Seagate ST380011AS, ST3120827AS, ST380817AS About the Serial ATA interface, Native Command Queuing

1.1About the Serial ATA interface

The Serial ATA interface provides several advantages over the traditional (parallel) ATA interface. The primary advantages include:

•Easy installation and configuration with true plug-and-play connectivity. It is not necessary to set any jump- ers or other configuration options.

•Thinner and more flexible cabling for improved enclosure airflow and ease of installation.

•Scalability to higher performance levels.

In addition, Serial ATA makes the transition from parallel ATA easy by providing legacy software support. Serial ATA was designed to allow you to install a Serial ATA host adapter and Serial ATA disc drive in your current system and expect all of your existing applications to work as normal.

The Serial ATA interface connects each disc drive in a point-to-point configuration with the Serial ATA host adapter. There is no master/slave relationship with Serial ATA devices like there is with parallel ATA. If two drives are attached on one Serial ATA host adapter, the host operating system views the two devices as if they were both “masters” on two separate ports. This essentially means both drives behave as if they are Device 0 (master) devices.

Note. The host adapter may, optionally, emulate a master/slave environment to host software where two devices on separate Serial ATA ports are represented to host software as a Device 0 (master) and Device 1 (slave) accessed at the same set of host bus addresses. A host adapter that emulates a master/slave environment manages two sets of shadow registers. This is not a typical Serial ATA environment.

The Serial ATA host adapter and drive share the function of emulating parallel ATA device behavior to provide backward compatibility with existing host systems and software. The Command and Control Block registers, PIO and DMA data transfers, resets, and interrupts are all emulated.

The Serial ATA host adapter contains a set of registers that shadow the contents of the traditional device regis- ters, referred to as the Shadow Register Block. All Serial ATA devices behave like Device 0 devices. For addi- tional information about how Serial ATA emulates parallel ATA, refer to the “Serial ATA: High Speed Serialized AT Attachment” specification. The specification can be downloaded from http://www.serialata.com.

1.2Native Command Queuing

Native Command Queuing (NCQ) is among the advanced features introduced in the Serial ATA II: Extensions to Serial ATA 1.0 Specification. NCQ is a powerful technology designed to increase performance and endur- ance by allowing the drive to internally optimize the execution order of workloads. Intelligent reordering of com- mands within the drive’s internal command queue helps improve performance of queued workloads by minimizing mechanical positioning latencies on the drive.

Operating systems such as Microsoft Windows and Linux are increasingly taking advantage of multi-threaded software or processor-based Hyper-Threading Technology. These features have a high potential to create workloads where multiple commands are outstanding to the drive at the same time. By utilizing NCQ, the potential disc performance is increased significantly for these workloads.

Native Command Queuing achieves high performance and efficiency through efficient command reordering. In addition, there are three new capabilities that are built into the Serial ATA protocol to enhance NCQ perfor- mance: racefree status return, interrupt aggregation, and First-Party DMA.

To learn more about NCQ, go to the Seagate Serial ATA resource site at:

www.seagate.com/products/interface/sata/.

2 Barracuda 7200.7 Serial ATA Product Manual, Rev. N