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The ATA (IDE) Interface

The interface used to connect a hard disk drive to a modern PC is typically called IDE (Integrated Drive Electronics) . However, the true name of the interface is ATA ( AT Attachment) . ATA refers to the fact that this interface originally was designed to connect a combined drive and controller directly to the bus of the 1984 vintage IBM AT (Advanced Technology) computer, otherwise known as the ISA (Industry Standard Architecture) or AT bus . IDE is a term originated by the marketing departments of some drive manufacturers to describe the drive/controller combination used in drives with the ATA interface. Integrated Drive Electronics refers to the fact that the interface electronics or controller is built in to the drive and is not a separate board, as with earlier drive interfaces.

Today, ATA is used to connect not only hard disks but also CD and DVD drives, high-capacity SuperDisk floppy drives, and tape drives. Even so, ATA is still thought of primarily as a hard disk interface, and it evolved directly from the separate controller and hard drive interfaces that were used prior to ATA. This chapter covers the ATA interface in detail, as well as the original interfaces from which ATA evolved. Because the ATA interface is directly integrated into virtually all motherboard chipsets, ATA is the primary storage interface used by most PCs, including both desktops and portables.

ATA is a 16-bit parallel interface, meaning that 16 bits are transmitted simultaneously down the interface cable. A new interface, called S erial ATA , was officially introduced in late 2000 and is being adopted in desktop systems starting in 2003 (it will likely be another year or so before it arrives in portables). Serial ATA (SATA) sends 1 bit down the cable at a time, enabling thinner and smaller cables to be used, as well as providing higher performance due to the higher cycling speeds allowed. SATA is a completely new and updated physical interface design, while remaining compatible on the software level with Parallel ATA. Throughout this book, ATA refers to the parallel version, whereas Serial ATA is explicitly referenced as SATA .

Note

Many people who use systems with ATA connectors on the motherboard believe that a hard disk controller is built in to their motherboards, but in a technical sense the controller is actually in the drive. Although the integrated ATA ports on a motherboard often are referred to as controllers , they are more accurately called host adapters (although you'll rarely hear this term). A host adapter can be thought of as a device that connects a controller to a bus.

The primary advantage of ATA drives over the older, separate controller “based interfaces and newer host bus interface alternatives, such as SCSI and IEEE 1394 (i.LINK or FireWire), is cost. Because the separate controller or host adapter is eliminated and the cable connections are simplified, ATA drives cost much less than a standard controller and drive combination.

ATA Standards

Today what we call the ATA interface is controlled by an independent group of representatives from major PC, drive, and component manufacturers. This group is called Technical Committee T13 (www.t13.org) and is responsible for all interface standards relating to the parallel AT Attachment storage interface. T13 is a part of the International Committee on Information Technology Standards (INCITS), which operates under rules approved by the American National Standards Institute (ANSI), a governing body that sets rules that control nonproprietary standards in the computer industry as well as many other industries. A second group, called the Serial ATA Working Group (www.serialata.org), has formed to create the Serial ATA standards that will also come under ANSI control. Although these are different groups, many of the same people are in both of them. It seems as if little further development will be done on Parallel ATA past the ATA-7 (ATA/133) specification. The further evolution of ATA will be in the Serial ATA form (discussed later in this chapter).

The rules these committees operate under are designed to ensure that voluntary industry standards are developed by the consensus of people and organizations in the affected industry. INCITS specifically develops Information Processing System standards, whereas ANSI approves the process under which they are developed and then publishes them. Because T13 is essentially a public organization, all the working drafts, discussions, and meetings of T13 are open for all to see.

The Parallel ATA interface has evolved into several successive standard versions, introduced as follows :

ATA-1 (1986 “1994)
ATA-2 (1995; also called Fast-ATA, Fast-ATA-2, or EIDE)
ATA-3 (1996)
ATA-4 (1997; also called Ultra-ATA/33)
ATA-5 (1998 “present; also called Ultra-ATA/66)
ATA-6 (2000 “present; also called Ultra-ATA/100)
ATA-7 (2001 “present; also called Ultra-ATA/133)

Each version of ATA is backward compatible with the previous versions. In other words, older ATA-1 or ATA-2 devices work fine on ATA-6 and ATA-7 interfaces. In cases in which the device version and interface version don't match, they work together at the capabilities of the lesser of the two. Newer versions of ATA are built on older versions and with few exceptions can be thought of as extensions of the previous versions. This means that ATA-7, for example, is generally considered equal to ATA-6 with the addition of some features.

Table 9.7 breaks down the various ATA standards. The following sections describe all the ATA versions in more detail.

