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Parallel ATA (PATA) is an interface standard for the connection of storage devices such as hard disks, solid-state drives, and CD-ROM drives in computers. The standard is maintained by X3/INCITS committee. It uses the underlying AT Attachment and AT Attachment Packet Interface (ATA/ATAPI) standards.


The current Parallel ATA standard is the result of a long history of incremental technical development. ATA/ATAPI is an evolution of the AT Attachment Interface, which was itself evolved in several stages from Western Digital's original Integrated Drive Electronics (IDE) interface. As a result, many near-synonyms for ATA/ATAPI and its previous incarnations exist, including abbreviations such as IDE which are still in common informal use. After the market introduction of Serial ATA in 2003, the original ATA was retroactively renamed Parallel ATA.

Parallel ATA only allows cable lengths up to 18 in (460 mm). Because of this length limit the technology normally appears as an internal computer storage interface. For many years ATA provided the most common and the least expensive interface for this application. By the beginning of 2007, it had largely been replaced by Serial ATA (SATA) in new systems.



History and terminology


The name of the standard was originally conceived as "PC/AT Attachment" as its primary feature was a direct connection to the 16-bit ISA bus introduced with the IBM PC/AT. The name was shortened to "AT Attachment" to avoid possible trademark issues. It is not spelled out as "Advanced Technology" anywhere in current or recent versions of the specification; it is simply "AT Attachment".


IDE and ATA-1


The first version of what is now called the ATA/ATAPI interface was developed by Western Digital under the name Integrated Drive Electronics (IDE). Together with Control Data Corporation (who manufactured the hard drive part) and Compaq Computer (into whose systems these drives would initially go), they developed the connector, the signalling protocols, and so on with the goal of remaining software compatible with the existing ST-506 hard drive interface. The first such drives appeared in Compaq PCs in 1986.

The term Integrated Drive Electronics (IDE) refers not just to the connector and interface definition, but also to the fact that the drive controller is integrated into the drive, as opposed to a separate controller on or connected to the motherboard. The integrated controller presented the drive to the host computer as an array of 512-byte blocks with a relatively simple command interface. This relieved the software in the host computer of the chores of stepping the disk head arm, moving the head arm in and out, and so on, as had to be done with earlier ST-506 and ESDI hard drives. All of these low-level details of the mechanical operation of the drive were now handled by the controller on the drive itself. This also eliminated the need to design a single controller that could handle many different types of drives, since the controller could be unique for the drive. The host need only ask for a particular sector, or block, to be read or written, and either accept the data from the drive or send the data to it.

The interface used by these IDE drives was standardized in 1994 as ANSI standard X3.221-1994, AT Attachment Interface for Disk Drives. After later versions of the standard were developed, this became known as "ATA-1".


The new ATA interface


Originally, there was only one ATA controller in early PCs, which could support up to two hard drives. At the time in combination with the floppy drive, this was sufficient for most people, and eventually it became common to have two hard drives installed. When the CDROM was developed, many computers were unable to accept them due to already having two hard drives installed. Adding the CDROM would have required removal of one of the drives.

SCSI was available as a CDROM expansion option at the time, but devices with SCSI were more expensive than ATA devices due to the need for a smart controller that is capable of bus arbitration. SCSI typically added US$ 100-300 to the cost of a storage device, in addition to the cost of a SCSI controller.

The less-expensive solution was the addition of the second ATA interface, typically included as an expansion option on a sound card. It was included on the sound card because early business PCs did not include support for more than simple beeps from the internal speaker, and tuneful sound playback was considered unnecessary for early business software. When the CDROM was introduced, it was logical to also add digital audio to the computer at the same time. An older business PC could be upgraded in this manner to meet the Multimedia PC standard for early software packages that used sound and colorful video animation.

The second drive interface initially was not well-defined. It was first introduced with modified controller interfaces specific to certain CDROM drives such as Mitsumi, Sony or Panasonic, and it was common to find early sound cards with two or three separate connectors each designed to match a certain brand of CDROM drive. This evolved into the standard ATA interface for ease of cross-compatibility, though the sound card ATA interface still usually supported only a single CDROM and not hard drives.

