Microsoft KB Archive/251369

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INFO: SCSI Pass Through Functionality and Limitations

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Q251369

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The information in this article applies to:


 * Microsoft Windows NT 4.0 Driver Development Kit (DDK)
 * Microsoft Windows 2000 Driver Development Kit (DDK)

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SUMMARY
On Windows NT and Windows 2000, the interface used in an application to send SCSI commands to a SCSI device is called SCSI Pass Through (SPT). A Win32 program uses DeviceIoControl with the IoControlCode of IOCTL_SCSI_PASS_THROUGH or IOCTL_SCSI_PASS_THROUGH_DIRECT. This article explains some of the functionality and limitations of the SCSI Pass Through interface.

Synchronous Requests
The SPT requests are always synchronous. Even if the caller has specified overlapped I/O (FILE_FLAG_OVERLAPPED) and passed the overlapped structure to the DeviceIoControl call, the ScsiPort driver will block the SPT request until the SCSI device completes it.

When completed by the SCSI device (success or error), ScsiPort will complete the SPT request and the calling thread will be unblocked.

Untagged Queuing
A SCSI command can be send to the SCSI device as either untagged or tagged. Untagged queuing permits a device to accept commands from an initiator while executing commands from another. Tagged queuing gives the device the ability to accept a series of commands from the same or different initiators. The SPT always uses untagged queuing while sending commands to the device.