Table 9.7. ATA Standards

Standard	Timeframe	PIO Modes	DMA Modes	UDMA Modes	Speed ^[1]	Features
ATA-1	1986 “1994	0 “2		”	8.33	Drives support up to 136.9GB. BIOS issues not addressed.
ATA-2	1995 “1996	0 “4	0 “2	”	16.67	Faster PIO modes. CHS/LBA BIOS translation defined up to 8.4GB. PC Card.
ATA-3	1996 “1997	0 “4	0 “2	”	16.67	SMART. Improved signal integrity. LBA support mandatory. Eliminated single-word DMA modes.
ATA-4	1997 “1998	0 “4	0 “2	0 “2	33.33	Ultra-DMA modes. BIOS support up to 136.9GB.
ATA-5	1998 “2000	0 “4	0 “2	0 “4	66.67	Faster UDMA modes. 80-pin cable with autodetection.
ATA-6	2000 “2002	0 “4	0 “2	0 “5	100.00	100MBps UDMA mode. Extended drive and BIOS support up to 144PB.
ATA-7	2002 “present	0 “4	0 “2	0 “6	133.00	133MBps UDMA mode.
SMART = Self-Monitoring, Analysis, and Reporting Technology
MB = megabyte (million bytes)
GB = gigabyte (billion bytes)
PB = petabyte (quadrillion bytes)
CHS = cylinder, head, sector
LBA = logical block address
PIO = Programmed I/O
DMA = Direct Memory Access
UDMA = Ultra DMA (Direct Memory Access)

^[1] Speed is megabytes per second

ATA-1

Although ATA-1 had been used since 1986 before being published as a standard, and although it was first published in 1988 in draft form, ATA-1 wasn't officially approved as a standard until 1994 (committees often work slowly). ATA-1 defined the original AT Attachment interface, which was an integrated bus interface between disk drives and host systems based on the ISA (AT) bus. Here are the major features introduced and documented in the ATA-1 specification:

40/44-pin connectors and cabling
Master/slave or cable select drive configuration options
Signal timing for basic PIO (Programmed I/O) and DMA (Direct Memory Access) modes
CHS (cylinder, head, sector) and LBA (logical block address) drive parameter translations supporting drive capacities up to 2 ²⁸ “2 ²⁰ (267,386,880) sectors, or 136.9GB

ATA-1 was officially published as "ANSI X3.221-1994, AT Attachment Interface for Disk Drives," and was officially withdrawn on August 6, 1999. ATA-2 and later are considered backward-compatible replacements .

Although ATA-1 supported theoretical drive capacities up to 136.9GB (2 ²⁸ “2 ²⁰ = 267,386,880 sectors), it did not address BIOS limitations that stopped at 528MB (1,024x16x63 = 1,032,192 sectors). The BIOS limitations would be addressed in subsequent ATA versions because, at the time, no drives larger than 528MB had existed.

ATA-2

Approved in 1996, ATA-2 was a major upgrade to the original ATA standard. Perhaps the biggest change was almost a philosophical one. ATA-2 was updated to define an interface between host systems and storage devices in general and not only disk drives. The major features added to ATA-2 as compared to the original ATA standard include the following:

Faster PIO and DMA transfer modes.
Support for power management.
Support for removable devices.
PCMCIA (PC card) device support.
More information reported from the Identify Drive command.
Defined standard CHS/LBA translation methods for drives up to 8.4GB in capacity.

The most important additions in ATA-2 were the support for faster PIO and DMA modes as well as the methods to enable BIOS support up to 8.4GB. The BIOS support was necessary because, although even ATA-1 was designed to support drives of up to 136.9GB in capacity, the PC BIOS could originally only handle drives of up to 528MB. Adding parameter-translation capability now allowed the BIOS to handle drives up to 8.4GB. This is discussed in more detail later in this chapter.

ATA-2 also featured improvements in the Identify Drive command, which enabled a drive to tell the software exactly what its characteristics are. This is essential for both Plug and Play (PnP) and compatibility with future revisions of the standard.

ATA-2 was also known by unofficial marketing terms, such as fast-ATA or fast-ATA-2 (Seagate/Quantum) and EIDE (Enhanced IDE, Western Digital). ATA-2 was officially published as "ANSI X3.279-1996 AT Attachment Interface with Extensions."

ATA-3

First published in 1997, ATA-3 was a comparatively minor revision to the ATA-2 standard that preceded it. It consisted of a general cleanup of the specification and had mostly minor clarifications and revisions. The most major changes included the following:

Eliminated single-word (8-bit) DMA transfer protocols.
Added SMART (Self-Monitoring, Analysis, and Reporting Technology) support for the prediction of device performance degradation.
LBA mode support was made mandatory (previously it had been optional).
Added security mode, allowing password protection for device access.
Made recommendations for source and receiver bus termination to solve noise issues at higher transfer speeds.

