Process control block

Process Control Block (PCB, also called Task Controlling Block, process table, 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".

Role

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 define 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.

Structure

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 is 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 is suspended, allowing the OS to restart it later and still execute correctly. This always includes the content of the CPU general-purpose registers, the CPU process status word, stack and frame pointers 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.

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

• The process scheduling state, 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/disallowed access to system resources.
• Process state: State may enter into new, ready, running, waiting, dead depending on CPU scheduling.
• Processor No: a unique identification number for each process in the operating system.
• Program counter: a pointer to the address of the next instruction to be executed for this process.
• CPU registers: indicates various register set of CPU where process need to be stored for execution for running state.
• CPU scheduling information: indicates the information of a process with which it uses the CPU time through scheduling.
• Memory management information: includes the information of page table, memory limits, Segment table depending on memory used by the operating system.
• Accounting information: includes the amount of CPU used for process execution, time limits, execution ID etc.
• IO status information: includes a list of I/O devices allocated to the process.

Location

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.

See also

• Thread control block (TCB)