[U-Boot] What shall u-boot do before passing 'root=/dev/ram0' to kernel?

[mike xu]

Hi All,

Do you know that what u-boot shall do after passing 'root=/dev/ram0'
to kernel? How kernel locates /dev/ram0?
I want to use one ramdisk as my rootfs, but the kernel failed to mount
the ramdisk, I set the u-boot parameter as below.
 setenv bootargs console=ttyS0,115200 root=/dev/ram0 ro mem=128M

kernel reports errors:
  No filesystem could mount root, tried:  ext2 cramfs squashfs romfs
  Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(1,0)

root=/dev/ram1 also failed... What's the difference between /dev/ram0
and /dev/ram1?

And are there any requirement for the ramdisk image? Such as
little/big endian, block size?

Thanks,
Mike