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Normally if the CHS fields for a partition are set to all 1's if they 
would otherwise wrap.  This typically causes BIOSes to properly detect 
that the disk should be put in Large mode.  If this option is specified 
the CHS fields will be set to wrapped values instead.  This option may 
be needed on very old PCs.

And from the fact that you have to open up your computer to boot from CD 
I'd say that you computer is quite old.

PS: the first sentence might need some work, as a non-native speaker it 
took me some time to figure it out.

--
Erik Wikström