Note: ATA-3 is not
the same as Ultra ATA, which is called by a host
of names including the similar-sounding "ATA-33". Some people assume that since
Ultra ATA is the next faster ATA version after ATA-2 that it is called ATA-3, but this is
not so.IDE/ATA Standards and Feature Sets
There are many different standards that are associated with IDE/ATA, which leads to a great deal of confusion as people try to make sense of them and figure out what is compatible with what. This problem is compounded by the fact that the manufacturers of hard drives can't seem to get along and agree on a single naming convention for their implementations and enhancements of the "real" ATA standards. The result is a confusing mish-mash of standards, some of which are real and others of which are just fancy-sounding marketing programs.
New standards are usually created to define new, higher-speed transfer modes, and other improvements to performance, reliability or compatibility. Most of these start off as informal standards; some are eventually codified into "official" ANSI standards. It can take several years for ANSI to approve and issue a formal standard.
The original, base specification for the family of interfaces described in this section is called both IDE and ATA. The name IDE is the more popular of the two, even though it is misleading since it stands for integrated drive electronics which refers to putting the logic board with the hard disk much more than it has anything to do with the interface. The more correct name is ATA or AT Attachment, which defines the standard interface as attached to an AT-style machine (which is the generic way of describing every PC since the old 8-bit XT). The IDE/ATA interface is 16 bits wide, just as the original IBM AT's data and I/O buses were. This size persists to this day, even in the higher-performance enhancements defined in ATA-2.
The ATA specification has been defined as an ANSI standard. A major reason for defining ATA as a formal standard was to eliminate some of the incompatibility problems that plagued early IDE/ATA drives. This particularly showed up when drives made by different manufacturers were placed as master and slave on the same IDE/ATA channel--often they would not work.
The original IDE/ATA standard defines the following features and transfer modes:
"Plain" ATA does not include support for enhancements such as ATAPI support for non-hard-disk IDE/ATA devices, block mode transfers, or logical block addressing.
While the original ATA standard served the early hard disks well, it was not suited to support the growing performance needs of a newer breed of hard disks. These disks required faster transfer rates and support for enhanced features.
The ATA-2 specification was defined and eventually approved through the establishment of an ANSI standard. It defines the following improvements over the base ATA standard (with which it is backward compatible):
ATA-2 is a "real" standard, because it is approved by ANSI, an independent standards body, and is therefore something tangible that you can refer to, and that cannot be easily changed. Unfortunately, few drives are marketed under the standard "ATA-2" but rather as "Fast ATA", "Fast ATA-2", or "Enhanced IDE", which are all marketing terms and not real standards. They all are based in one way or another on ATA-2, but differ in terms of what is supported and how.
Fast ATA and Fast ATA-2 are the marketing terms used by peripheral manufacturers Seagate and Quantum to refer to different portions of the (real) ATA-2 specification. I believe that these may be the most confusing of all of the strange terminology used regarding the IDE/ATA interface families, despite the protestations from these two companies that Western Digital's EIDE "standard" is confusing. It is, but so are these two, because their names make it sound like Fast ATA-2 is related to ATA-2, and Fast ATA is related to regular ATA. This is not so, as both really build on ATA-2. Also, the two are virtually the same.
For all intents and purposes, Fast ATA-2 is identical to ATA-2; see its description for a list of its improvements over regular ATA. Fast ATA is also the same, except that it does not support the highest transfer modes; it includes PIO mode 3, but not 4; and multiword DMA mode 1, but not 2.
Quantum and Seagate will claim that their standards are meant to define "only" faster transfer rates, but this is really not the case. Quantum for example claims in their criticism of the Enhanced IDE (EIDE) standard that LBA (which is part of EIDE) is a "BIOS and device driver function" and therefore doesn't belong in a hard disk standard, but then later in the same article make specific mention of their drives supporting LBA. EIDE is confusing but really, all of these marketing "standards" are misleading and really need to be cleaned up once and for all and a single, true standard agreed upon and established. I'm not holding my breath.
The ATA-3 standard is a minor revision of ATA-2. It defines the following improvements compared to ATA-2 (with which it is backward compatible):
The manufacturers of hard disks are generally avoiding the use of the term ATA-3 even though they are adding its component features (such as SMART) to their drives. In a way this is probably for the best, since the interfacing naming business is confusing enough as it is. ATA-3 is basically ignored for the simple reason that it doesn't define any new high-performance transfer modes, which are of primary interest to both manufacturers and buyers.
