Background
Comparative Performance
The IDE Standard
Development of Enhanced IDE
Features of Enhanced IDE
Implementing Enhanced IDE
Enhanced IDE Adapters
For several years now, IDE has been the preferred interface for hard drives on volume PCs, due to its low cost and ease of implementation. It has shrugged off competition from competing interfaces, such as SCSI, to become the effective standard for PC disk I/O.
The capacity and throughput required by today's Windows-based applications however, particularly in the multimedia arena, threaten to demand more than IDE can deliver. Its support for only two devices, each limited to a maximum 528MB, and restricted host transfer rates of up to just 3 MB/second, create capacity and performance bottlenecks which will prove unacceptable in tomorrow's standard PC configurations, and which already restrict the performance of multitasking, 32-bit operating systems such as OS/2 and Windows NT.
Enhanced IDE (E-IDE) builds upon the existing IDE standard to remove these bottlenecks and create a new drive interface standard capable of delivering capacity and performance far in excess of current norms. Its compatibility with existing IDE devices and small price premium virtually guarantee that E-IDE will supplant standard IDE as the chosen drive interface for volume PCs within a matter of months, probably before the end of 1994.
This paper has been written to provide a convenient reference document for system builders and others who need to understand and take advantage of this new technology which is set to revolutionise drive subsystems in volume PCs.
Perhaps the most significant benefit of Enhanced IDE is its vastly improved data throughput compared to standard IDE. E-IDE outperforms Fast SCSI-2 and, with certain drives, even rivals the latest Fast Wide SCSI-P. This level of performance is achieved without the use of an onboard cache, and, combined with its other new features, allows E-IDE to compete on equal terms (but at a much more attractive price point) with SCSI as a high-end drive interface.
Typical comparative data throughput rates from the drive to the host PC on VESA Local Bus systems are shown in the graph below. These figures were obtained on a 33MHz 486DX VL-Bus PC.
IDE (Integrated Drive Electronics) describes a class of hard disk subsystem which was designed for AT-compatible PCs to overcome the performance limitations of earlier subsystem standards such as ST506 and ESDI.
In an IDE subsystem, the logic which controls the operation of the hard disk is integrated into the hard disk unit itself. This approach allowed hard disk manufacturers to optimise disk I/O performance without being tied to a fixed controller standard. Instead, a simple interface was defined through which the drive unit could interchange data with the host computer and receive and respond to instructions.
The IDE concept was originally proposed in 1986 by Western Digital and Compaq. While the term IDE in itself does not necessarily represent a standard, their proposals were incorporated into an industry agreed interface specification known as AT Attachment (ATA). This defines a command and register set for the interface through which the drive unit communicates with the host PC. It is estimated that at least 95% of hard disks installed in PCs today are ATA-compatible drives (also nown as AT-compatible drives).
Since the primary purpose of the ATA specification was to decouple the controller logic from the interface, thus making it possible for drive manufacturers to enhance drive performance independently, no enhanced performance features were designed into the ATA interface. The emphasis was on compatibility with the host PC.
Even on local bus machines, an ATA interface therefore remains a 16-bit channel with these additional limitations:
As drive manufacturers began to develop their products in response to the opportunity offered by adoption of the ATA standard, it became evident that drive performance and capacity would ultimately reach a level at which the interface would cease to be a facilitator and become a bottleneck. At the same time, the spread of local bus architectures offered a bandwidth on the host PC which the ATA interface was unable to take advantage of.
The race was on to develop an enhanced version of the ATA interface which would support those key features lacking in the original specification. In 1993, Western Digital proposed E-IDE as an enhanced but compatible extension to the existing ATA standard, and incorporated E-IDE features into their own range of Caviar drives. PC manufacturers were now free to design systems incorporating E-IDE interfaces, and as Western Digital's proposals were incorporated into the industry's agreed ATA specifications, other drive manufacturers began to develop and release drives with E-IDE support.
Because Enhanced IDE adds to the traditional ATA standard, drives with E-IDE support are fully backwards compatible with earlier ATA drives. The major drive manufacturers have therefore included support for E-IDE in their standard ATA-compatible drives rather than offer two separate ranges.
