Parallel ATA (PATA), originally AT Attachment, is an interface
standard for the connection of storage devices such as hard disk
drives, floppy disk drives, and optical disc drives in computers. The
standard is maintained by the X3/
INCITS committee. It uses the
underlying AT Attachment (ATA) and AT Attachment Packet Interface
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, in particular Extended IDE (EIDE) and
Ultra ATA (UATA). After the introduction of
Serial ATA (SATA) in 2003,
the original ATA was renamed to Parallel ATA, or PATA for short.
Parallel ATA cables have a maximum allowable length of 18 in
(457 mm). 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 It has largely been replaced by SATA in newer systems.
1 History and terminology
1.1 IDE and ATA-1
1.2 Second ATA interface
1.3 EIDE and ATA-2
UDMA and ATA-4
1.6 Ultra ATA
1.7 Current terminology
BIOS size limitations
1.9 Interface size limitations
1.10 Primacy and obsolescence
Parallel ATA interface
2.1 44 pin variant
2.2 Differences between connectors on 80-conductor cables
2.3 Multiple devices on a cable
2.4 Cable select
2.4.1 Master and slave clarification
2.5 Serialized, overlapped, and queued operations
2.6 Two devices on one cable—speed impact
2.6.1 "Lowest speed"
2.6.2 "One operation at a time"
2.7 HDD passwords and security
2.8 External parallel ATA devices
3 Compact Flash interface
4 ATA standards versions, transfer rates, and features
4.1 Features introduced with each ATA revision
4.2 Speed of defined transfer modes
5 Related standards, features, and proposals
5.1 ATAPI Removable Media Device (ARMD)
5.2 ATA over Ethernet
6 See also
8 External links
History and terminology
The standard was originally conceived as the "AT Bus Attachment,"
officially called "AT Attachment" and abbreviated "ATA" because
its primary feature was a direct connection to the 16-bit ISA bus
introduced with the IBM PC/AT. The original ATA specifications
published by the standards committees use the name "AT
Attachment". The "AT" in the
IBM PC/AT referred to "Advanced
Technology" so ATA has also been referred to as "Advanced Technology
Attachment". When a newer
Serial ATA (SATA) was
introduced in 2003, the original ATA was renamed to Parallel ATA, or
PATA for short.
IDE and ATA-1
Example of a 1992 80386 PC motherboard with nothing built in other
than memory, keyboard, processor, cache, realtime clock, and slots.
Such basic motherboards could have been outfitted with either the
ST-506 or ATA interface, but usually not both. A single 2-drive ATA
interface and a floppy interface was added to this system via the
16-bit ISA card.
The first version of what is now called the ATA/ATAPI interface was
Western Digital under the name Integrated Drive
Electronics (IDE). Together with
Control Data Corporation
Control Data Corporation (the hard
drive manufacturer) and
Computer (the initial customer), they
developed the connector, the signaling protocols and so on, with the
goal of remaining software compatible with the existing
drive interface. The first such drives appeared in
Compaq PCs in
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 to 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
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 "XT-IDE", "XTA" or "XT
Second ATA interface
Oak Technology Mozart 16 16-bit ISA sound card, from when the CDROM
drive interface had not yet been standardized. This card offers four
separate interface connectors for IDE, Panasonic, Mitsumi, and Sony
CDROM drives, but only one connector could be used since they all
shared the same interface wiring.
SoundBlaster 32 16-bit ISA sound card, from after connector
standardization had occurred, with an IDE interface for the CDROM
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 users. 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.[dubious –
SCSI was available as a
CD-ROM expansion option at the time, but
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, which was typically included as an expansion option on a
sound card. PC motherboards initially did not come with support for
more than simple beeps from internal speakers; thus, sound cards (such
as the Sound Blaster Pro) were available for use with games, operating
system and software event sounds, or to listen to audio CDs. Also,
sound cards commonly included a gameport joystick/gamepad port along
with interfaces to control a
CD-ROM and transmit CD audio to the
Initially, the second drive interface was not well defined. It was
first introduced with interfaces specific to certain
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
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
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.
— AT Attachment Interface with Extensions (ATA-2), page 2
Main article: ATA Packet Interface
As mentioned in the previous sections, ATA was originally designed
for, and worked only with hard disk drives and devices that could
emulate them. The introduction of ATAPI (ATA Packet Interface) by a
group called the
Small Form Factor committee (SFF) allowed ATA to be
used for a variety of other devices that require functions beyond
those necessary for hard disk drives. 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.
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
SCSI devices with an ATA/ATAPI to
SCSI protocol converter added
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
DVD-ROM drives, tape drives, and
large-capacity floppy drives such as the
Zip drive and SuperDisk
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).
UDMA and ATA-4
See also: UDMA
The ATA/ATAPI-4 standard 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 new 80-wire cables to reduce crosstalk. The latest versions
Parallel ATA support up to 133 MByte/s.
