AT Attachment
Encyclopedia
Parallel ATA originally ATA, is an interface
standard
for the connection of storage
devices such as hard disk
s, solid-state drive
s, floppy drives
, and optical disc drives in computer
s. The standard is maintained by X3/INCITS
committee. It uses the underlying AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) standards.
The Parallel ATA standard is the result of a long history of incremental technical development, which began with the original AT Attachment interface, developed for use in early PC AT equipment. The ATA interface 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 are still in common informal use. After the introduction of Serial ATA
in 2003, the original ATA was renamed
Parallel ATA, PATA for short.
Parallel ATA cables have a maximum allowable length of only 18 inch. Because of this 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. It has largely been replaced by Serial ATA (SATA)
in newer systems.
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 signaling 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 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 interface cards used to connect a parallel ATA drive to, for example, a PCI slot are not drive controllers, they are merely bridges between the host bus and the ATA interface. Since the original ATA interface is essentially just a 16-bit ISA bus in disguise, the bridge was especially simple in case of an ATA connector being located on an ISA interface card. 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 mainboard and interface cards 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 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".
A short-lived, seldom-used implementation of ATA was created for the IBM XT and similar machines that used the 8-bit version of the ISA bus. It has been referred to as "XTA" or "XT Attachment."
When PC motherboard makers started to include onboard ATA interfaces in place of the earlier ISA plug-in cards, there was usually only one ATA connector on the board, 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 CD-ROM
was developed, many computers would have been unable to accept these drives if they had been ATA devices, due to already having two hard drives installed. Adding the CD-ROM drive would have required removal of one of the drives.
SCSI
was available as a CD-ROM expansion option at the time, but devices with SCSI were more expensive than ATA devices due to the need for a smart interface 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 host adapter.
The less-expensive solution was the addition of a dedicated CD-ROM 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 CD-ROM was introduced, it was logical to also add digital audio to the computer at the same time (for the same reason, sound cards tended to include a gameport interface for joysticks). An older business PC could be upgraded in this manner to meet the Multimedia PC
standard for early software packages that used sound (which required the sound card) and colorful video animation (which required the CD-ROM as floppy disks simply did not have the necessary data capacity).
The second drive interface initially was not well-defined. It was first introduced with interfaces specific to certain CD-ROM drives such as Mitsumi, Sony or Panasonic drives, and it was common to find early sound cards with two or three separate connectors each designed to match a certain brand of CD-ROM 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 CD-ROM 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.
Called the "primary" and "secondary" ATA interfaces, they were assigned to base addresses
0x1F0 and 0x170 on ISA bus systems.
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:
(SFF) 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 drive
s, 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. One commonly used set is defined in the MMC
SCSI command set.
ATAPI was adopted as part of ATA in INCITS 317-1998, AT Attachment with Packet Interface Extension (ATA/ATAPI-4).
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 most Parallel 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.
Due to a lack of foresight by motherboard manufacturers, the system BIOS was often hobbled by artificial C/H/S size limitations due to the manufacturer assuming certain values would never exceed a certain numerical maximum.
The first of these BIOS limits occurred when ATA drives reached sizes in excess of . Because some motherboard BIOS would not allow C/H/S values above , , and . Multiplied by 512 bytes per sector, this totals which divided by per megabyte, equals 504 megabytes.
The second of these BIOS limitations occurred at 1024 cylinders, 256 heads, and 63 sectors, but a bug in MS-DOS
and MS-Windows 95
limit the number heads to 255. This totals to , commonly referred to as the 8.4 gigabyte barrier. This is also a limit imposed by x86 BIOSes, and not a limit imposed by the ATA interface.
It was eventually determined that these size limitations could be overridden with a tiny program loaded at startup from a hard drive's boot sector. Some hard drive manufacturers such as Western Digital started including these override utilities with new large hard drives to help overcome these problems. However, if the computer were booted in some other manner without loading the special utility, the invalid BIOS settings would be used, and the drive could either be inaccessible or could appear to be damaged to the operating system.
Later an extension to the x86 BIOS disk services called the "Extended Disk Drive" (EDD) were made available which makes it possible to address drives as large as .
(137 GB
).
ATA-6 introduced 48-bit addressing, increasing the limit to 128 PiB
(144 PB
). As a consequence, any ATA drive of capacity larger than about 137 gigabyte
s 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 interface.
Some operating systems, including Windows XP pre-SP 1, and Windows 2000, disable 48-bit LBA
by default, requiring the user to take extra steps to use the entire capacity of an ATA drive larger than about 137 gigabyte
s. Older operating systems, such as Windows 98, do not support 48-bit LBA at all.
After the introduction of SATA (Serial ATA
), use of Parallel ATA declined, and new motherboards had only a single PATA connector, for up to two PATA devices- typically optical drives- along with (typically) six or more SATA connectors for hard drives and other devices. In new computers, the parallel ATA interface is rarely used, and several PC chipset
s have removed support for PATA, and motherboard vendors still wishing to offer ATA with those chipsets must include an additional interface chip.