Note: ATA-3 is not
the same as Ultra ATA, which is called by a host
of names including the similar-sounding "ATA-33". Some people assume that since
Ultra ATA is the next faster ATA version after ATA-2 that it is called ATA-3, but this is
not so.
Ultra ATA / Ultra DMA / ATA-33 / DMA-33
The newest ATA-related standard is Ultra ATA, which also goes by several other names including Ultra DMA, ATA-33, and DMA-33. It is also sometimes called ATA-3, which it is not. Ultra ATA is not a formal standard but rather a term that refers to the use of the higher-speed DMA-33 transfer mode (multiword DMA mode 3), running at 33.3 MB/s. Special error detection and correction logic (CRC) is used to support the use of this high-speed mode over a standard IDE/ATA ribbon cable (which has not changed since transfer rates were below 5 MB/s and can now be a problem in terms of corruption when used at very high speeds). Ultra ATA maintains backward compatibility with the older standards upon which it is based (ATA-2 and ATA). If you use an Ultra ATA hard disk in a system that does not support Ultra ATA, it will still work, just at the slower speeds that system can handle.
Drives that support Ultra ATA allow the use of the high-speed DMA-33 transfer mode, but are otherwise the same as other ATA-2/EIDE drives. Ultra ATA requires both a hard disk and a system BIOS/chipset that support the Ultra ATA protocol.
Originally, the IDE/ATA interface was designed to work only with hard disks. CD-ROMs and tape drives used either proprietary interfaces (often implemented on sound cards) or the floppy disk interface (which is slow and cumbersome). A few years ago it became apparent that there would be enormous advantages to using the standard IDE/ATA interface to support devices other than hard disks, due to its high performance, relative simplicity, and universality.
However, because of how the ATA command structure works, it wasn't possible to simply put non-hard-disk devices on the IDE channel and expect it to work. Therefore, a special protocol was developed called the ATA Packet Interface or ATAPI. The ATAPI standard is used for devices like CD-ROM and tape drives. It enables them to plug into the standard IDE cable used by IDE/ATA hard disks, and be configured as master or slave, etc. just like a hard disk would be.
Note: When you see a
CD-ROM or other non-hard-disk peripheral advertised as being an "IDE device" or
working with IDE, it is really using the ATAPI protocol.
Internally, however, the ATAPI protocol is not identical to the standard ATA (ATA-2, etc.) command set used by hard disks. A special ATAPI driver is used to communicate with these devices. This driver must be loaded into memory before the device can be accessed (some operating systems like Windows 95 support ATAPI internally and in essence, load their own drivers for the interface). The name "packet interface" comes from the fact that commands to ATAPI devices are sent in groups called packets; ATAPI in general is a much more complex interface than regular ATA. The actual transfers over the channel use regular PIO or DMA modes, just like hard disks, although support for the various modes differs much more widely by device than it does for hard disks.
For the mostpart, ATAPI devices will coexist with IDE/ATA devices and from the user's perspective, they behave as if they are regular IDE/ATA hard disks on the channel. Newer BIOSes will even allow booting from ATAPI CD-ROM drives.
Enhanced IDE, also called EIDE, is the term that Western Digital uses to describe its advanced drives that incorporate the performance improvements defined in ATA-2, along with several other refined protocols. EIDE has become a widely-accepted term in the industry, which would be great if not for the fact that it is used to stand for so many things, some of them completely different. For this reason, the term has been criticized--not least by Western Digital's competitors--for being confusing and vague.
Some of this criticism is valid. EIDE is a marketing term and not a real standard. One problem with it is that since it is Western Digital's, they can change it as they see fit, and they have. For example, at one time EIDE only included PIO modes up to mode 3, while now it includes mode 4 as well. Another problem is the wide scope of the standard.
Western Digital describes its Enhanced IDE program as including the following improvements over ATA:
Most of the criticism of EIDE is that its scope is too wide, and that it encompasses support that is really a function of the BIOS. For example, support for dual IDE/ATA host adapters, meaning a secondary IDE/ATA channel, has nothing to do with the interface or the hard disk itself. And ATAPI is a standard that is defined for use with CD-ROMs and other non-hard-disk devices, which again requires BIOS and driver support and has nothing to do with the hard disk. This is why other hard disk manufacturers have not only not included these as part of their standards (Fast ATA, Fast ATA-2), they have made a point of saying that they have omitted them intentionally because they feel they don't belong.
