A process is an instance of a running program, and a program is an executable stored on a persistent storage (e.g., disk). Suppose we want to design an OS so that for any program, there can be only one instance of it running at any given time. If the user attempts to execute the program again while an instance of it is already running, the OS simply tells the user that the program is already running and does nothing. To achieve the above, the OS needs to maintain some additional information. Let's assume such information is maintained inside PCBs. Please explain what information needs to be maintained in PCBs to allow for the above semantics. Only include what is relevant to achieve this goal, and explain why it is relevant.