You may want to reboot a running Linux system using the shutdown -r now command. In this case, the Linux kernel returns control to Milo (via the HALT CallPAL entrypoint). Milo leaves a compressed copy of itself in memory for just this reason and detects that the system is being rebooted from information held in the HWRPB (Hardware Restart Parameter Block). In this case it starts to reboot using exactly the same command that was used to boot the Linux kernel the last time. There is a 30 second timeout that allows you to interrupt this process and boot whatever kernel you wish in whatever way you wish.