IDE (aka ATA) is the technology used for most inexpensive disks and CDROMs. Most PCs built after 1995 have two IDE controllers each supporting two devices. Ideally, the controllers will be entirely separate even though implemented on the same card or chip. In practice, the controllers may be "serialized". They may share portions of the circuitry "serially". This has at least one, possibly two, negative affects.
One well documented problem occurs when the devices attached to the two controllers require incompatible software drivers. Usually this occurs when a CDROM requiring a "Real Mode" driver is attached to one of the controllers. It may be impossible for it to coexist with some (faulty) protected mode hard disk drivers. This problem is exacerbated by Windows 95 and 98 which set a NOIDE flag in the Registry if the protected mode drivers fail during initialization. NOIDE then causes a low performance -- one size fits all -- real mode "DOS Compatibility driver" to be used for all disks. "32 bit" disk options will be disabled in DOS compatibility mode. Win95/98 have no automatic provision to reset NOIDE. The Registry must be manually edited with REGEDIT in WIN95. WIN98 provides an INF file that will do the editing.
It appears that some Real Mode drivers handle serialization better than others and that a driver upgrade and clearing NOIDE may get some machines out of DOS compatibility mode without requiring a hardware change.
The Key in question is HKEY_LOCAL_MACHINE\SYSTEM\CURRENTCONTROLSET\SERVICES\VXD\IOS\NOIDE. NOIDE can either be removed or set to zero. In Win98, it can be cleared by double clicking \TOOLS\MTSUTIL\NOIDE.INF on the CDROM.
A second, less well substantiated problem is that even if serialization does not cause NOIDE to be set, it may be necessary to slow faster devices to the speed of the slower ones in order to accommodate the configuration required by the serialized hardware. I have been unable to confirm that this problem is real and is actually due to serialization.
In addition to triggering NOIDE setting, Serialization has been reported to cause 'Code 9 -- BIOS not Reporting Resource Correctly' errors in Windows 9x.
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