Process Control Block

• The OS must know specific information about processes in order to manage, control them and also to implement the process model, the OS maintains a table (an array of structures), called the process table, with one entry per process.
• These entries are called process control blocks (PCB) - also called a task control block.
• This entry contains information about the process'state, its program counter, stack pointer, memory allocation, the status of its open files, its accounting and scheduling information, and everything else about the process that must be saved when the process is switched from running to ready or blocked state so that it can be restarted later as if it had never been stopped. A PCB is shown in Fig. 5.

  Figure 5: Process control block (PCB).

  
• Such information is usually grouped into two categories: Process State Information and Process Control Information. Including these:
  • Process state. The state may be new, ready, running, waiting, halted, and so on.
  • Program counter. The counter indicates the address of the next instruction to be executed for this process.
  • CPU registers. The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information.
  • CPU-scheduling information. This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters.
  • Memory-management information. This information may include such information as the value of the base and limit registers, the page tables, or the segment tables, depending on the memory system used by the OS.
  • Accounting information. This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on.
  • I/O status information. This information includes the list of I/O devices allocated to the process, a list of open files, and so on.
• Figure 6 shows some of the more important fields in a typical system. The fields in the first column relate to process management. The other two columns relate to memory management and file management, respectively.

  Figure 6: Some of the fields of a typical process table entry.

  
• Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterward (see Fig. 7).

  Figure 7: Diagram showing CPU switch from process to process.