This second ATA interface on the sound card eventually evolved into the second motherboard ATA interface which was long included as a standard component in all PCs. For a long period of time, ATA ruled as the primary storage device interface and in some systems a third and fourth motherboard interface was provided (Promise Ultra-100), for up to eight ATA devices attached to the motherboard.

After the introduction of SATA or Serial ATA, use of ATA declined and motherboards began to be shipped with only a single interface, for up to two ATA optical drives, along with two or more SATA connectors for hard drives. Optical drives are now available with SATA, so the ATA interface often goes unused.

EIDE and ATA-2

In 1994, about the same time that the ATA-1 standard was adopted, Western Digital introduced drives under a slightly new name, Enhanced IDE (EIDE). These included most of the features of the forthcoming ATA-2 specification and several additional enhancements. Other manufacturers introduced their own variations of ATA-1 such as "Fast ATA" and "Fast ATA-2".

The new version of the ANSI standard, AT Attachment Interface with Extensions ATA-2 (X3.279-1996), was approved in 1996. It included most of the features of the manufacturer-specific variants.

ATA-2 also was the first to note that devices other than hard drives could be attached to the interface:

3.1.7 Device: Device is a storage peripheral. Traditionally, a device on the ATA interface has been a hard disk drive, but any form of storage device may be placed on the ATA interface provided it adheres to this standard.



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As mentioned in the previous sections ATA was originally designed for and worked only with hard disks and devices that could emulate them. The introduction of ATAPI (ATA Packet Interface) by a group called the Small Form Factor committee allowed ATA to be used for a variety of other devices that require functions beyond those necessary for hard disks. For example, any removable media device needs a "media eject" command, and a way for the host to determine whether the media is present, and these were not provided in the ATA protocol.

The Small Form Factor committee approached this problem by defining ATAPI, the "ATA Packet Interface". ATAPI is actually a protocol allowing the ATA interface to carry SCSI commands and responses; therefore all ATAPI devices are actually "speaking SCSI" other than at the electrical interface. In fact, some early ATAPI devices were simply SCSI devices with an ATA/ATAPI to SCSI protocol converter added on. The SCSI commands and responses are embedded in "packets" (hence "ATA Packet Interface") for transmission on the ATA cable. This allows any device class for which a SCSI command set has been defined to be interfaced via ATA/ATAPI.

ATAPI devices are also "speaking ATA", as the ATA physical interface and protocol are still being used to send the packets. On the other hand, ATA hard drives and solid state drives do not use ATAPI.

ATAPI devices include CD-ROM and DVD-ROM drives, tape drives, and large-capacity floppy drives such as the Zip drive and SuperDisk drive.

The SCSI commands and responses used by each class of ATAPI device (CD-ROM, tape, etc.) are described in other documents or specifications specific to those device classes and are not within ATA/ATAPI or the T13 committee's purview.

ATAPI was adopted as part of ATA in INCITS 317-1998, AT Attachment with Packet Interface Extension (ATA/ATAPI-4).


UDMA and ATA-4


The ATA/ATAPI-4 also introduced several "Ultra DMA" transfer modes. These initially supported speeds from 16 MByte/s to 33 MByte/second. In later versions faster Ultra DMA modes were added, requiring a new 80-wire cable to reduce crosstalk. The latest versions of Parallel ATA support up to 133 MByte/s.

Current terminology

The terms "integrated drive electronics" (IDE), "enhanced IDE" and "EIDE" have come to be used interchangeably with ATA (now Parallel ATA). However the terms "IDE" and "EIDE" are at best imprecise. Every ATA drive is an "integrated drive electronics" drive, but SCSI drives could also legitimately be described as having "integrated drive electronics". However the abbreviation IDE is rarely if ever used for SCSI drives.

In addition there have been several generations of "EIDE" drives marketed, compliant with various versions of the ATA specification. An early "EIDE" drive might be compatible with ATA-2, while a later one with ATA-6.

Nevertheless a request for an "IDE" or "EIDE" drive from a computer parts vendor will almost always yield a drive that will work with modern systems' ATA interfaces.

