reclaimed 8M space (yes, more than 8MB) by removing build-in game bounce and raging thunder (you can remove others also if not needed inside core)
some system files cannot be deleted even you don need them. however, some of them checked by the system for existence only, so this trick is to replace these files with empty files
howto:
(bounce, for example)
create these empty files and save them in rofs3
\sys\bin\Bounce.exe
\resource\apps\Bounce.rsc
\resource\apps\Bounce_aif.mif
\private\10003a3f\import\apps\Bounce_reg.rsc
and finally, the trick is ....
create an empty file in private and named it 2000C4D4 (it's \private\2000C4D4 that occupy 8MB)
YES!! an empty file is enough to prevent the 2000C4D4 directory inside core being copied to Z!! (due to the priority rule for building)
DONE!!
more info
