represents the IDE channel; zero is the primary channel, one is the secondary channel).

Notice that the initiator ID is always 255 on IDE/ATAPI channels. There will be a DWORD value named DMAEnabled that will be set to zero if DMA is not active, and one, if DMA is active.

Enabling DMA Detection Level 1
The ATAPI driver can be set to enable DMA use on a per-channel basis. Under the current SCSIPORT model, it is not possible to use DMA on one device and PIO on another if the devices are on the same channel.

Enabling DMA use can be done by adding the following keys under:

NOTE: The above registry key is one path; it has been wrapped for readability.

When the keys have been created, you must create a DriverParameter STRING value and set it to DMADetectionLevel = 0x1.

Disabling DMA Detection Level Zero
The ATAPI driver can be set to disable DMA use on a per-channel basis. This can be done by adding the following keys under:

NOTE: The above registry key is one path; it has been wrapped for readability.

When you create the keys, you must also create a DriverParameter STRING value and set it to DMADetectionLevel = 0x0.

Additional query words: DMA ATAPI IDE

Keywords: kbhowto KB182532

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