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What Is a Process Control Block?

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  • Written By: Alex Newth
  • Edited By: Angela B.
  • Last Modified Date: 12 April 2014
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A block of coding included in most computer operating systems (OSs) to hold information on performing a range of simple to complex tasks is a process control block. Each process control block has an identification number, and the developer predetermines tasks for each block. To keep the computer running smoothly, most blocks do not use the central processing unit (CPU) until an action corresponding to their task is performed. If too many blocks are active at once, the CPU begins to thrash, or only provides power to the blocks and not to the actual processes. Some blocks need files to be opened to complete their task, and these blocks are given authority to automatically open the necessary files.

The process control block can be made to do a variety of things, such as adding text to a screen or moving a mouse icon on the screen. Each block, during the creation of an OS, is given an identifier that distinguishes it from other blocks. Along with the identifier, each block is made for a specific task, unlike other types of blocks that can adapt to any task.

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If each process control block took up CPU power simultaneously, then most computers would have a hard time performing any actions. To correct this, and to make computers run smoothly, inactive blocks are essentially turned off, and they take up little or no CPU power in this state. When the user performs an action attributed to that block, the block becomes active and begins requesting power from the CPU. An active block will normally turn back to inactive once the user stops performing the action.

Using this status system to keep the CPU from using too much power generally works, but the CPU may encounter a state known as thrashing if too many blocks are active at once. The CPU normally can easily balance power and processing but, if many blocks are requesting power, then the CPU can only send power. This means processing stops and the computer lags. Depending on the amount of thrashing, the CPU may be able to clear this up in a few seconds or minutes, or the computer may have to be restarted.

During its operation, a process control block may require files to properly perform its duty. The OS typically gives authority to the block to automatically open these files, even if the user does not manually agree to open the files. If the user had to manually agree, then this would slow down processing while the user selected whether he or she wanted the files opened.

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