Parallel ATA cables transfer data 16 bits at a time. The traditional cable uses 40-pin connectors attached 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.
ATA's 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.
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, black, and gray for controller, master drive, and slave drive respectively) as opposed to uniformly colored 40-wire cable's connectors (commonly all gray). The gray connector on 80-conductor cables has pin 28 CSEL not connected, making it the slave position for drives configured cable select.
Round parallel ATA cables (as opposed to ribbon cables) were eventually made available for 'case modders' for cosmetic reasons, as well as claims of improved computer cooling and were easier to handle; however, only ribbon cables are supported by the ATA specifications.
Pin 20
In the ATA standard pin 20 is defined as (mechanical) key and is not used. This socket on the female connector is often obstructed, requiring pin 20 to be omitted from the male cable or drive connector, making it impossible to plug it in the wrong way round; a male connector with pin 20 present cannot be used. However, some flash memory
drives can use pin 20 as VCC_in to power the drive without requiring a special power cable; this feature can only be used if the equipment supports this use of pin 20.
Pin 28
Pin 28 of the gray (slave/middle) connector of an 80 conductor cable is not attached to any conductor of the cable. It is attached normally on the black (master drive end) and blue (motherboard end) connectors.
Pin 34
Pin 34 is connected to ground inside the blue connector of an 80 conductor cable but not attached to any conductor of the cable. It is attached normally on the gray and black connectors. See page 315 of.
The image shows PATA connectors after removal of strain relief, cover, and cable. Pin one is at bottom left of the connectors, pin 2 is top left, etc., except that the lower image of the blue connector shows the view from the opposite side, and pin one is at top right.
Each contact comprises a pair of points which together pierce the insulation of the ribbon cable with such precision that they make a connection to the desired conductor without harming the insulation on the neighboring wires. The center row of contacts are all connected to the common ground bus and attached to the odd numbered conductors of the cable. The top row of contacts are the even-numbered sockets of the connector (mating with the even-numbered pins of the receptacle) and attach to every other even-numbered conductor of the cable. The bottom row of contacts are the odd-numbered sockets of the connector (mating with the odd-numbered pins of the receptacle) and attach to the remaining even-numbered conductors of the cable.
Note the connections to the common ground bus from sockets 2 (top left), 19 (center bottom row), 22, 24, 26, 30, and 40 on all connectors. Also note (enlarged detail, bottom, looking from the opposite side of the connector) that socket 34 of the blue connector does not contact any conductor but unlike socket 34 of the other two connectors, it does connect to the common ground bus. On the gray connector, note that socket 28 is completely missing, so that pin 28 of the drive attached to the gray connector will be open. On the black connector, sockets 28 and 34 are completely normal, so that pins 28 and 34 of the drive attached to the black connector will be connected to the cable. Pin 28 of the black drive reaches pin 28 of the host receptacle but not pin 28 of the gray drive, while pin 34 of the black drive reaches pin 34 of the gray drive but not pin 34 of the host. Instead, pin 34 of the host is grounded.
The standard dictates color-coded connectors for easy identification by both installer and cable maker. All three connectors are different from one another. The blue (host) connector has the socket for pin 34 connected to ground inside the connector but not attached to any conductor of the cable. Since the old 40 conductor cables do not ground pin 34, the presence of a ground connection indicates that an 80 conductor cable is installed. The wire for pin 34 is attached normally on the other types and is not grounded. Installing the cable backwards (with the black connector on the system board, the blue connector on the remote device and the gray connector on the center device) will ground pin 34 of the remote device and connect host pin 34 through to pin 34 of the center device. The gray center connector omits the connection to pin 28 but connects pin 34 normally, while the black end connector connects both pins 28 and 34 normally.
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 will often work reliably even though configured as the slave drive (most often seen where a CD ROM has a channel to itself).
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 inches (457.2 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.
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 onboard controller operates independently of the other.
The terms "master" and "slave" have not been without controversy. In 2003, the County of Los Angeles, California
, US
requested that, when possible, suppliers stop using the terms because the county found them unacceptable in light of its "cultural diversity and sensitivity".
The ATA-4 and subsequent versions of the specification have included an "overlapped feature set" and a "queued feature set" as optional features, both being given the name "Tagged Command Queuing
", a reference to a set of features from SCSI which the ATA version attempts to emulate. However, support for these is extremely rare in actual parallel ATA products and device drivers because these feature sets were implemented in such a way as to maintain software compatibility with its heritage as originally an extension of the ISA bus. This implementation resulted in excessive CPU utilization which largely negated the advantages of command queuing. By contrast, overlapped and queued operations have been common in other storage buses, in particular, SCSI's version of tagged command queuing had no need to be software compatible with ISA's APIs, allowing it to attain high performance with low overhead on buses which supported first party DMA like PCI. 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. Many SATA/II hard drives sold today support NCQ, while no removable (CD/DVD) drives do because the ATAPI command set used to control them prohibits queued operations.
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.
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.
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 Master 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.)
