Editing Native Command Queuing (section)

== History ==
Native Command Queuing was preceded by [[Parallel ATA]]'s version of [[Tagged Command Queuing]] (TCQ). ATA's attempt at integrating TCQ was constrained by the requirement that ATA host bus adapters use [[Industry Standard Architecture|ISA]] bus device protocols to interact with the [[operating system]]. The resulting high CPU overhead and negligible performance gain contributed to a lack of market acceptance for ATA TCQ.

NCQ differs from TCQ in that, with NCQ, each command is of equal importance, but NCQ's host bus adapter also programs its own [[Direct memory access#Bus mastering|first party DMA]] engine with CPU-given [[Direct memory access|DMA]] parameters during its command sequence whereas TCQ interrupts the CPU during command queries and requires it to modulate the ATA host bus adapter's [[Direct memory access#Third-party|third party DMA]] engine. NCQ's implementation is preferable because the drive has more accurate knowledge of its performance characteristics and is able to account for its rotational position. Both NCQ and TCQ have a maximum queue length of 32 outstanding commands.<ref name = "sata">[https://web.archive.org/web/20101218131508/http://seagate.com/content/pdf/whitepaper/D2c_tech_paper_intc-stx_sata_ncq.pdf PDF white paper on NCQ from Intel and Seagate]</ref><ref name = "pata-7Vol1">{{Cite web |url=http://www.t13.org/Documents/UploadedDocuments/docs2004/d1532v1r4b-ATA-ATAPI-7.pdf |title=Volume 1 of the final draft of the ATA-7 standard |access-date=2007-01-02 |archive-date=2012-02-06 |archive-url=https://web.archive.org/web/20120206032939/http://www.t13.org/Documents/UploadedDocuments/docs2004/d1532v1r4b-ATA-ATAPI-7.pdf |url-status=dead }}</ref> Because the ATA TCQ is rarely used, [[Parallel ATA]] (and the IDE mode of some chipsets) usually only support one outstanding command per port.

For NCQ to be enabled, it must be supported and enabled in the SATA host bus adapter and in the hard drive itself. The appropriate driver must be loaded into the operating system to enable NCQ on the host bus adapter.<ref>[http://download.intel.com/support/chipsets/imsm/sb/sata2_ncq_overview.pdf "SATA II Native Command Queuing Overview", Intel Whitepaper, April 2003. ]</ref>

Many newer chipsets support the [[Advanced Host Controller Interface]] (AHCI), which allows operating systems to universally control them and enable NCQ.  [[DragonFly BSD]] has supported AHCI with NCQ since 2.3 in 2009.<ref>{{cite web |author= Matthew Dillon |author-link= Matthew Dillon |date= 2009-06-04 |url= http://www.dragonflybsd.org/mailarchive/kernel/2009-06/msg00004.html |title= "Re: DragonFly-2.3.1.165.g25822 master sys/dev/disk/ahci Makefile TODO ahci.c ahci.h ahci_attach.c ahci_cam.c ahci_dragonfly.c ahci_dragonfly.h atascsi.h" }}</ref><ref>{{cite web |author= Matthew Dillon |author-link= Matthew Dillon |date= 2009 |url= http://bxr.su/d/share/man/man4/ahci.4 |title= ahci(4) — Advanced Host Controller Interface for Serial ATA |website= BSD Cross Reference |publisher= [[DragonFly BSD]]}}
*{{cite book |section=ahci - Advanced Host Controller Interface for Serial ATA |title=DragonFly On-Line Manual Pages |url=http://mdoc.su/d/ahci.4 }}</ref> [[Linux kernel]]s support AHCI natively since version 2.6.19, and [[FreeBSD]] fully supports AHCI since version 8.0. [[Windows Vista]] and [[Windows 7]] also natively support AHCI, but their AHCI support (via the msahci service) must be manually enabled via registry editing if controller support was not present during their initial install. [[Windows 7]]'s AHCI enables not only NCQ but also [[Trim (computing)|TRIM]] support on [[SSD]] drives (with their supporting [[firmware]]).  Older operating systems such as [[Windows XP]] require the installation of a vendor-specific driver (similar to installing a [[RAID]] or [[SCSI]] controller) even if AHCI is present on the host bus adapter, which makes initial setup more tedious and conversions of existing installations relatively difficult as most controllers cannot operate their ports in mixed AHCI–SATA/IDE/legacy mode.