When a machine is turned on, it goes through a Power On Self Test, POST, and reads the Master Fixed Disk Boot Record (MFDBR) from disk drive zero to get the proper boot partition. It goes to that partition, loads the system and executes. After the host is powered up, the D3 Monitor can be invoked.
PCBs are associated with each job running on the system. They tell the system all the variables, what they currently are, and which resources are allocated to it. The system saves this information during round robin timeslices, so it can restart jobs.