[Buildroot] where is the initrd image?

Dear All Guru :

     I can generate the target achi rootfs , and kernel
but I can't find the initrd file

Thus I met the booting problem , VFS cannot open root device "hdxx"

Do I nust have the initrd file , or not ?
If do , how and where to build it ?

Thanks !!

ps: I am using buildroot snapshot , June 23 '07
and the target archi is i386
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