I never noticed the mem requirements ...
So are you compiling the JDK just as a curiosity or do you not like to install binaries? I've never tried to actually compile the JDK itself... Besides, shouldn't you be working on Pearl and not trying to compile Java VM's
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As far as requiring the Sun JDK, I suspect it requires "JDK 1.4" (rather than specifically the Sun JDK) ... Which probably means it will run on OS X as well. I haven't checked to see if IBM has released a 1.4 version yet (I assume they probably have). Sun's the only one that you have a chance of seeing the source for though.
ms