In a sophisticated multiprogramming system, a great deal of information about each process is required for process management. This information can be considered to reside in a process control block. Different systems will organize this information in different ways.
Process control block information into the following three categories they are Process identification, Processor state information, and Process control information. With respect to process identification, in virtually all operating systems, each process is assigned a unique numeric identifier.
The identifier may simply bee an index into the primary process table. If there is no numeric identifier, there must be a mapping that allows the operating system to locate the appropriate tables on the basis of the process identifier. This identifier is useful in a variety of ways.
Many of the other tables controlled by the operating system may use process identifiers to cross reference process tables. The memory tables may be organized in such a way as to provide a map of main memory with an indication of which process is assigned to each region of memory.
Similar reference will appear in I/O and file tables. When the process communicates with one another, the process identifier is used to inform the operating system of the destination of a particular communication. When processes are allowed to create other processes, identifiers are used to indicate the parent and descendants of each process.
In addition to these process identifiers, a process may be assigned a user identifier that indicates the user who is responsible for the job.
The next major collection of information is processor state information. This, essentially, consists of the contents of processor registers. While a process is running, of course, the information is in the registers. When a process is interrupted, all this register information must be saved so that it can be restored when the process resumes execution.
The nature and number of registers involved depend on the design of the processor. Typically, the register set includes user-visible registers, control, and status registers, and stacks pointers. User-visible registers are those that are accessible to user programs and are to be used for temporary storage of data. Most processors include from 8 to 32 such registers. Some recent reduced-instruction set computer architectures have more than 100 such registers.
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A variety of control and status registers are employed to control the operation of the processor. Most of these, on most machines are not visible to the user.
