[U-Boot-Users] how to boot a root.cramfs in ram

i use uboot loadb command to place kenerl on 0x32000000, root.cramfs on 0x30800000,and when i type bootm 32000000 30800000, it just boot as there is a ramdisk image.Does uboot support boot cramfs rootfs in ram? what's the command?

thanks all

zzh