While the ATA disk lock is intended to be impossible to defeat without a valid password, there are workarounds to unlock a drive. Many data recovery companies offer unlocking services, so while the disk lock will deter a casual attacker, it is not secure against a qualified adversary.
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.
The ATA connector specification does not include pins for supplying power to a CF device, so power is inserted into the connector from a separate source.
CF devices can be designated as master or slave on an ATA interface, though since most CF devices offer only a single socket, it is not necessary to offer this selection to end users.
Although CF can be hot pluggable with additional design methods, by default when wired directly to an ATA interface, it is not intended to be hot-pluggable.
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 are usually already buffered by the operating system.
As of April 2010 mechanical hard disk drives can transfer data at up to 157 MB/s, which is beyond the capabilities of the PATA/133 specification. High-performance flash drives
can transfer data at up to 308 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.
. These can be supported as bootable devices by a BIOS complying with the
ATAPI Removable Media Device BIOS Specification, originally developed by Compaq Computer Corporation
and Phoenix Technologies
. It specifies provisions in the BIOS
of a personal computer
to allow the computer to be bootstrapped
from devices such as Zip drives, Jaz drives, SuperDisk
(LS-120) drives, and similar devices.
These devices have removable media like floppy disk drives
, but capacities more commensurate with hard drives, and programming requirements unlike either. Due to limitations in the floppy controller interface most of these devices were ATAPI
devices, connected to one of the host computer's ATA interfaces, similarly to a hard drive or CD-ROM
device. However, existing BIOS standards did not support these devices. An ARMD-compliant BIOS allows these devices to be booted from and used under the operating system without requiring device-specific code in the OS.
A BIOS implementing ARMD allows the user to include ARMD devices in the boot search order. Usually an ARMD device is configured earlier in the boot order than the hard drive. Similarly to a floppy drive, if bootable media is present in the ARMD drive, the BIOS will boot from it; if not, the BIOS will continue in the search order, usually with the hard drive last.
There are two variants of ARMD, ARMD-FDD and ARMD-HDD. Originally ARMD caused the devices to appear as a sort of very large floppy drive, either the primary floppy drive device 00h or the secondary device 01h. Some operating systems required code changes to support floppy disks with capacities far larger than any standard floppy disk drive. Also, standard-floppy disk drive emulation proved to be unsuitable for certain high-capacity floppy disk drives such as Iomega Zip drives. Later the ARMD-HDD, ARMD-"Hard disk device", variant was developed to address these issues. Under ARMD-HDD, an ARMD device appears to the BIOS and the operating system as a hard drive.
protocol to carry ATA commands over Ethernet
instead of directly connecting them to a PATA host adapter. This permitted the established block protocol to be reused in storage area network
(SAN) applications.
Electrical connector
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
standard
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
for the connection of storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
devices such as hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
s, solid-state drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...
s, floppy drives
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, and optical disc drives in computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s. The standard is maintained by X3/INCITS
INCITS
The InterNational Committee for Information Technology Standards, or INCITS , is an ANSI-accredited forum of IT developers. It was formerly known as the X3 and NCITS....
committee. It uses the underlying AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) standards.
The Parallel ATA standard is the result of a long history of incremental technical development, which began with the original AT Attachment interface, developed for use in early PC AT equipment. The ATA interface itself evolved in several stages from Western Digital
Western Digital
Western Digital Corporation is one of the largest computer hard disk drive manufacturers in the world. It has a long history in the electronics industry as an integrated circuit maker and a storage products company. Western Digital was founded on April 23, 1970 by Alvin B...
's original Integrated Drive Electronics (IDE) interface. As a result, many near-synonyms for ATA/ATAPI and its previous incarnations are still in common informal use. After the introduction of Serial ATA
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
in 2003, the original ATA was renamed
Retronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
Parallel ATA, PATA for short.
Parallel ATA cables have a maximum allowable length of only 18 inch. Because of this 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. It has largely been replaced by Serial ATA (SATA)
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
in newer systems.
History and terminology
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 "AT" in "IBM PC/AT" is an initialism for "Advanced Technology," but that term does not appear in current or recent versions of the ATA specification; it is simply "AT Attachment". This name was chosen to avoid possible trademark issues.IDE and ATA-1
The first version of what is now called the ATA/ATAPI interface was developed by Western DigitalWestern Digital
Western Digital Corporation is one of the largest computer hard disk drive manufacturers in the world. It has a long history in the electronics industry as an integrated circuit maker and a storage products company. Western Digital was founded on April 23, 1970 by Alvin B...
under the name Integrated Drive Electronics (IDE). Together with Control Data Corporation
Control Data Corporation
Control Data Corporation was a supercomputer firm. For most of the 1960s, it built the fastest computers in the world by far, only losing that crown in the 1970s after Seymour Cray left the company to found Cray Research, Inc....
(who manufactured the hard drive part) and Compaq Computer
Compaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....
