How can I halt the PC during boot if CPU is running too fast?

[William Ballard <40311.nospam@comcast.net>]

I can O/C my computer stable to 230mhz FSB in Windows but it's not 
stable in Linux -- I get ReiserFS corruption.  Up to 220mhz is stable 
everywhere.  (Actually I can push it farther but don't want too much 
heat).

I'd like  to check /proc/cpuinfo early in the runlevel init 
scripts, preferably before the root is mounted, and halt if the MHz 
number is too high.  However since 'sed', 'cut', 'bash', and 'halt' are 
only available on the filesystem, I'd either need a special tool like 
busybox available somewhere or else I have to compile this into the 
kernel itself.

(1) How can I stop the computer from booting during boot process, based 
on detecting the CPU MHz?

(2) How can I run code before the root volume is mounted?

(3) Is this only possible during the Kernel itself and therefore I need 
to ask on a kernel mailing list?

Thanks