Ultra ATA, abbreviated UATA, is a designation that has been primarily
Western Digital for different speed enhancements to the
ATA/ATAPI standards. For example, in 2000
Western Digital published a
document describing "Ultra ATA/100", which brought performance
improvements for the then-current ATA/ATAPI-5 standard by improving
maximum speed of the
Parallel ATA interface from 66 to
100 MB/s. Most of Western Digital's changes, along with
others, were included in the ATA/ATAPI-6 standard (2002).
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
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"
BIOS size limitations
See also: Enhanced BIOS
Initially, the size of an ATA drive was stored in the system x86 BIOS
using a type number (1 through 45) 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 was 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 on, an "identify drive" command was implemented that can be
sent and which will return all drive parameters.
Owing 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 particular
The first of these
BIOS limits occurred when ATA drives reached sizes
in excess of 504 megabytes, because some motherboard BIOSes would not
allow C/H/S values above 1024 cylinders, 16 heads, and 63 sectors.
Multiplied by 512 bytes per sector, this totals 528482304 bytes which,
divided by 1048576 bytes per megabyte, equals 504 megabytes.
The second of these
BIOS limitations occurred at 1024 cylinders, 256
heads, and 63 sectors, and a bug in
MS-DOS and MS-
Windows 95 limited
the number of heads to 255. This totals to 8422686720 bytes, commonly
referred to as the 8.4 gigabyte barrier. This is again 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 was 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 appear to the operating system to be damaged.
Later, an extension to the x86
BIOS disk services called the "Enhanced
Disk Drive" (EDD) was made available, which makes it possible to
address drives as large as 264 sectors.
Interface size limitations
The first drive interface used 22-bit addressing mode which resulted
in a maximum drive capacity of two gigabytes. Later, the first
formalized ATA specification used a 28-bit addressing mode through
LBA28, allowing for the addressing of 228 (268435456) sectors (blocks)
of 512 bytes each, resulting in a maximum capacity of 128 GiB
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 GB 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
Some operating systems, including
Windows XP pre-SP1, and Windows 2000
LBA48 by default, requiring the user to take extra
steps to use the entire capacity of an ATA drive larger than about 137
Older operating systems, such as Windows 98, do not support 48-bit LBA
at all. However, members of the third-party group MSFN have
Windows 98 disk drivers to add unofficial support for
48-bit LBA to
Windows 95 OSR2, Windows 98,
Windows 98 SE and Windows
Some 16-bit and 32-bit operating systems supporting
LBA48 may still
not support disks larger than 2
TiB due to using 32-bit arithmetics
only; a limitation also applying to many boot sectors.
Primacy and obsolescence
Parallel ATA (then simply called ATA or IDE) became the primary
storage device interface for PCs soon after its introduction. In some
systems, a third and fourth motherboard interface was provided,
allowing up to eight ATA devices to be attached to the motherboard.
Often, these additional connectors were implemented by inexpensive
Soon after the introduction of
Serial ATA (SATA) in 2003, use of
Parallel ATA declined. The first motherboards with built-in SATA
interfaces usually had only a single PATA connector (for up to two
PATA devices), along with multiple SATA connectors.
As of 2007, some PC chipsets, for example the Intel ICH10, had removed
support for PATA.
Motherboard vendors still wishing to offer Parallel
ATA with those chipsets must include an additional interface chip. In
more recent computers, the
Parallel ATA interface is rarely used even
if present, as four or more
Serial ATA connectors are usually provided
on the motherboard and SATA devices of all types are common.
With Western Digital's withdrawal from the PATA market, hard disk
drives with the PATA interface were no longer in production after
December 2013 for other than specialty applications.
Parallel ATA interface
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 storage devices, most commonly hard disk drives
or optical 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 UDMA/66 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,
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: 40-wire ribbon cable (top), and 80-wire ribbon cable
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 the 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.
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 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 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.
44 pin variant
A 44 pin variant PATA interface is used for 2.5 inch drives inside
laptops. The pins are closer together and the connector is physically
smaller than the 40 pin interface. The extra pins carry power.
Differences between connectors on 80-conductor cables
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Differences between connectors
The image on the right 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
The connector is an insulation-displacement connector—in other
words, 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 are attached 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
Intel 486 era and older), the drives are often referred to by
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 an optical drive is the only device on the secondary
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
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 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 same ATA
interface. 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.
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" (TCQ), 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
Serial ATA standard has supported native command queueing (NCQ)
since its first release, but it is an optional feature for both host
adapters and target devices. Many obsolete PC motherboards do not
support NCQ, but modern SATA hard disk drives and SATA solid-state
drives usually support NCQ, which is not the case for removable
(CD/DVD) drives 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".
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 applies only 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 will not 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
"ATA Secure Erase" redirects here. For ATA Secure Erase with flash
memory, see Write amplification § Secure erase. For general use,
see Disk formatting § Recovery of data from a formatted disk.