(into whose systems these drives would initially go), they developed the connector, the signaling protocols, and so on with the goal of remaining software compatible with the existing ST-506
ST-506
The ST-506 was the first 5.25 inch hard disk drive. Introduced in 1980 by Seagate Technology , it stored up to 5 megabytes after formatting and cost $1500. The similar 10 MB ST-412 was introduced in late 1981. Both used MFM encoding...
hard drive interface. The first such drives appeared in Compaq PCs in 1986.
The term Integrated Drive Electronics 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
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
. The interface cards used to connect a parallel ATA drive to, for example, a PCI slot are not drive controllers, they are merely bridges between the host bus and the ATA interface. Since the original ATA interface is essentially just a 16-bit ISA bus in disguise, the bridge was especially simple in case of an ATA connector being located on an ISA interface card. 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 mainboard and interface cards 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
ST-506
The ST-506 was the first 5.25 inch hard disk drive. Introduced in 1980 by Seagate Technology , it stored up to 5 megabytes after formatting and cost $1500. The similar 10 MB ST-412 was introduced in late 1981. Both used MFM encoding...
and ESDI
Enhanced Small Disk Interface
Enhanced Small Disk Interface was a disk interface designed by Maxtor Corporation in the early 1980s to be a follow-on to the ST-506 interface...
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 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".
A short-lived, seldom-used implementation of ATA was created for the IBM XT and similar machines that used the 8-bit version of the ISA bus. It has been referred to as "XTA" or "XT Attachment."
Second ATA interface
CD-ROM
A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
was developed, many computers would have been unable to accept these drives if they had been ATA devices, due to already having two hard drives installed. Adding the CD-ROM drive would have required removal of one of the drives.
SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
was available as a CD-ROM expansion option at the time, but devices with SCSI were more expensive than ATA devices due to the need for a smart interface 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 host adapter.
The less-expensive solution was the addition of a dedicated CD-ROM interface, typically included as an expansion option on a sound card
Sound card
A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...
. 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 CD-ROM was introduced, it was logical to also add digital audio to the computer at the same time (for the same reason, sound cards tended to include a gameport interface for joysticks). An older business PC could be upgraded in this manner to meet the Multimedia PC
Multimedia PC
The Multimedia PC, or MPC, was a recommended configuration for a PC with a CD-ROM drive. The standard was set and named by the "Multimedia PC Marketing Council", which was a working group of the Software Publishers Association . The MPMC comprised companies including Microsoft, Creative Labs,...
standard for early software packages that used sound (which required the sound card) and colorful video animation (which required the CD-ROM as floppy disks simply did not have the necessary data capacity).
The second drive interface initially was not well-defined. It was first introduced with interfaces specific to certain CD-ROM drives such as Mitsumi, Sony or Panasonic drives, and it was common to find early sound cards with two or three separate connectors each designed to match a certain brand of CD-ROM 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 CD-ROM 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.
Called the "primary" and "secondary" ATA interfaces, they were assigned to base addresses
Input/Output Base Address
In the x86 architecture, an input/output base address is a base address of an I/O port. In other words, this is the first address of a range of consecutive I/O port addresses that device uses....
0x1F0 and 0x170 on ISA bus systems.
EIDE and ATA-2
In 1994, about the same time that the ATA-1 standard was adopted, Western Digital introduced drives under a newer 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:
ATAPI
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 committeeSmall Form Factor committee
The Small Form Factor committee is an ad hoc electronics industry group formed to quickly develop interoperability specifications ....
(SFF) 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
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
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 drive
Tape drive
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
s, and large-capacity floppy
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drives such as the Zip drive
Zip drive
The Zip drive is a medium-capacity removable disk storage system that was introduced by Iomega in late 1994. Originally, Zip disks launched with capacities of 100 MB, but later versions increased this to first 250 MB and then 750 MB....
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. One commonly used set is defined in the MMC
MultiMedia Commands
MultiMedia Commands defines a SCSI/ATA based command set for accessing multimedia features on devices capable of such functionality. T10 subcommittee is responsible for developing it as well as other SCSI command set standards.-See also:...
SCSI command set.
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, or PATA).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 most Parallel 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.
x86 BIOS size limitations
Initially the size of an ATA drive was stored in the system x86 BIOS using a type number that predefined the C/H/S parameters and also often the landing zone, in which the drive heads are parked while not in use. Later a "user definable" format called C/H/S or cylinders, heads, sectors were made available. These numbers were important for the earlier ST-506 interface, but were generally meaningless for ATA—the CHS parameters for later ATA large drives often specified impossibly high numbers of heads or sectors that did not actually define the internal physical layout of the drive at all. From the start and up to ATA-2 every user had to specify explicitly how large every attached drive was. From ATA-2 an "identify drive" command were implemented that can be sent and which will return all drive parameters.Due to a lack of foresight by motherboard manufacturers, the system BIOS was often hobbled by artificial C/H/S size limitations due to the manufacturer assuming certain values would never exceed a certain numerical maximum.
The first of these BIOS limits occurred when ATA drives reached sizes in excess of . Because some motherboard BIOS would not allow C/H/S values above , , and . Multiplied by 512 bytes per sector, this totals which divided by per megabyte, equals 504 megabytes.
