An Operating System Process is a program in execution. It includes the value in the program counter and in the processor's registers. A process also includes a stack, data section, and a heap. Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences.
each process is represented in the operating system by a Process Control Block (PCB) also called a task control block. A PCB is shown in Figure 3.3. It contains many pieces of information associated with a specific process, including these:
• Process state. The state may be new, ready, running, waiting, and so on.
• Program counter. The counter indicates the address of the next instruction to be executed for this process.
• CPU registers. The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general purpose registers, plus any condition-code information. Along with the program counter, this state information must be saved when an interupt occurs, to allow the process to be continued correctly afterward.
• CPU-scheduling information. This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters.
• Memory-management information. This information may include such items as the value of the base and limit registers and the page tables, or the segment tables, depending on the memory syetm used by the operating syatem.
• Accounting information. This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on.
• I/O status information. This information includes the list of I/O devices allocated to the process, a list of open files, and so on.
• Thread information. Most modern operating systems have extended the process concept to allow a process to have multiple threads of execution and thus to perform more than one taask at a time. This feature is especially beneficial on multicore systems, where multiple threads can run in parallel. On a system that supports threads, the PCB is expanded to include information for each thread.
In brief, the PCB simply serves as the repository for any information that may vary from process to process.
About the Authors
Abraham Silberschatz is the Sidney J. Weinberg Professor of Computer Science at Yale University. Prior to joining Yale, he was the Vice President of the Information Sciences Research Center at Bell Laboratories. Prior to that, he held a chaired professorship in the Department of Computer Sciences at the University of Texas at Austin.
Professor Silberschatz is a Fellow of the Association of Computing Machinery (ACM), a Fellow of Institute of Electrical and Electronic Engineers (IEEE), a Fellow of the American Association for the Advancement of Science (AAAS), and a member of the Connecticut Academy of Science and Engineering.
Greg Gagne is chair of the Computer Science department at Westminster College in Salt Lake City where he has been teaching since 1990. In addition to teaching operating systems, he also teaches computer networks, parallel programming, and software engineering.
Operating System Concepts, now in its ninth edition, continues to provide a solid theoretical foundation for understanding operating systems. The ninth edition has been thoroughly updated to include contemporary examples of how operating systems function. The text includes content to bridge the gap between concepts and actual implementations. End-of-chapter problems, exercises, review questions, and programming exercises help to further reinforce important concepts. A new Virtual Machine provides interactive exercises to help engage students with the material.
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