Process Control Structures

by : Jesse Miller

Consider what the operating system must know if it is to manage and control a process. First, it must know where the process is located, and second, it must know the attributes of the process that are necessary for its management.

At a minimum, a process must include a program or set of programs to be executed. Associated with these programs is a set of data locations for local and global variables and any defined constants. Thus, a process will consist of at least sufficient memory to hold the programs and the data of that process. In addition, the execution of a program typically involves a stack that is used to keep track of procedure calls and parameters passing between procedures. Finally, associated with each process are a number of attributes used by the operating system for process control. Typically, the collection of attributes is referred to as process control block. This collection of program, data, stack and attributes are referred as process image.

The location of a process image depends on the memory management scheme being used. In the simplest possible case, the process image is maintained as a contiguous block of memory. This block is maintained in secondary memory, usually disk. So that the operating system can manage the process, at least a small portion of the image containing information of use to the operating system must be maintained in main memory. To execute the process, the entire process image must be loaded into main memory. Thus, the operating system needs to know the location of each process on disk and for each process that is in main memory, the location of the process in main memory. When a process is swapped out with CTSS part of the process image may remain in main memory. Thus, the operating system must keep track of which portions of the image of each process are still in main memory.

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Most modern operating systems use some sort of memory management scheme in which a process image consists of a set of blocks that need not be stored contiguously. Depending on the type of scheme used, these blocks may be of variable length (called segments) or fixed length (called pages), or a combination. In any case, such schemes allow the operating system to bring in only a portion of any particular process. Thus, at any given time, a portion of a process image may be in main memory.