2 Introduction

The IDE field has seen a great number of changes since a few years ago. These novelties are commonly known under the names Enhanced IDE (EIDE) and Ultra-ATA. EIDE has caused a lot of confusion since it is merely a marketing program from Western Digital which, in turn, builds on a couple of real standards: ATA-2 and ATAPI. Fast-ATA, launched by Seagate and endorsed by Quantum in response to WD's marketing is similar. It builds on ATA-2 only. Ultra-ATA is a widely supported extension of ATA-2 (and ATA-3) adding high speed DMA modes.

IDE (Integrated Drive Electronics--or numerous other interpretations) and ATA (AT Attachment) are one and the same thing: a disk drive implementation designed to integrate the controller onto the drive itself, thereby reducing interface costs, and making firmware implementations easier. This low cost/easy integration created a boom in the disk drive industry, as PC integrators readily ate up the low-cost alternative. Since the late 80's, ATA (as it is properly called) has become the drive of choice for the cost inhibited buyer.

When it became clear that improvements in harddrive technology and the increasing demands made by software would eventually strain the capacity of the ATA interface, the drive industry, in the form of the Small Form Factor (SFF) Committee, created a compatible extension of ATA called ATA-2. This standard not only adds faster PIO modes and DMA modes, but also improves upon Plug'n'Play and compatibility with future revisions of the standard.

While there is also a new way of addressing sectors on the harddisk (LBA), this is merely a simplification. Contrary to common myth LBA proper has nothing to do with breaking the famous 504MB (528 million bytes) barrier. In fact, even in the old ATA/IDE standard the capacity limit is well over 100GB.

The latest revision of the standard is ATA-3. Key features include: improved reliability, especially of PIO mode 4; a simple password-based security scheme; more sophisticated power management; and Self Monitoring Analysis and Report Technology (S.M.A.R.T.), allowing the drive to warn you about certain types of impending failure.

ATA-3 does not define any faster modes. Even though some manufacturers were once marketing "mode 5" equipment, there will never be a PIO mode beyond ATA-2 PIO mode 4.

One of the disadvantages of ATA is that it was designed for harddisks only. That was fine back when a high end PC shipped with just a floppy drive and a 40MB harddisk, but today CD-ROM and tape drives are commonplace devices that should preferably run off a single low-cost interface. The ATA Packet Interface (ATAPI) is a standard designed for devices such as CD-ROMs and tape drives that plug into an ordinary ATA (IDE) port.

The principal advantage of ATAPI hardware is that it's cheap and works on on every PC with an IDE or 'EIDE' adapter. ATAPI tape drives can enjoy superior performance and reliability compared to the popular QIC117 'floppy' tape devices.

Beware that although ATAPI devices plug into the IDE interface, they differ considerably from an IDE harddisk. Caching controllers and other intelligent interfaces will not work unless they're ATAPI aware. Booting from an ATAPI CD-ROM is only possible with the latest BIOSes.

Ultra-ATA is somewhat similar to Ultra-SCSI in the sense that it bridges the gap between the current standard (ATA-3) and ATA-4, which isn't quite finished yet. Ultra-ATA adds a new, high performance mode: DMA/33 with 33MB/s bandwidth, twice that of DMA mode 2. It is supported by Intel's Pentium II chipset, the TX Pentium chipset, the latest Intel competitors and a few add-on (PCI) cards.

Work on the next standard, predictably called ATA-4, has already started. First and foremost it is a much-needed attempt to merge ATA-3 and ATAPI into one. Faster transfer modes are also on the agenda, of course: it will incorporate Ultra-ATA's DMA/33, at least.

Regarding other features, the future for strong command overlap looks rather bleak: the current proposal is a hideous animal and important players like Microsoft do not plan on supporting it in its present form. More limited forms of overlap stand a somewhat better chance of surviving.

A rather different issue was the 504MB (or, equivalently, 528 million bytes) capacity limit that becomes apparent when accessing IDE drives through the BIOS. It is caused by the disk geometry (cylinders, heads, sectors) supported by the combination of an IDE drive and the BIOS' software interface ('int13'). Both IDE/ATA and the BIOS are capable of supporting huge disks, but their combined limitations conspire to restrict the useful capacity of the drive to 504MB. Since only MSDOS still uses the BIOS for harddisk access, this is sometimes erroneously thought to be an MSDOS limitation; other operating systems experience the same restrictions at boot time though.

An Enhanced BIOS works around this problem by representing the drive to the software using a different geometry than the native geometry of the drive itself. This juggling act is called 'translation'. For example, if your drive has 1500 cylinders and 16 heads, a translating BIOS will make software programs think that the drive has 750 cylinders and 32 heads.

You do not need an "EIDE" adapter to use harddisks greater than 504MB.

The de facto standard is described in the Microsoft/IBM "INT 13 Extensions" document. Phoenix has presented a superset in their "Phoenix Enhanced BIOS" specification. Phoenix, AMI, Award and MR BIOSes are based on the Microsoft/IBM specification.

Last but not least, the usual limit of two devices was far too restrictive if CD-ROMs and tape drives were to be connected to the IDE interface in addition to the harddisk(s). Fortunately the solution was already known in the form of a so-called secondary harddisk interface. The possibility of such an interface has been there for a long time, but support was lacking.

There is nothing special about a secondary channel; it is an ordinary (E)IDE port that uses a different interrupt and I/O addresses to avoid clashes with the ordinary (primary) one. A secondary interface allows you to connect another ribbon cable with two more ATA devices (harddisk, ATAPI CD-ROM or ATAPI tape). Today, many interfaces combine both primary and secondary port on a single board to make a dual-ported interface that handles up to four devices.

To use harddisks on the secondary port with DOS and Windows 3.x, you will need BIOS (either system BIOS or adapter BIOS) or driver support. You can recognize a BIOS with four drive support by the fact that it allows for four sets of drive parameter in the BIOS setup.