The second of these BIOS limitations occurred at 1024 cylinders, 256 heads, and 63 sectors, but a bug in MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
and MS-Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
limit the number heads to 255. This totals to , commonly referred to as the 8.4 gigabyte barrier. This is also a limit imposed by x86 BIOSes, and not a limit imposed by the ATA interface.
It was eventually determined that these size limitations could be overridden with a tiny program loaded at startup from a hard drive's boot sector. Some hard drive manufacturers such as Western Digital started including these override utilities with new large hard drives to help overcome these problems. However, if the computer were booted in some other manner without loading the special utility, the invalid BIOS settings would be used, and the drive could either be inaccessible or could appear to be damaged to the operating system.
Later an extension to the x86 BIOS disk services called the "Extended Disk Drive" (EDD) were made available which makes it possible to address drives as large as .
Interface size limitations
The first drive interface used 22-bit addressing mode which resulted in a maximum drive capacity of . Later the first formalized ATA specification used a 28-bit addressing mode, allowing for the addressing of 228 (blocks) of 512 bytes each, resulting in a maximum capacity of 128 GiBGibibyte
The gibibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB....
(137 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
).
ATA-6 introduced 48-bit addressing, increasing the limit to 128 PiB
Pebibyte
The pebibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The pebibyte unit symbol is PiB....
(144 PB
Petabyte
A petabyte is a unit of information equal to one quadrillion bytes, or 1000 terabytes. The unit symbol for the petabyte is PB...
). As a consequence, any ATA drive of capacity larger than about 137 gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
s 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 interface.
Some operating systems, including Windows XP pre-SP 1, and Windows 2000, disable 48-bit LBA
Logical block addressing
Logical block addressing is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks....
by default, requiring the user to take extra steps to use the entire capacity of an ATA drive larger than about 137 gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
s. Older operating systems, such as Windows 98, do not support 48-bit LBA at all.
Obsolescence
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 (for example, Promise Ultra-100), for up to eight ATA devices attached to the motherboard.After the introduction of SATA (Serial ATA
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
), use of Parallel ATA declined, and new motherboards had only a single PATA connector, for up to two PATA devices- typically optical drives- along with (typically) six or more SATA connectors for hard drives and other devices. In new computers, the parallel ATA interface is rarely used, and several PC chipset
Chipset
A chipset, PC chipset, or chip set refers to a group of integrated circuits, or chips, that are designed to work together. They are usually marketed as a single product.- Computers :...
s have removed support for PATA, and motherboard vendors still wishing to offer ATA with those chipsets must include an additional interface chip.
Parallel ATA interface
Ribbon cable
A ribbon cable is a cable with many conducting wires running parallel to each other on the same flat plane. As a result the cable is wide and flat. Its name comes from the resemblance of the cable to a piece of ribbon.Ribbon cables are usually seen for internal peripherals in computers, such as...
. 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.
ATA's 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
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
wires, interleaved with the previously defined wires to reduce the effects of capacitive coupling
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
between neighboring signal wires, reducing crosstalk
Crosstalk (electronics)
In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel...
. 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.
Round parallel ATA cables (as opposed to ribbon cables) were eventually made available for 'case modders' for cosmetic reasons, as well as claims of improved computer cooling and were easier to handle; however, only ribbon cables are supported by the ATA specifications.
Pin 20
In the ATA standard pin 20 is defined as (mechanical) key and is not used. This socket on the female connector is often obstructed, requiring pin 20 to be omitted from the male cable or drive connector, making it impossible to plug it in the wrong way round; a male connector with pin 20 present cannot be used. However, some flash memory
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
drives can use pin 20 as VCC_in to power the drive without requiring a special power cable; this feature can only be used if the equipment supports this use of pin 20.
Pin 28
Pin 28 of the gray (slave/middle) connector of an 80 conductor cable is not attached to any conductor of the cable. It is attached normally on the black (master drive end) and blue (motherboard end) connectors.
Pin 34
Pin 34 is connected to ground inside the blue connector of an 80 conductor cable but not attached to any conductor of the cable. It is attached normally on the gray and black connectors. See page 315 of.
Differences between connectors on 80-conductor cables
Each contact comprises a pair of points which together pierce the insulation of the ribbon cable with such precision that they make a connection to the desired conductor without harming the insulation on the neighboring wires. The center row of contacts are all connected to the common ground bus and attached to the odd numbered conductors of the cable. The top row of contacts are the even-numbered sockets of the connector (mating with the even-numbered pins of the receptacle) and attach to every other even-numbered conductor of the cable. The bottom row of contacts are the odd-numbered sockets of the connector (mating with the odd-numbered pins of the receptacle) and attach to the remaining even-numbered conductors of the cable.
