The article is firstly posted here https://askubuntu.com/questions/1094854/how-to-modify-initrd-initial-ramdisk-of-ubuntu-18-10-cosmic-cuttlefish/1094855 because this change is pretty new and it seems that nobody has asked on the internet. To post there should help many people in the follow months after 18.10 release.
Besides, the quote from Debian wiki is also useful as background knowledge.
- If an uncompressed cpio archive exists at the start of the initramfs, extract and load the microcode from it to CPU.
- If an uncompressed cpio archive exists at the start of the initramfs, skip that and set the rest of file as the basic initramfs. Otherwise, treat the whole initramfs as the basic initramfs.
- unpack the basic initramfs by treating it as compressed (currently gzipped) cpio archive into a RAM-based disk.
- mount and use the RAM-based disk as the initial root filesystem.