ATA devices may support an optional security feature which is defined
in an ATA specification, and thus not specific to any brand or device.
The security feature can be enabled and disabled by sending special
ATA commands to the drive. If a device is locked, it will refuse all
access until it is unlocked.
A device can have two passwords: A User Password and a Master
Password. Either or both may be set; there is a Master Password
identifier feature which if supported and used can identifies with out
disclosing the current Master Password.
A device 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 device 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 device can be unlocked only with the
User password. If the User password is not available, the only
remaining way to get at least the bare hardware 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. Word 89 in the IDENTIFY response indicates how
long the operation will take.
While the ATA lock is intended to be impossible to defeat without a
valid password, there are purported workarounds to unlock a device.
External parallel ATA devices
USB Adapter. It is mounted on the rear of a DVD-RW optical
drive inside an external case
Due to a short cable length specification and shielding issues it is
extremely uncommon to find external PATA devices that directly use
PATA for connection to a computer. 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.
External hard disk drives or optical disk drives that have an internal
PATA interface, 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
Compact Flash interface
Compact flash is a miniature ATA interface, slightly modified to be
able to also supply power to the CF device.
Compact Flash in its IDE mode is essentially a miniaturized ATA
interface, intended 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. The exception to this is when the CF device is
connected to a 44-pin ATA bus designed for 2.5-inch hard disk drives,
commonly found in notebook computers, as this bus implementation must
provide power to a standard hard disk drive.
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
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 2010[update], 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 solid
state 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
Features introduced with each ATA revision
New transfer modes
Maximum disk size
(512 byte sector)
Other new features
2 GiB (2.1 GB)
22-bit logical block addressing (LBA)
PIO 0, 1, 2
Single-word DMA 0, 1, 2
Multi-word DMA 0
128 GiB (137 GB)
28-bit logical block addressing (LBA)
(obsolete since 1999)
EIDE, Fast ATA, Fast IDE, Ultra ATA
PIO 3, 4
Multi-word DMA 1, 2
PCMCIA connector. Identify drive command.
(obsolete since 2001)
Single-word DMA modes dropped
S.M.A.R.T., Security, 44 pin connector for 2.5" drives
(obsolete since 2002)
ATA-4, Ultra ATA/33
Ultra DMA 0, 1, 2,
also known as 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 (HPA),
CompactFlash Association (CFA) feature set
for solid state drives
ATA-5, Ultra ATA/66
Ultra DMA 3, 4,
also known as UDMA/66
ATA-6, Ultra ATA/100
also known as UDMA/100
128 PiB (144 PB)
Device Configuration Overlay (DCO),
Automatic Acoustic Management (AAM)
ATA-7, Ultra ATA/133
also known as UDMA/133
SATA 1.0, Streaming feature set, long logical/physical sector feature
set for non-packet devices
INCITS 397-2005 (vol 1)
INCITS 397-2005 (vol 2)
INCITS 397-2005 (vol
Hybrid drive featuring non-volatile cache to speed up critical OS
Data Set Management, Extended Power Conditions, CFast, additional
Speed of defined transfer modes
Maximum transfer rate
240 ns ÷ 2
160 ns ÷ 2
2 (Ultra ATA/33)
120 ns ÷ 2
90 ns ÷ 2
4 (Ultra ATA/66)
60 ns ÷ 2
5 (Ultra ATA/100)
40 ns ÷ 2
6 (Ultra ATA/133)
30 ns ÷ 2
7 (Ultra ATA/167)
24 ns ÷ 2
Related standards, features, and proposals
ATAPI Removable Media Device (ARMD)
Main article: ATA Packet Interface
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 system. These can be supported as
bootable devices by a
BIOS complying with the ATAPI Removable Media
BIOS Specification, originally developed by
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,
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.
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
ATA over Ethernet 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.
Advanced Host Controller Interface (AHCI)
ATA over Ethernet (AoE)
BIOS Boot Specification (BBS)
CE-ATA Consumer Electronics (CE) ATA
FATA (hard drive)
INT 13H for
Enhanced Disk Drive Specification (SFF-8039i)
IT8212, a low-end
Parallel ATA controller
Computer System Interface)
List of device bandwidths
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Technical and de facto standards for wired computer buses
Network on a chip
Plug and play
List of bus bandwidths
Europe Card Bus
HP Precision Bus
HP GSC bus
PCI Extended (PCI-X)
PCI Express (PCIe)
Direct Media Interface (DMI)
Intel QuickPath Interconnect
Intel UltraPath Interconnect
Parallel ATA (PATA)
Serial ATA (SATA)
PCI Express (via
AHCI or NVMe logical device interface)
Apple Desktop Bus
IEEE-1284 (parallel port)
IEEE 1394 (FireWire)
Intel HD Audio
Interfaces are listed by their speed in the (roughly) ascending order,
so the interface at the end of each section should be the fastest.