Note the connections to the common ground bus from sockets 2 (top left), 19 (center bottom row), 22, 24, 26, 30, and 40 on all connectors. Also note (enlarged detail, bottom, looking from the opposite side of the connector) that socket 34 of the blue connector does not contact any conductor but unlike socket 34 of the other two connectors, it does connect to the common ground bus. On the gray connector, note that socket 28 is completely missing, so that pin 28 of the drive attached to the gray connector will be open. On the black connector, sockets 28 and 34 are completely normal, so that pins 28 and 34 of the drive attached to the black connector will be connected to the cable. Pin 28 of the black drive reaches pin 28 of the host receptacle but not pin 28 of the gray drive, while pin 34 of the black drive reaches pin 34 of the gray drive but not pin 34 of the host. Instead, pin 34 of the host is grounded.
The standard dictates color-coded connectors for easy identification by both installer and cable maker. All three connectors are different from one another. The blue (host) connector has the socket for pin 34 connected to ground inside the connector but not attached to any conductor of the cable. Since the old 40 conductor cables do not ground pin 34, the presence of a ground connection indicates that an 80 conductor cable is installed. The wire for pin 34 is attached normally on the other types and is not grounded. Installing the cable backwards (with the black connector on the system board, the blue connector on the remote device and the gray connector on the center device) will ground pin 34 of the remote device and connect host pin 34 through to pin 34 of the center device. The gray center connector omits the connection to pin 28 but connects pin 34 normally, while the black end connector connects both pins 28 and 34 normally.
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 BIOSBIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
and/or operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
Jumper (computing)
In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit...
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 will often work reliably even though configured as the slave drive (most often seen where 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
Jumper (computing)
In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit...
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 inches (457.2 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 extremely common use, the terms "master" and "slave" do not actually appear in current versions of the ATA specifications. The two devices are simply referred to as "device 0" and "device 1", respectively, in ATA-2 and later.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 onboard controller operates independently of the other.
The terms "master" and "slave" have not been without controversy. In 2003, the County of Los Angeles, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, US
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
requested that, when possible, 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, both being given the name "Tagged Command Queuing
Tagged Command Queuing
Tagged Command Queuing is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient native command queuing used by SATA drives...
", a reference to a set of features from SCSI which the ATA version attempts to emulate. However, support for these is extremely rare in actual parallel ATA products and device drivers because these feature sets were implemented in such a way as to maintain software compatibility with its heritage as originally an extension of the ISA bus. This implementation resulted in excessive CPU utilization which largely negated the advantages of command queuing. By contrast, overlapped and queued operations have been common in other storage buses, in particular, SCSI's version of tagged command queuing had no need to be software compatible with ISA's APIs, allowing it to attain high performance with low overhead on buses which supported first party DMA like PCI. This has long been seen as a major advantage of SCSI.
The Serial ATA standard has supported native command queueing
Native Command Queuing
Native Command Queuing is a technology designed to increase performance of SATA hard disks under certain conditions by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed...
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. Many SATA/II hard drives sold today support NCQ, while no removable (CD/DVD) drives do because the ATAPI command set used to control them prohibits queued operations.
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. The disk lock can be enabled and disabled by sending special ATA commands to the drive. If a disk is locked, it will refuse all access until it is unlocked.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
Default password
Where a device needs a username and/or password to login, a default password is usually provided that allows the device to be accessed during its initial setup...
. 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 Master 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.)
While the ATA disk lock is intended to be impossible to defeat without a valid password, there are workarounds to unlock a drive. Many data recovery companies offer unlocking services, so while the disk lock will deter a casual attacker, it is not secure against a qualified adversary.
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.
Compact Flash interface
Compact flash is essentially just a miniaturized ATA interface, for use on devices that use flash memory storage. No interfacing chips or circuitry are required, other than to directly adapt the smaller CF socket onto the larger ATA connector.The ATA connector specification does not include pins for supplying power to a CF device, so power is inserted into the connector from a separate source.
CF devices can be designated as master or slave on an ATA interface, though since most CF devices offer only a single socket, it is not necessary to offer this selection to end users.
Although CF can be hot pluggable with additional design methods, by default when wired directly to an ATA interface, it is not intended to be hot-pluggable.
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", defined by ATA-5) 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
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
— a very unusual situation, especially considering that such data are usually already buffered by the operating system.
As of April 2010 mechanical hard disk drives can transfer data at up to 157 MB/s, which is beyond the capabilities of the PATA/133 specification. High-performance flash drives
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...
can transfer data at up to 308 MB/s.
Only the Ultra DMA modes use CRC
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
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.