The final step was the release of E-IDE adapter cards which would allow system builders to incorporate the technology into PCs built with off-the-shelf motherboards, and take advantage of the E-IDE support built into the drives they already buy. These cards are now becoming available for both VL-Bus and PCI architectures.
Enhanced IDE introduces five separate enhancements to the original ATA standard. Some confusion has been caused by certain manufacturers introducing interfaces labelled as Enhanced IDE that provide partial support for these enhancements, for instance supporting only the faster data transfer rates. True E-IDE provides all five of these features.
Enhanced IDE incorporates specifications for new transfer modes which allow high-speed data transfer to take advantage of the higher bandwidths available on VESA and PCI local bus architectures. Whereas host transfer rates on ISA are limited to around 2-3 MB/sec, these new transfer modes permit transfers at rates ranging from 10-20 MB/sec, comparable to those achieved using high-end SCSI interfaces.
E-IDE to host transfers are controlled by the drive, under an operation called Flow Control Using I/O Channel Ready (IORDY). They are achieved using new definitions for Programmed Input/Output (PIG), a standard method for transferring data between a drive and the host PC which is already supported within existing BIOS structures and operating system device drivers. There are also definitions for transfers using Direct Memory Access (DMA), an alternative with applications on the PCI local bus, but which requires significant BIOS changes and external device drivers.
The E-IDE standard today defines PIO Mode 3, and most currently-available drives incorporate this level of support. Mode 3 allows for data transfer rates up to a ceiling of 11.1 Megabytes per second.
Modes 4 and 5 will follow shortly, allowing even faster data transfer rates. Mode 4 drives are expected to be available in the autumn, capable of up to 16.7 MB/sec, while tests on Mode 5 drives, due to be released in 1995, have recorded transfer rates in excess of 20 MB/sec.
This aspect of Enhanced IDE breaks through the 528MB barrier which had previously prevented IDE competing with SCSI for high capacity applications such as network servers and advanced multimedia, providing support for up to 8.4GB storage using a technique called BIOS translation.
The 528MB limitation is due to a mismatch of permitted field sizes for the cylinder, head and sector (CHS) entries allowed by IDE and by the PC BIOS's Interrupt 13h (INT13). E-IDE defines two approaches to side-stepping this obstacle.
The first solution uses the traditional CHS method by translating drive parameters into a pseudo CHS array. A more sophisticated solution translates the parameters to a non-drive specific 28-bit Logical Block Address (LBA). Both methods raise the drive capacity ceiling to the 8.4GB permitted by INT13.
The LBA solution is regarded as a simple and efficient means by which operating systems can access E-IDE drives, and is already supported by 32-bit operating systems such as Windows NT and OS12 2.1. External drivers are required for other operating systems. Future versions of Windows may also include an IBM/Microsoft variant of the LBA system, called the INT13 Extensions Functional Specification, which defines packets of LBA information that are provided as input when INT13 is called.
Enhanced IDE incorporates a feature which permits a second ATA channel to coexist alongside the primary channel. Since both channels will support up to two devices, this feature doubles IDE support to a maximum four devices.
The primary channel occupies the same port address (1FOh) and interrupt as a standard IDE interface to assure backward compatibility. The secondary channel's port address (typically 170h) is set by a jumper on the host adapter or motherboard and this channel will normally occupy system interrupt IR015. The purpose of the secondary channel is to provide additional capacity for attachment of non-hard disk devices, although hard disks are also supported, and it may be connected either to the ISA bus or the local bus.
Dual channel support is i mplemented within the interface logic, and therefore is not part of the ATA specification, which deals solely with the definition of the channel itself. Drives do not therefore need to explicitly support this feature to be able to take advantage of it. Major PC BIOS vendors already offer support for the feature, while most 32-bit operating systems and the latest version of DOS include native support. Where native support does not yet exist, BIOS enhancements and external drivers are available.
Enhanced IDE drives provide additional information to the 8108 and software drivers to enable full auto-detection of E-IDE features without user intervention.
The final element of Enhanced IDE is a specification allowing devices other than hard disks to be attached to the interface, completing a range of features which permit IDE to compete far more effectively against SCSI, and providing a significant role for the secondary channel.
