Enhanced IDE (EIDE) Technology

Original IDE drives were connected to the system board by an adapter card in an expansion slot. Now, most system boards support IDE by providing one or two IDE connections directly on the system board. To alert a user when a hard disk is being accessed, a system board has a two-pin connection that connects to an LED on the front

of the computer case. A lit LED means a hard drive is being accessed. An IDE drive has its controller mounted directly on top of the drive. The operating system does not communicate directly with the IDE drive, as is the case with drives that use older technology.

The operating system passes its requests to the drive controller, which is responsible for keeping up with where and how data is stored on the drive. As far as the operating system is concerned, an IDE drive is simply a very long list of logical sectors, each 512 bytes long. The operating system doesn’t care where on the drive these sectors are located, since the controller maintains that information.

Setup for IDE drives is very simple; the most important fact that setup and the operating system need to know is how many sectors there are on the drive. It’s important not to overestimate the number of sectors because you don’t want the operating system requesting use of a sector that does not exist. However, you can tell setup that you have fewer sectors than are present. If you do, some sectors will remain unused.

Enhanced IDE (EIDE) Technology

IDE drives follow the standard AT interface known as ATA (AT Attachment interface).

These IDE drives translate the sector coordinates to the coordinates expected by DOS and BIOS, but are still limited to the 528 MB ceiling on BIOS because they follow the ATA standard that most BIOS at that time also followed.

To break this 528 MB limitation, a new standard was developed called ATA-2, which is used by the Enhanced IDE (EIDE) drives.

The ATA standard only allowed up to two devices on the same controller and they both had to be hard drives. The ATA-2 standard allows for up to four devices on the same controller, and these devices may be hard drives, CD-ROMs, or tape drives as well as other devices. The ATA standard assumed that the number of tracks or cylinders would not exceed 1,024, a limitation ATA-2 does not have. Enhanced IDE drives, accordingly, can hold more than 528 MB of data, can have more than 1,024 tracks or cylinders, and can have more than two devices on a single controller.

As standards were developed, different hard drive manufacturers adopted different names for them, which can be confusing. Standards today specify data transfer speed more than any other single factor.

When selecting a hard drive standard, select the fastest standard you can, but keep in mind that the operating system, BIOS on the system board, and the hard drive must all support this standard. If one of three does not support the standard, the other two will probably revert to using a slower standard that all three can use. The most common method that EIDE drives use to exceed the 528 MB limit is logical block addressing rather than the traditional CHS (cylinders, heads, sectors) method. With LBA, DOS and BIOS see a drive only as a list of sectors, each with a 28-bit address.

With 28-bit addressing, you can have as many as 268 million sectors of 512 bytes each, which allows for a maximum disk capacity of about 128 GB. The following table lists the different ANSI standards for hard drives.