Features introduced with each ATA revision
| Standard | Other names | New transfer modes | Maximum disk size (512 byte sector) |
Other new features | ANSI reference |
|---|---|---|---|---|---|
| IDE (pre-ATA) | IDE | PIO Programmed input/output Programmed input/output is a method of transferring data between the CPU and a peripheral such as a network adapter or an ATA storage device.... 0 |
2 GiB Gibibyte The gibibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB.... (2.1 GB Gigabyte The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is... ) |
22-bit logical block addressing Logical block addressing Logical block addressing is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks.... (LBA) |
- |
| ATA-1 | ATA, IDE | Multi-word DMA 0 |
128 GiB Gibibyte The gibibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB.... (137 GB Gigabyte The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is... ) |
28-bit logical block addressing (LBA) | [ftp://ftp.t10.org/t13/project/d0791r4c-ATA-1.pdf X3.221-1994] (obsolete since 1999) |
| ATA-2 | EIDE, , , | PIO 3, 4 Multi-word DMA WDMA (computer) The Word DMA interface was the fastest method used to transfer data between the computer and an ATA device until UDMA.... 1, 2 |
PCMCIA connector. Identify drive command. | X3.279-1996 (obsolete since 2001) |
|
| ATA-3 | EIDE | Single-word DMA WDMA (computer) The Word DMA interface was the fastest method used to transfer data between the computer and an ATA device until UDMA.... modes dropped |
S.M.A.R.T., Security, 44 pin connector for 2.5" drives | X3.298-1997 (obsolete since 2002) |
|
| ATA/ATAPI-4 | ATA-4, | aka UDMA/33 |
AT Attachment Packet Interface (ATAPI) (support for CD-ROM, tape drives etc.), Optional overlapped and queued command set features, Host Protected Area Host Protected Area The host protected area, sometimes referred to as hidden protected area, is an area of a hard drive that is not normally visible to an operating system .- History :HPA was first introduced in the ATA-4 standard cxv .-How it works:... (HPA), CompactFlash CompactFlash CompactFlash is a mass storage device format used in portable electronic devices. Most CompactFlash devices contain flash memory in a standardized enclosure. The format was first specified and produced by SanDisk in 1994... Association (CFA) feature set for solid state drives |
NCITS 317-1998 | |
| ATA/ATAPI-5 | ATA-5, | Ultra DMA 3, 4 aka UDMA/66 |
80-wire cables; CompactFlash CompactFlash CompactFlash is a mass storage device format used in portable electronic devices. Most CompactFlash devices contain flash memory in a standardized enclosure. The format was first specified and produced by SanDisk in 1994... connector |
NCITS 340-2000 | |
| ATA/ATAPI-6 | ATA-6, | UDMA 5 aka UDMA/100 |
128 PiB Pebibyte The pebibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The pebibyte unit symbol is PiB.... (144 PB Petabyte A petabyte is a unit of information equal to one quadrillion bytes, or 1000 terabytes. The unit symbol for the petabyte is PB... ) |
48-bit LBA, Device Configuration Overlay Device configuration overlay Device configuration overlay is a hidden area on many of today’s hard disk drives . Usually when information is stored in either the DCO or host protected area , it is not accessible by the BIOS, OS, or the user. However, certain tools can be used to modify the HPA or DCO... (DCO), Automatic Acoustic Management Automatic Acoustic Management Automatic acoustic management is a method for reducing acoustic emanations in AT Attachment mass storage devices, such as ATA hard disk drives and ATAPI optical disc drives... (AAM) |
NCITS 361-2002 |
| ATA/ATAPI-7 | ATA-7, | UDMA 6 aka UDMA/133 SATA/150 |
SATA Serial ATA Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives... 1.0, Streaming feature set, long logical/physical sector feature set for non-packet devices |
NCITS 397-2005 (vol 1)NCITS 397-2005 (vol 2)NCITS 397-2005 (vol 3) | |
| ATA/ATAPI-8 | ATA-8 | — | Hybrid drive Hybrid drive A Hybrid Drive, Hybrid Hard Drive , or Hybrid Hard Disk Drive is a type of large-buffer computer hard disk drive. It is different from standard hard drives in that it integrates a cache using non-volatile memory or even a small solid-state drive... featuring non-volatile cache to speed up critical OS files |
In progress |
Speed of defined transfer modes
| Mode | # | Maximum transfer rate (MB/s) |
cycle time |
|---|---|---|---|
| PIO Programmed input/output Programmed input/output is a method of transferring data between the CPU and a peripheral such as a network adapter or an ATA storage device.... |
0 | 3.3 | 600 ns |
| 1 | 5.2 | 383 ns | |
| 2 | 8.3 | 240 ns | |
| 3 | 11.1 | 180 ns | |
| 4 | 16.7 | 120 ns | |
| Single-word DMA WDMA (computer) The Word DMA interface was the fastest method used to transfer data between the computer and an ATA device until UDMA.... |
0 | 2.1 | 960 ns |
| 1 | 4.2 | 480 ns | |
| 2 | 8.3 | 240 ns | |
| Multi-word DMA WDMA (computer) The Word DMA interface was the fastest method used to transfer data between the computer and an ATA device until UDMA.... |
0 | 4.2 | 480 ns |
| 1 | 13.3 | 150 ns | |
| 2 | 16.7 | 120 ns | |
| 3 | 20 | 100 ns | |
| 4 | 25 | 80 ns | |
| Ultra DMA | 0 | 16.7 | 240 ns ÷ 2 |
| 1 | 25.0 | 160 ns ÷ 2 | |
| 2 (Ultra ATA/33) | 33.3 | 120 ns ÷ 2 | |
| 3 | 44.4 | 90 ns ÷ 2 | |
| 4 (Ultra ATA/66) | 66.7 | 60 ns ÷ 2 | |
| 5 (Ultra ATA/100) | 100 | 40 ns ÷ 2 | |
| 6 (Ultra ATA/133) | 133 | 30 ns ÷ 2 | |
| 7 (Ultra ATA/167) | 167 | 24 ns ÷ 2 |
ATAPI Removable Media Device (ARMD)
ATAPI devices with removable media, other than CD and DVD drives, are classified as ARMD (ATAPI Removable Media Device) and can appear as either a super-floppy (non-partitioned media) or a hard drive (partitioned media) to the operating systemOperating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
. These can be supported as bootable devices by a BIOS complying with the
ATAPI Removable Media Device BIOS Specification, originally developed by Compaq Computer Corporation
Compaq
Compaq Computer Corporation is a personal computer company founded in 1982. Once the largest supplier of personal computing systems in the world, Compaq existed as an independent corporation until 2002, when it was acquired for US$25 billion by Hewlett-Packard....