On the other hand, EIDE is criticized for specifying LBA support as part of its standard, which is really part of ATA-2. Seagate and Quantum, which use Fast ATA and Fast ATA-2 instead of EIDE for their marketing programs, also support LBA despite claiming that this support doesn't belong as part of an ATA standard. So it is all rather muddy.
I believe that Western Digital was trying to define an encompassing standard that would specify changes not just in terms of how hard disks worked but also the BIOS and other parts of the system. A laudable goal, but they may have bitten off a bit more than they could chew. One problem is that the term "EIDE" is now put on so many different things that it is hard to understand what it really means. For example, an "EIDE" hard disk may support LBA, but it won't work unless the BIOS also supports it. Some hard disk controllers now call themselves "EIDE" controllers, but that doesn't necessarily mean that they support all the provisions of EIDE. You have to break down their claims into the individual features they support in order to do a proper evaluation.
Finally, there is the matter of "enhanced IDE" vs. "enhanced BIOS". This one can really be confusing. Because of the fact that both of these phrases use the word "enhanced", and because EIDE defines BIOS support standards, many people have come to think of the terms as interchangeable when they really are not. This has lead to claims that you need an enhanced IDE interface to support disks over 504 MB, when you don't--you just need an enhanced BIOS, as described here. The fact that some companies advertise add-in cards with enhanced BIOSes as "enhanced IDE cards" makes this whole problem worse.
Making Sense of IDE/ATA Standards and Compatibility
OK, so where does the collection of different "standards" leave you? Even though there are a number of standards and the terminology is quite confusing at times, in practice the situation isn't all that bad. One way to make sense of ATA, ATA-2, Fast ATA, EIDE etc. is simple: don't use them. Break them down into their components and find out what the drive supports that way. This means figuring out if the drive will support multiword DMA mode 2 by looking at specifications or asking a reputable dealer, instead of looking for an EIDE sticker and making assumptions. It's always preferable to get the facts.
If you don't want to deal with the details, you can use some simple rules of thumb. First of all, virtually all newer drives support the features that are defined under ATA-2, whether they are marketed as being Fast ATA-2 or EIDE. These drives are interchangeable with one another and will work together and in any system that has an enhanced BIOS (for support of disks over 504 MB). Second, the Fast ATA-2 standard doesn't mention support for ATAPI, but this is more due to marketing games. Fast ATA-2 drives will work just fine used with ATAPI devices.
In virtually every case, the newer standards are fully backward compatible with the older ones. This means that ATA-2 drives will normally work fine with older ATA drives, and motherboards that support ATA-2 will support the older technology as well. There can be issues in configuring two drives on the same channel when one is older than the other, but these matters have more to do with manufacturers that have not strictly adhered to the IDE/ATA standards, especially with much older hard disks.
If you are looking at newer drives, one of the few distinguishing characteristics from an interface standpoint is whether or not Ultra ATA is supported, so you will want to find out about that. Remember that you need support from the motherboard (actually the chipset) in order to take advantage of Ultra ATA.
The table below provides a quick summary of the different IDE/ATA interface variants and what they support:
Interface Standard |
Standard Type |
PIO Modes |
DMA Modes |
Special Features or Enhancements Introduced Relative to IDE/ATA |
IDE / ATA |
ANSI |
0, 1, 2 |
Single word 0, 1, 2; multiword 0 |
-- |
ATA-2 |
ANSI |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2 |
Block transfers, logical block addressing, Improved identify drive command |
Fast ATA |
Marketing |
0, 1, 2, 3 |
Single word 0, 1, 2; multiword 0, 1 |
As for ATA-2 |
Fast ATA-2 |
Marketing |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2 |
As for ATA-2 |
ATA-3 |
Unofficial |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2 |
As for ATA-2, plus improved reliability, SMART |
Ultra ATA |
Unofficial |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2, 3 (DMA-33) |
As for ATA-3 |
ATAPI |
ANSI |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2 |
Support for non-hard-disk devices |
EIDE |
Marketing |
0, 1, 2, 3, 4 |
Single word 0, 1, 2; multiword 0, 1, 2 |
As for ATA-2, plus ATAPI and dual host adapters |
Next: IDE/ATA
Transfer Modes and Protocols
| The PC Guide Disk Edition - Version
1.8 - January 1999 Based on The PC Guide web site (http://www.PCGuide.com) Visit online for the latest updates, or to order Disk Edition upgrades. ©
Copyright 1997-1999 Charles M. Kozierok. All
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