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The original IBM PC/AT defined support for two hard disk controllers and allowed support for up to four disk drives via a primary and secondary controller. The original BIOS and operating system drivers, however, only supported the primary controller, limiting the standard PC configuration to two disk drives. Today's operating systems now offer both primary and secondary controller support providing an opportunity to extend peripheral attachment capabilities with IDE. The addition of a second connector via a hardware change is a simple, low cost solution that allows for multiple IDE peripheral connectivity.
The cost of a second IDE connector is less than $1.00. Most core logic and Super I/O devices have already integrated the capability to support either the primary or secondary address decode logic and therefore the cost of the secondary port is simply the 40 pin connector and surrounding transceivers and resistors. For $1.00, dual IDE connectors offer support for four IDE devices and satisfy the expansion needs of the majority of the mainstream personal computer market, a very cost effective alternative to connectivity via SCSI.
Western Digital's Enhanced IDE program works with system manufacturers to understand the BIOS implications of a secondary channel for support of two additional IDE devices. The BIOS must be able to determine the physical location of the drive based on the Int 13h drive number . Since DOS 3.0 and later will support up to seven disk drives, only the system BIOS Interrupt 13h needs to be modified to support primary and secondary IDE. Windows 3.1 accesses the disk via Interrupt 13h calls to the BIOS. Again, all that is required is modification to the system BIOS to support dual channel IDE. IBM OS/2 2.0 and 2.1 as well as MS/IBM OS/2 1.31 all support four IDE drives on dual IDE connectors via their drivers. Netware is hardcoded to support four IDE connectors or 8 IDE devices. Dual channel IDE support will be in the final release of Windows NT.
Dual channel IDE not only enables the cost effective and easy implementation to support multiple disk drives, it presents the opportunity to expand IDE into non-disk peripheral support. A slow speed channel and a high speed channel can be developed for efficient implementation of storage solutions via high performance hard disk drives and mass data storage vehicles such as CD-ROM and tape drives.
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