Because there is no hard drive in many embedded systems based on Linux, the initrd also serves as the permanent root file system.I'm using a standard Linux desktop so you can follow along without an embedded target. Other than cross-compilation, the concepts (as they apply to initrd construction) are the same for an embedded target.
Utility (mkird) to create a custom initrd
# Create an empty ramdisk image
dd if=/dev/zero of=/tmp/ramdisk.img bs=$BLKSIZE count=$RDSIZE
# Make it an ext2 mountable file system
/sbin/mke2fs -F -m 0 -b $BLKSIZE /tmp/ramdisk.img $RDSIZE
# Mount it so that we can populate
mount /tmp/ramdisk.img /mnt/initrd -t ext2 -o loop=/dev/loop0
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