thanks for your help.
by the way,have you ever tried yaffs on SD card?
i want to use sd card to replace nand flash.As  i found that sd is not only
a lot of nand flash but also include a controller.Some vender have
integreted wear-leveling , bad block manangement and some other works that
yaffs do itself. So if i want use yaffs on different sd card, can i success
or what should i do to accomplish this work?
thanks.
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Regards,
Ezio.