The ATA Packet Interface (ATAPI) specification, finalised early in 1994, defines the new command set required to address such devices through the ATA interface. The first wave of ATAPI devices now being introduced into the marketplace are CD-ROM drives, providing a much-needed alternative to the current options of either SCSI or proprietary interfaces for CD-ROM attachment. Tape drives are expected to appear before the end of 1994.
ATAPI connection of CD-ROM drives provides significant cost savings compared to SCSI or proprietary interfaces, since the E-IDE interface is already required in the PC to support hard disk storage, without the price penalty involved in implementing SCSI. Connection is achieved through the standard IDE cable, providing a further cost saving. An additional benefit is that system builders are freed from reliance on the CD-ROM manufacturers' own proprietary interfaces. Any sound card can be used with an ATAPI CD-ROM drive. A standard audio cable provides the audio link from the drive to the card.
BIOS and operating system support for ATAPI will either be incorporated native or supplied as BIOS enhancements and drivers.
Enhanced IDE has been designed to extend traditional IDE's ease of integration to encompass larger, faster hard disks and other devices. Once E-IDE has replaced standard IDE as the industry's preferred choice for mainstream PC drive subsystems, installation of E-IDE hard disks, ATAPI CD- ROMs and other devices will be as simple as IDE hard disk installation is today. In the transition period, successful implementation of Enhanced IDE into a PC configuration relies on selection of E- IDE compatible system components.
A VESA or PCI local bus Enhanced IDE interface is a prerequisite for support of E-IDE features. These interfaces are now available as host adapters for both local bus architectures.
Hard disks selected should support PIO Mode 3 or above in order to benefit from the significantly increased drive performance available with E-IDE. Drives currently available in the market support Mode 3. Mode 4 devices are expected to become available during the autumn of 1994, with Mode 5 devices emerging in the middle of 1995.
Manufacturers who currently offer Mode 3 hard disks include:
| Western Digital | Quantum |
| Conner | Maxtor |
| Seagate* | IBM |
The authors are able to provide, on request, a regularly updated list of hard disk models available from these and other manufacturers with Mode 3, 4 and 5 support.
* Seagate
have chosen to describe their E-IDE hard disks as "Fast ATA"
devices. While Fast ATA includes a Seagate-specific multiple block read/write
feature which, with appropriate BIOS support, can nudge conventional IDE
performance from 3 MB/sec up to 4 MB/sec, the remainder of the Fast ATA
specification consists of E-IDE features.
Non-hard disk devices must be ATAPI compatible in order to work with the E-IDE interface. Devices designed for other interfaces, such as SCSI or proprietary interfaces, will not work with E-IDE unless such support has been explicitly incorporated.
Currently-available operating systems do not include support for all the features of Enhanced IDE, and therefore drivers for the intended operating system are essential. Drivers must be supplied by the adapter vendor, and it is important to note that not all adapter vendors will offer drivers for the more advanced operating systems such as NetWare, Windows NT, OS/2 and Unix.
An up-to-date BIOS is required to utilise all the features of Enhanced IDE.
Enhanced IDE adapter cards are now becoming available in the market with a variety of features and levels of support for E-IDE. When selecting an adapter for use in their PC configurations, it is important for system builders to review their requirements and ensure that the product selected matches their needs.
The following checklists provide an overview of the features which may or may not be included in specific E-IDE adapters, and may be used as an aid in the selection process.
Alongside development of Enhanced IDE, a specification has also been agreed for power management of ATA-compatible drives known as the ATA2 Power Management Feature Set.
The significantly improved performance and capacity of Enhanced IDE provides a synergy with other features and functions which adapter vendors may chose to build into their E-IDE products.
As is common with traditional IDE adapters, I/O functions will often be included on the adapter. Some vendors are chosing to incorporate enhanced I/O features such as 16550 UARTs to provide support for high-speed serial connection, which, for example, is essential to support the very high data throughput of 28800 bps modems now becoming available.
Other vendors include support for file and data server applications such as disk mirroring under NetWare and RAID under Windows and Window NT.