Another common usage is to refer to the specification version by the fastest mode supported. For example, ATA-4 supported Ultra DMA modes 0 through 2, the latter providing a maximum transfer rate of 33 megabytes per second. ATA-4 drives are thus sometimes called "UDMA-33" drives, and sometimes "ATA-33" drives. Similarly, ATA-6 introduced a maximum transfer speed of 100 megabytes per second, and some drives complying to this version of the standard are marketed as "PATA/100" drives.

Drive size limitations

The original ATA specification used a 28-bit addressing mode, allowing for the addressing of 228 (268,435,456) sectors (blocks) of 512 bytes each, resulting in a maximum capacity of about 137 gigabytes. (This is displayed by Windows operating systems as "128 GB".) The BIOS in early PCs imposed smaller limits such as 8.46 GB, with a maximum of 1024 cylinders, 256 heads and 63 sectors, but this was not a limit imposed by the ATA interface.

ATA-6 introduced 48-bit addressing, increasing the limit to 144 petabytes. As a consequence, any ATA drive of capacity larger than 137 gigabytes must be an ATA-6 or later drive. Connecting such a drive to a host with an ATA-5 or earlier interface will limit the usable capacity to the maximum of the controller.

Some OSs, including Windows 2000 pre-SP4 and Windows XP pre-SP 1, disable 48-bit LBA by default, requiring the user to take extra steps to use the entire capacity of an ATA drive larger than 137 gigabytes.


Parallel ATA interface

Ribbon cable with two connectors

Until the introduction of Serial ATA, 40-pin connectors generally attached drives to a ribbon cable. Each cable has two or three connectors, one of which plugs into an adapter interfacing with the rest of the computer system. The remaining connector(s) plug into drives. Parallel ATA cables transfer data 16 bits at a time.

ATA's ribbon cables have had 40 wires for most of its history (44 conductors for the smaller form-factor version used for 2.5" drives - the extra four for power), but an 80-wire version appeared with the introduction of the Ultra DMA/33 (UDMA) mode. All of the additional wires in the new cable are ground wires, interleaved with the previously defined wires to reduce the effects of capacitive coupling between neighboring signal wires, reducing crosstalk. Capacitive coupling is more of a problem at higher transfer rates, and this change was necessary to enable the 66 megabytes per second (MB/s) transfer rate of UDMA4 to work reliably. The faster UDMA5 and UDMA6 modes also require 80-conductor cables.

ATA cables


Though the number of wires doubled, the number of connector pins and the pinout remain the same as 40-conductor cables, and the external appearance of the connectors is identical. Internally the connectors are different; the connectors for the 80-wire cable connect a larger number of ground wires to a smaller number of ground pins, while the connectors for the 40-wire cable connect ground wires to ground pins one-for-one. 80-wire cables usually come with three differently colored connectors (blue — controller, gray — slave drive, and black — master drive) as opposed to uniformly colored 40-wire cable's connectors (all black). The gray connector on 80-conductor cables has pin 28 CSEL not connected; making it the slave position for drives configured cable select.


Multiple devices on a cable


If two devices attach to a single cable, one must be designated as device 0 (commonly referred to as master) and the other as device 1 (slave). This distinction is necessary to allow both drives to share the cable without conflict. The master drive is the drive that usually appears "first" to the computer's BIOS and/or operating system. On old BIOSes (Intel 486 era and older), the drives are often referred to by the BIOS as "C" for the master and "D" for the slave following the way DOS would refer to the active primary partitions on each.

The mode that a drive must use is often set by a jumper setting on the drive itself, which must be manually set to master or slave. If there is a single device on a cable, it should be configured as master. However, some hard drives have a special setting called single for this configuration (Western Digital, in particular). Also, depending on the hardware and software available, a single drive on a cable can work reliably even though configured as the slave drive (this configuration is most often seen when a CD ROM has a channel to itself).



Cable select


A drive mode called cable select was described as optional in ATA-1 and has come into fairly widespread use with ATA-5 and later. A drive set to "cable select" automatically configures itself as master or slave, according to its position on the cable. Cable select is controlled by pin 28. The host adapter grounds this pin; if a device sees that the pin is grounded, it becomes the master device; if it sees that pin 28 is open, the device becomes the slave device.

This setting is usually chosen by a jumper setting on the drive called "cable select", usually marked CS, which is separate from the "master" or "slave" setting.

