Process control block

Process Control Block (PCB, also called Task Controlling Block, Task Struct, or Switchframe) is a data structure in the operating system kernel containing the information needed to manage a particular process. The PCB is "the manifestation of a process in an operating system".^[1]

If the mission of the operating system is to manage computing resources on behalf of processes, then it must be continuously informed about the status of each process and resource. The approach commonly followed to represent this information is to create and update status tables for each relevant entity, like memory, I/O devices, files and processes. Memory tables, for example, may contain information about the allocation of main and secondary (virtual) memory for each process, authorization attributes for accessing memory areas shared among different processes, etc. I/O tables may have entries stating the availability of a device or its assignment to a process, the status of I/O operations being executed, the location of memory buffers used for them, etc. File tables provide info about location and status of files (of course, what else? more on this later). Finally, process tables store the data the OS needs to manage processes. At least part of the process control data structure is always maintained in main memory, though its exact location and configuration varies with the OS and the memory management technique it uses. In the following we'll refer by process image to the complete physical manifestation of a process, which includes instructions, program data areas (both static and dynamic - e.g. at least a stack for procedure calls and parameter passing) and the process management information. We'll call this last set the process control block (PCB).

The role of the PCBs is central in process management: they are accessed and/or modified by most OS utilities, including those involved with scheduling, memory and I/O resource access and performance monitoring. It can be said that the set of the PCBs defines the current state of the operating system. Data structuring for processes is often done in terms of PCBs. For example, pointers to other PCBs inside a PCB allow the creation of those queues of processes in various scheduling states ("ready", "blocked", etc.) that we previously mentioned.

In modern sophisticated multitasking systems the PCB stores many different items of data, all needed for correct and efficient process management. Though the details of these structures are obviously system-dependent, we can identify some very common parts, and classify them in three main categories:

Process identification data; Processor state data; Process control data; Process identification data always include a unique identifier for the process (almost invariably an integer number) and, in a multiuser-multitasking system, data like the identifier of the parent process, user identifier, user group identifier, etc. The process id is particularly relevant, since it's often used to cross-reference the OS tables defined above, e.g. allowing to identify which process is using which I/O devices, or memory areas.

Processor state data are those pieces of information that define the status of a process when it's suspended, allowing the OS to restart it later and still execute correctly. This always include the content of the CPU general-purpose registers, the CPU process status word, stack and frame pointers etc.

Process control information is used by the OS to manage the process itself. This includes:

The process scheduling state (different from the task state above discussed), e.g. in terms of "ready", "suspended", etc., and other scheduling information as well, like a priority value, the amount of time elapsed since the process gained control of the CPU or since it was suspended. Also, in case of a suspended process, event identification data must be recorded for the event the process is waiting for. Process structuring information:process's children id's, or the id's of other processes related to the current one in some functional way, which may be represented as a queue, a ring or other data structures. Interprocess communication information: various flags, signals and messages associated with the communication among independent processes may be stored in the PCB. Process privileges, in terms of allowed/unallowed access to system resources.

Accounting information.

Included information

Implementations differ, but in general a PCB will include, directly or indirectly:

• The identifier of the process (a process identifier, or PID)
• Register values for the process including, notably, the program counter and stack pointer values for the process.
• The address space for the process
• Priority (in which higher priority process gets first preference. e.g., nice value on Unix operating systems)
• Process accounting information, such as when the process was last run, how much CPU time it has accumulated, etc.
• Pointer to the next PCB i.e. pointer to the PCB of the next process to run
• I/O Information (i.e. I/O devices allocated to this process, list of opened files, etc.)

During context switch, the running process is stopped and another process is given a chance to run. The kernel must stop the execution of the running process, copy out the values in hardware registers to its PCB, and update the hardware registers with the values from the PCB of the new process.

A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including

• The current state of the process i.e., whether it is ready, running, waiting etc.
• Unique identification of the process in order to track "which is which" information.
• A pointer to parent process.
• Similarly, a pointer to child process (if it exists).
• The priority of process (a part of CPU scheduling information).
• Pointers to locate memory of processes.
• A register save area.
• The processor it is running on.

The PCB is a certain store that allows the operating systems to locate key information about a process. Thus, the PCB is the data structure that defines a process to the operating systems.

Location of the PCB

Since PCB contains the critical information for the process, it must be kept in an area of memory protected from normal user access. In some operating systems the PCB is placed in the beginning of the kernel stack of the process since that is a convenient protected location.^[2]

Notes