ATA-3 has been officially published as "ANSI X3.298-1997, AT Attachment 3 Interface."

ATA-3, which builds on ATA-2, adds improved reliability, especially of the faster PIO Mode 4 transfers; however, ATA-3 does not define any faster modes. ATA-3 also adds a simple password-based security scheme, more sophisticated power management, and SMART. This enables a drive to keep track of problems that might result in a failure and thus avoid data loss. SMART is a reliability prediction technology that was initially developed by IBM.

ATA/ATAPI-4

First published in 1998, ATA-4 included several important additions to the standard. It included the Packet Command feature, known as the AT Attachment Packet Interface ( ATAPI ) , which allowed devices such as CD-ROM and CD-RW drives, LS-120 SuperDisk floppy drives, tape drives, and other types of storage devices to be attached through a common interface. Until ATA-4 came out, ATAPI was a separately published standard. ATA-4 also added the 33MBps transfer mode known as Ultra- DMA or Ultra-ATA . ATA-4 is backward compatible with ATA-3 and earlier definitions of the ATAPI. The major revisions added in ATA-4 were as follows:

Ultra-DMA (UDMA) transfer modes up to Mode 2, which is 33MBps (called UDMA/33 or Ultra-ATA/33).
Integral ATAPI support.
Advanced power-management support.
Defined an optional 80-conductor, 40-pin cable for improved noise resistance.
Compact Flash Adapter (CFA) support.
Introduced enhanced BIOS support for drives over 9.4ZB (zettabytes, or trillion gigabytes) in size (even though ATA was still limited to 136.9GB).

ATA-4 was published as "ANSI NCITS 317-1998, ATA-4 with Packet Interface Extension."

The speed and level of ATA support in your system is mainly dictated by your motherboard chipset. Most motherboard chipsets come with a component called either a South Bridge or an I/O controller hub that provides the ATA interface (as well as other functions) in the system. Check the specifications for your motherboard or chipset to see whether yours supports the faster ATA/33, ATA/66, ATA/100, or ATA/133 mode.

ATA-4 made ATAPI support a full part of the ATA standard; therefore, ATAPI was no longer an auxiliary interface to ATA but rather was merged completely within. This promoted ATA for use as an interface for many other types of devices. ATA-4 also added support for new Ultra-DMA modes (also called Ultra-ATA ) for even faster data transfer. The highest-performance mode, called UDMA/33 , had 33MBps bandwidth ”twice that of the fastest programmed I/O mode or DMA mode previously supported. In addition to the higher transfer rate, because UDMA modes relieve the load on the processor, further performance gains were realized.

An optional 80-conductor cable (with cable select) is defined for UDMA/33 transfers. Although this cable was originally defined as optional, it would later be required for the faster ATA/66, ATA/100, and ATA/133 modes in ATA-5 and later.

Also included was support for queuing commands, similar to that provided in SCSI-2. This enabled better multitasking as multiple programs make requests for ATA transfers.

ATA/ATAPI-5

This version of the ATA standard was approved in early 2000 and builds on ATA-4. The major additions in the standard include the following:

Ultra-DMA (UDMA) transfer modes up to Mode 4, which is 66MBps (called UDMA/66 or Ultra-ATA/66).
80-conductor cable now mandatory for UDMA/66 operation.
Added automatic detection of 40- or 80-conductor cables.
UDMA modes faster than UDMA/33 are enabled only if an 80-conductor cable is detected .

ATA-5 includes Ultra-ATA/66 (also called Ultra- DMA or UDMA/66 ), which doubles the Ultra-ATA burst transfer rate by reducing setup times and increasing the clock rate. The faster clock rate increases interference, which causes problems with the standard 40-pin cable used by ATA and Ultra-ATA. To eliminate noise and interference, the new 40-pin, 80-conductor cable has now been made mandatory for drives running in UDMA/66 or faster modes. This cable was first announced in ATA-4 but is now mandatory in ATA-5 to support the Ultra-ATA/66 mode. This cable adds 40 additional ground lines between each of the original 40 ground and signal lines, which help shield the signals from interference. Note that this cable works with older non-Ultra-ATA devices as well because it still has the same 40-pin connectors.

For reliability, Ultra-DMA modes incorporate an error-detection mechanism known as cyclical redundancy checking (CRC) . CRC is an algorithm that calculates a checksum used to detect errors in a stream of data. Both the host (controller) and the drive calculate a CRC value for each Ultra-DMA transfer. After the data is sent, the drive calculates a CRC value, and this is compared to the original host CRC value. If a difference is reported, the host might be required to select a slower transfer mode and retry the original request for data.