Note that if two drives are configured as master and slave manually, this configuration does not need to correspond to their position on the cable. Pin 28 is only used to let the drives know their position on the cable; it is not used by the host when communicating with the drives.

With the 40-wire cable it was very common to implement cable select by simply cutting the pin 28 wire between the two device connectors; putting the slave device at the end of the cable, and the master on the middle connector. This arrangement eventually was standardized in later versions. If there is just one device on the cable, this results in an unused stub of cable, which is undesirable for physical convenience and electrical reasons. The stub causes signal reflections, particularly at higher transfer rates.

Starting with the 80-wire cable defined for use in ATAPI5/UDMA4, the master device goes at the end of the 18-inch (460 mm) cable--the black connector--and the slave device goes on the middle connector--the gray one--and the blue connector goes onto the motherboard. So, if there is only one (master) device on the cable, there is no cable stub to cause reflections. Also, cable select is now implemented in the slave device connector, usually simply by omitting the contact from the connector body.


Master and slave clarification


Although they are in common use, the terms master and slave do not actually appear in current versions of the ATA specifications. The two devices are correctly referred to as device 0 (master) and device 1 (slave), respectively. It is a common myth that the controller on the master drive assumes control over the slave drive, or that the master drive may claim priority of communication over the other device on the channel. In fact, the drivers in the host operating system perform the necessary arbitration and serialization, and each drive's controller operates independently.

The terms "master" and "slave" have not been without controversy. In 2003, the County of Los Angeles, California, US demanded that suppliers stop using the terms because the county found them unacceptable in light of its "cultural diversity and sensitivity."


Serialized, overlapped, and queued operations


The parallel ATA protocols up through ATA-3 require that once a command has been given on an ATA interface, it must complete before any subsequent command may be given. Operations on the devices must be serialized—with only one operation in progress at a time—with respect to the ATA host interface. A useful mental model is that the host ATA interface is busy with the first request for its entire duration, and therefore can not be told about another request until the first one is complete. The function of serializing requests to the interface is usually performed by a device driver in the host operating system.

The ATA-4 and subsequent versions of the specification have included an "overlapped feature set" and a "queued feature set" as optional features. However, support for these is extremely rare in actual parallel ATA products and device drivers. By contrast, overlapped and queued operations have been common in other storage buses. In particular, tagged command queuing is characteristic of SCSI; this has long been seen as a major advantage of SCSI.

The Serial ATA standard has supported native command queueing since its first release, but it is an optional feature for both host-adapters and target-devices. Many less expensive PC motherboards do not support NCQ. Nearly all SATA/II hard drives sold today support NCQ, while very few removable (CD/DVD) drives do.

Two devices on one cable — speed impact

There are many debates about how much a slow device can impact the performance of a faster device on the same cable. There is an effect, but the debate is confused by the blurring of two quite different causes, called here "Lowest speed" and "One operation at a time".


"Lowest speed"


It is a common misconception that, if two devices of different speed capabilities are on the same cable, both devices' data transfers will be constrained to the speed of the slower device.

For all modern ATA host adapters this is not true, as modern ATA host adapters support independent device timing. This allows each device on the cable to transfer data at its own best speed. Even with older adapters without independent timing, this effect only applies to the data transfer phase of a read or write operation. This is usually the shortest part of a complete read or write operation.


"One operation at a time"


This is caused by the omission of both overlapped and queued feature sets from most parallel ATA products. Only one device on a cable can perform a read or write operation at one time, therefore a fast device on the same cable as a slow device under heavy use will find it has to wait for the slow device to complete its task first.

However, most modern devices will report write operations as complete once the data is stored in its onboard cache memory, before the data is written to the (slow) magnetic storage. This allows commands to be sent to the other device on the cable, reducing the impact of the "one operation at a time" limit.

The impact of this on a system's performance depends on the application. For example, when copying data from an optical drive to a hard drive (such as during software installation), this effect probably doesn't matter: Such jobs are necessarily limited by the speed of the optical drive no matter where it is. But if the hard drive in question is also expected to provide good throughput for other tasks at the same time, it probably should not be on the same cable as the optical drive.


HDD Passwords and Security


The disk lock is a built-in security feature in the disk. It is part of the ATA specification, and thus not specific to any brand or device.

