 All systems also support a limited mode of operation called "Basic Test".In Basic Test mode, Nucleus is disabled, I/O is performed differently, and only a single program can run, restricted to the bottom 64KiB of store, but all other non-nucleus and non-PAS instructions operate normally.This mode is used very early during booting to set up the system tables required by Nucleus, before obeying a "Switch Full Nucleus" instruction.