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
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
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
Today what we call the ATA interface is controlled by an independent group of representatives from major PC, drive, and component manufacturers. This
The rules these
The Parallel ATA interface has evolved into several successive standard versions, introduced as
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.
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
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:
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
Although ATA-1 supported theoretical drive capacities up to 136.9GB (2
= 267,386,880 sectors), it did not address BIOS limitations that
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:
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
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:
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,
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:
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
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
) for even faster data transfer. The highest-performance mode, called
, had 33MBps bandwidth ”twice that of the
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
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:
ATA-5 includes Ultra-ATA/66 (also called
), which doubles the Ultra-ATA burst transfer rate by reducing setup times and increasing the clock rate. The faster clock rate
For reliability, Ultra-DMA modes
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
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:
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.