A disk always has two passwords: A User password and a Master password. Most disks support a Master Password Revision Code. Reportedly some disks can report if the Master password has been changed, or if it still the factory default. The revision code is word 92 in the IDENTIFY response. Reportedly on some disks a value of 0xFFFE means the Master password is unchanged. The standard does not distinguish this value.

A disk can be locked in two modes: High security mode or Maximum security mode. Bit 8 in word 128 of the IDENTIFY response shows which mode the disk is in: 0 = High, 1 = Maximum.

In High security mode, the disk can be unlocked with either the User or Master password, using the "SECURITY UNLOCK DEVICE" ATA command. There is an attempt limit, normally set to 5, after which the disk must be power cycled or hard-reset before unlocking can be attempted again. Also in High security mode the SECURITY ERASE UNIT command can be used with either the User or Master password.

In Maximum security mode, the disk cannot be unlocked without the User password - the only way to get the disk back to a usable state is to issue the SECURITY ERASE PREPARE command, immediately followed by SECURITY ERASE UNIT. In Maximum security mode the SECURITY ERASE UNIT command requires the User password and will completely erase all data on the disk. The operation is slow, it may take longer than half an hour or more, depending on the size of the disk. (Word 89 in the IDENTIFY response indicates how long the operation will take.)


External Parallel ATA devices


It is extremely uncommon to find external PATA devices that directly use the interface for connection to a computer. PATA is primarily restricted to devices installed internally, due to the short data cable specification. A device connected externally needs additional cable length to form a U-shaped bend so that the external device may be placed alongside, or on top of the computer case, and the standard cable length is too short to permit this.

For ease of reach from motherboard to device, the connectors tend to be positioned towards the front edge of motherboards, for connection to devices protruding from the front of the computer case. This front-edge position makes extension out the back to an external device even more difficult. Ribbon cables are poorly shielded, and the standard relies upon the cabling to be installed inside a shielded computer case to meet RF emissions limits.

All external PATA devices, such as external hard drives, use some other interface technology to bridge the distance between the external device and the computer. USB is the most common external interface, followed by Firewire. A bridge chip inside the external devices converts from the USB interface to PATA, and typically only supports a single external device without cable select or master/slave.

A side effect of the PATA bridge chip is that most bridges do not properly support PATA device idle and power save, which causes external hard drives to spin continuously even when the connected computer is in standby or turned off. This can result in shortened hard drive lifespan due to continuous operation at all times when the external device is powered.


ATA standards versions, transfer rates, and features


The following table shows the names of the versions of the ATA standards and the transfer modes and rates supported by each. Note that the transfer rate for each mode (for example, 66.7 MB/s for UDMA4, commonly called "Ultra-DMA 66") gives its maximum theoretical transfer rate on the cable. This is simply two bytes multiplied by the effective clock rate, and presumes that every clock cycle is used to transfer end-user data. In practice, of course, protocol overhead reduces this value.

Congestion on the host bus to which the ATA adapter is attached may also limit the maximum burst transfer rate. For example, the maximum data transfer rate for conventional PCI bus is 133 MB/s, and this is shared among all active devices on the bus.

In addition, no ATA hard drives existed in 2005 that were capable of measured sustained transfer rates of above 80 MB/s. Furthermore, sustained transfer rate tests do not give realistic throughput expectations for most workloads: They use I/O loads specifically designed to encounter almost no delays from seek time or rotational latency. Hard drive performance under most workloads is limited first and second by those two factors; the transfer rate on the bus is a distant third in importance. Therefore, transfer speed limits above 66 MB/s really affect performance only when the hard drive can satisfy all I/O requests by reading from its internal cache — a very unusual situation, especially considering that such data is usually already buffered by the operating system.

As of April 2009 mechanical hard disk drives can transfer data at up to 131 MB/s, which is within the capabilities of the older PATA/133 specification. However, high-performance flash drives can transfer data at up to 201 MB/s.

Only the Ultra DMA modes use CRC to detect errors in data transfer between the controller and drive. This is a 16 bit CRC, and it is used for data blocks only. Transmission of command and status blocks do not use the fast signaling methods that would necessitate CRC. For comparison, in Serial ATA, 32 bit CRC is used for both commands and data.



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