Custom kernel won't boot - unable to mount root fs

[Dave Sherohman <esper@sherohman.org>]

I've got a SCSI-based quad P3 system which starts up just fine off the stock
2.2.19pre17 kernel from potato, but refuses to boot the 2.4.5 kernel that
I've built for it.  It dies just after initializing the NET4 subsystem,
saying:

NET4: Unix domain sockets 1.0/SMP for Linux NET4.0.
request_module[block-major-8]: Root fs not mounted
VFS: Cannot open root device "801" or 08:01
Please append a correct "root=" boot option
Kernel panic: VFS: Unable to mount root fs on 08:01

The SCSI controller (Adaptec AIC7899) is detected, although it looks like my
2.4.5 only detects one channel while the stock 2.2.19pre17 sees two channels.
But that could just be a difference in how 2.2 and 2.4 report things.

There's obviously something that I need to include in my kernel, but I've
double- and triple-checked menuconfig and there's nothing which obviously
should be required that isn't included.  What am I missing?

-- 
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