MSDOS Compatibility Mode: MSDOS Compatibility Mode is an operating mode that Win9x systems may default into if Windows 9x is unable to configure the IDE drivers properly. A typical cause is that one device could use a (faster) protected mode driver, but another device needs a real mode driver. This can be a conflict between the Primary and Secondary devices on one channel or by devices on different channels that use serialization of the two channels. MSDOS Compatibility Mode uses simple/unaggressive real mode drivers for all IDE devices. It is usually slow compared to dedicated drivers.

Unfortunately, the Registry key (NOIDE) that forces MSDOS Compatibility Mode is not cleared automatically after the cause of the problem is corrected. Therefore, in order to get a machine out of MSDOS Compatibility Mode, it may be necessary to:

Microsoft is prone to move Registry Keys around between OS versions (No, I have no idea why). The NOIDE may always be at HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\IOS\NOIDE or it may not. There is a TOOLS\MTSUTIL\NOIDE.INF file in Windows 98 and probably other versions that will clear NOIDE.

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