Microsoft KB Archive/103435

= Advantages of the Small Computer System Interface (SCSI) =

Article ID: 103435

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Advanced Server 3.1
 * Microsoft Windows NT Workstation 3.1

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This article was previously published under Q103435



SUMMARY
The following are advantages of the small computer system interface (SCSI):


 * 1) Unlike other interfaces, when you interface with different device types using SCSI, the interfacing is done through the same cable. In a non-SCSI environment, devices such as a proprietary tape controller, disk controller, and so on, must be used to connect their respective devices to the system bus.
 * 2) SCSI peripheral devices of the same type have similar characteristics (this makes it easy to replace old devices with new ones).
 * 3) SCSI peripheral devices are intelligent and independent: a controller is built onto each SCSI device. This allows the computer to do other work.
 * 4) SCSI I/O is independent of the system bus. This allows peripheral devices to work with different computer types, which preserves a company's hardware investment.
 * 5) SCSI is fast (10 megabytes (MB)/second on 8 bit bus, 20 MB/second on 16 bit bus).
 * 6) Multi-threaded operating systems, such as Windows NT, can take full advantage of the multi-tasking capabilities of the SCSI bus.

For example, when a Windows NT thread requests to read a logical block on SCSI disk 1, and at the same time, a second thread requests to write some data to SCSI disk 2, the following may occur:


 * The SCSI host adapter will process the first request made by Windows NT executive by arbitrating the SCSI bus and making a connection to disk 1.
 * After the connection is made, disk 1 will disconnect and give up the bus (bus free) so that other requests can be made by the host.
 * The first thread will stop executing and will wait while the slow I/O device completes a data transfer.
 * As the seek is carried out on disk 1, the second thread request will be processed in the same manner as the first, because Windows NT can issue a "context switch" to allow for a thread of execution while another is still being completed.
 * Because the bus is free at this time, the host (initiator) will be able to make a connection with disk 2. Disk 2 will then disconnect and perform a write of some data to a logical location on the disk. At the same time, disk 1 may still be seeking the block to read. The two devices are therefore performing a task ( read, write) simultaneously.

Additional query words: prodnt

Keywords: kbhardware KB103435

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