ATA/ATAPI-6

ATA-6 began development during 2000 and was officially published as a standard early in 2002. The major changes or additions in the standard include the following:

Ultra-DMA (UDMA) Mode 5 added, which allows 100MBps transfers (called UDMA/100, Ultra-ATA/100, or just ATA/100).
Sector count per command increased from 8 bits (256 sectors or 131KB) to 16 bits (65,536 sectors or 33.5MB), allowing larger files to be transferred more efficiently .
LBA addressing extended from 2 ²⁸ to 2 ⁴⁸ (281,474,976,710,656) sectors, supporting drives up to 144.12PB (petabyte = quadrillion bytes).
CHS addressing made obsolete. Drives must use 28-bit or 48-bit LBA addressing only.

ATA-6 includes Ultra-ATA/100 (also called Ultra- DMA or UDMA/100 ), which increases the Ultra-ATA burst transfer rate by reducing setup times and increasing the clock rate. As with ATA-5, the faster modes require the improved 80-conductor cable. Using the ATA/100 mode requires both a drive and motherboard interface that supports that mode.

Besides adding the 100MBps UDMA Mode 5 transfer rate, ATA-6 also extended drive capacity greatly, and just in time. ATA-5 and earlier standards supported drives of up to only 137GB in capacity, which was becoming a limitation as larger drives became available. Commercially available 3.5-inch drives exceeding 137GB were introduced during 2001 but originally were available only in SCSI versions because SCSI doesn't share the same limitations as ATA. With ATA-6, the sector addressing limit has been extended from (2 ²⁸ ) sectors to (2 ⁴⁸ ) sectors. What this means is that LBA addressing previously could use only 28-bit numbers, but with ATA-6 LBA addressing can use larger, 48-bit numbers if necessary. With 512 bytes per sector, this raises the maximum supported drive capacity to 144.12PB. That is equal to more than 144.12 quadrillion bytes! Note that the 48-bit addressing is optional and necessary only for drives larger than 137GB. Drives 137GB or less can use either 28-bit or 48-bit addressing.

ATA/ATAPI-7

Work on ATA-7 began late in 2001 and is still underway at the present. As with the previous ATA standards, ATA-7 is built on the previous standard (ATA-6), with some additions.

The primary additions to ATA-7 include the following:

Ultra-DMA (UDMA) Mode 6 added, which allows for 133MBps transfers (called UDMA/133, Ultra-ATA/133, or just ATA/133). As with UDMA Mode 5 (100MBps) and UDMA Mode 4 (66MBps), the use of an 80-conductor cable is required.
Added support for long physical sectors, which allows a device to be formatted so that there are multiple logical sectors per physical sector. Each physical sector stores an ECC field, so long physical sectors allow increased format efficiency with fewer ECC bytes used overall.
Added support for long logical sectors, which allows additional data bytes to be used per sector (520 or 528 bytes instead of 512 bytes) for server applications. Devices using long logical sectors are not backward compatible with devices or applications that use 512-byte sectors, meaning standard desktop and laptop systems.
Incorporated Serial ATA as part of the ATA-7 standard.
Split the ATA-7 document into three volumes : Volume 1 covers the command set and logical registers, Volume 2 covers the parallel transport protocols and interconnects, and Volume 3 covers the serial transport protocols and interconnects.

The ATA/133 transfer mode was actually proposed by Maxtor, and so far it is the only drive manufacturer to adopt this mode. Other drive manufacturers have not adopted the 133MBps interface transfer rate because most drives have actual media transfer rates that are significantly slower than that. VIA, ALi, and SiS have added ATA/133 support to their latest chipsets, but Intel has decided to skip ATA/133 in lieu of adding Serial ATA (150MBps) instead. This means that even if a drive can transfer at 133MBps from the circuit board on the drive to the motherboard, data from the drive media (platters) through the heads to the circuit board on the drive moves at less than half that rate. For that reason, running a drive capable of UDMA Mode 6 (133MBps) on a motherboard capable of only UDMA Mode 5 (100MBps) really won't slow things down much, if at all. Likewise, upgrading your ATA host adapter from one that does 100MBps to one that can do 133MBps won't help much if your drive can only read data off the disk platters at half that speed. Always remember that the media transfer rate is far more important than the interface transfer rate when selecting a drive, because the media transfer rate is the limiting factor.

ATA-7 is still a work in progress, so further changes may come. As a historical note, ATA-7 represents the combining of the venerable Parallel ATA standard and the newer Serial ATA standard under a single specification.