PCBs and Hardware State
When a process is running its program counter, stack pointer, registers, etc., are loaded on the CPU (i.e., the processor hardware registers contain the values)
When the OS is ready to start executing a new process, it loads the hardware registers from the values stored in that process’ PCB.
The process of switching the CPU from one process to another is called a context switch. Timesharing systems may do 100s or 1000s of context switches a second!