and Phoenix Technologies
Phoenix Technologies
Phoenix Technologies Ltd designs, develops and supports core system software for personal computers and other computing devices. Phoenix's products — commonly referred to as BIOS or firmware — support and enable the compatibility, connectivity, security and management of the various components and...
. It specifies provisions in the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
of a personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
to allow the computer to be bootstrapped
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
from devices such as Zip drives, Jaz drives, SuperDisk
SuperDisk
The SuperDisk, sometimes marketed as LS-120 and a later variant LS-240, is a high-speed, high-capacity alternative to the 90 mm , 1.44 MB floppy disk. The Superdisk hardware was introduced by 3M's storage products group circa 1997...
(LS-120) drives, and similar devices.
These devices have removable media like floppy disk drives
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, but capacities more commensurate with hard drives, and programming requirements unlike either. Due to limitations in the floppy controller interface most of these devices were ATAPI
AT Attachment
Parallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...
devices, connected to one of the host computer's ATA interfaces, similarly to a hard drive or CD-ROM
CD-ROM
A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
device. However, existing BIOS standards did not support these devices. An ARMD-compliant BIOS allows these devices to be booted from and used under the operating system without requiring device-specific code in the OS.
A BIOS implementing ARMD allows the user to include ARMD devices in the boot search order. Usually an ARMD device is configured earlier in the boot order than the hard drive. Similarly to a floppy drive, if bootable media is present in the ARMD drive, the BIOS will boot from it; if not, the BIOS will continue in the search order, usually with the hard drive last.
There are two variants of ARMD, ARMD-FDD and ARMD-HDD. Originally ARMD caused the devices to appear as a sort of very large floppy drive, either the primary floppy drive device 00h or the secondary device 01h. Some operating systems required code changes to support floppy disks with capacities far larger than any standard floppy disk drive. Also, standard-floppy disk drive emulation proved to be unsuitable for certain high-capacity floppy disk drives such as Iomega Zip drives. Later the ARMD-HDD, ARMD-"Hard disk device", variant was developed to address these issues. Under ARMD-HDD, an ARMD device appears to the BIOS and the operating system as a hard drive.
ATA over Ethernet
In August 2004, Sam Hopkins and Brantley Coile of Coraid specified a lightweight ATA over EthernetATA over Ethernet
ATA over Ethernet is a network protocol developed by the Brantley Coile Company, designed for simple, high-performance access of SATA storage devices over Ethernet networks. It is used to build storage area networks with low-cost, standard technologies.- Protocol description :AoE runs on layer 2...
protocol to carry ATA commands over Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
instead of directly connecting them to a PATA host adapter. This permitted the established block protocol to be reused in storage area network
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
(SAN) applications.
See also
- Master/slave (technology)
- Advanced Host Controller InterfaceAdvanced Host Controller InterfaceThe Advanced Host Controller Interface is a technical standard defined by Intel that specifies the operation of Serial ATA host bus adapters in a non-implementation-specific manner....
(AHCI) - List of device bandwidths
- CE-ATACE-ATAConsumer Electronics ATA is an interface standard for the connection of storage devices and hosts in consumer electronic device such as mobile and handheld devices...
Consumer Electronics (CE) ATA http://www.ce-ata.org/ - Serial ATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
- FATA (hard drive)
- SCSISCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
- BIOSBIOSIn IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
for BIOS Boot Specification (BBS) - INT 13H for BIOS Enhanced Disk Drive Specification (SFF-8039i)
- ATA over EthernetATA over EthernetATA over Ethernet is a network protocol developed by the Brantley Coile Company, designed for simple, high-performance access of SATA storage devices over Ethernet networks. It is used to build storage area networks with low-cost, standard technologies.- Protocol description :AoE runs on layer 2...
(AoE) - IT8212IT8212The IT8212, or more correctly the IT8212F, is a low-end Parallel ATA controller designed by the Taiwanese company , more commonly known as ITE Inc. Depending on the implemented BIOS and configuration the IT8212F functions in either a RAID or an ATAPI mode, supporting up to four devices using dual...
, a low-end Parallel ATA controller