Process Management - Process concept




 A batch system executes jobs, whereas a time-shared system has user programs, or tasks. Even on a single-user system such as Microsoft Windows, a user may be able to run several programs at one time: a word processor, a web browser, and an e-mail package. Even if the user can execute only one program at a time, the operating system may need to suppoft its own internal programmed activities, such as memory management. In many respects, all these activities are similar, so we call all of them processes.

The terms job and process are used almost interchangeably in this text. Although we personally prefer the term process, much of operating-system theory and terminology was developed during a time when the major activity of operating systems was job processing. It would be misleading to avoid the use of commonly accepted terms that include the word job (such as job scheduling) simply because process has superseded job.

The Process

Informally, as mentioned earlier, a process is a program in execution. A process is more than the program code, which is sometimes known as the text section.
It also includes the current activity, as represented by the value of the program counter and the contents of the processor's registers. A process generally also
includes the process stack, which contains temporary data (such as function parameters, return addresses, and local variables), and a data section, which
contains global variables. A process may also include a heap, which is memory that is dynamically allocated during process run time. The structure of a process
in memory is shown in Figure. We emphasize that a program by itself is not a process; a program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file), whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources. A program becomes a process when an executable file is loaded into memory. Two common techniques for loading executable files are double-clicking an icon representing the executable file and entering the name of the executable file on the command line (as in prog. exe or a. out.)
Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. For instance,

 

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Process Management - Process concept

  process in memory

several users may be running different copies of the mail program, or the same user may invoke many copies of the web browser program. Each of these is a
separate process; and although the text sections are equivalent, the data, heap, and stack sections vary. It is also common to have a process that spawns many
processes as it runs. 

Process State


As a process executes, it changes state. The state of a process is defined in part by the current activity of that process. Each process may be in one of the
following states:
• New. The process is being created.
• Running. Instructions are being executed.
• Waiting. The process is waiting for some event to occur (such as an I/O completion or reception of a signal).
• Ready. The process is waiting to be assigned to a processor.
• Terminated. The process has finished execution. These names are arbitrary, and they vary across operating systems. The states that they represent are fotind on all systems, however. Certain operating systems also more finely delineate process states. It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and limiting, however. The state diagram corresponding to these states is presented in Figure 3.2.

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Process Management - Process concept

 process states

Process Control Block


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 . It contains
many pieces of information associated with a specific process, including these:
• Process state. The state may be new, ready, running, waiting, halted, 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 interrupt 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 information as the value of the base and limit registers, the page tables, or the segment tables, depending on the memory system used by the operating system .
• Accounting information. This information includes the amount of CPU and real time used, time limits, account mimbers, 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. In brief, the PCB simply serves as the repository for any information that may vary from process to process.

Process Management - Process concept

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process control block

 

Threads

 

The process model discussed so far has implied that a process is a program that performs a single thread of execution. For example, when a process is running a word-processor program, a single thread of instructions is being executed. This single thread of control allows the process to perform only one task at one time. The user cannot simultaneously type in characters and run the spell checker within the same process, for example. Many 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 task at a time. 

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Process Management - Process concept

  CPU switch from process to process



Frequently Asked Questions

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Ans: System Calls and Programs All the present-day operating systems support the following two modes of operation for the CPU: User mode Kernel mode view more..
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Ans: the idea was to combine the best functionalities of all old approaches and hence this design is termed as the hybrid structured operating system. view more..
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Ans: The basic ideology in this architecture is to keep the kernel as small as possible. We know that kernel is the core part of the operating system and hence it should be meant for handling the most important services only. view more..
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Ans: A batch system executes jobs, whereas a time-shared system has user programs, or tasks. Even on a single-user system such as Microsoft Windows, a user may be able to run several programs at one time: a word processor, a web browser, and an e-mail package. Even if the user can execute only one program at a time, the operating system may need to suppoft its own internal programmed activities, such as memory management. view more..
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Ans: Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. view more..
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Ans: Process scheduling selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling. The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. ... Time-sharing operating systems have no long term scheduler. view more..
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Ans: Inter process communication (IPC) is a mechanism which allows processes to communicate each other and synchronize their actions. The communication between these processes can be seen as a method of co-operation between them. Processes can communicate with each other using these two ways: Shared Memory. Message passing. view more..
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Ans: Direct memory access (DMA) is a method that allows an input/output (I/O) device to send or receive data directly to or from the main memory, bypassing the CPU to speed up memory operations. The process is managed by a chip known as a DMA controller (DMAC) view more..
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Ans: An instruction cycle (sometimes called a fetch–decode–execute cycle) is the basic operational process of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction dictates, and carries out those actions. view more..
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Ans: System programs provide a convenient environment for program development and execution. Some of them are simply user interfaces to system calls; others are considerably more complex view more..
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Ans: After an operating system is generated, it must be made available for use by the hardware. But how does the hardware know where the kernel is or how to load that kernel? The procedure of starting a computer by loading the kernel is known as booting the system. view more..
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Ans: A system as large and complex as a modern operating system must be engineered carefully if it is to function properly and be modified easily. A common approach is to partition the task into small components rather than have one monolithic system. Each of these modules should be a well-defined portion of the system, with carefully defined inputs, outputs, and functions. view more..
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Ans: System calls provide an interface to the services made available by an operating system. These calls are generally available as routines written in C and C++, although certain low-level tasks (for example, tasks where hardware must be accessed directly), may need to be written using assembly-language instructions. view more..
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Ans: Before we can explore the details of how computer systems operate, we need a general knowledge of the structure of a computer system. In this section, we look at several parts of this structure to round out our background knowledge. view more..
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Ans: It is possible to design, code, and implement an operating system specifically for one machine at one site. More commonly, however, operating systems are designed to run on any of a class of machines at a variety of sites with a variety of peripheral configurations. The system must then be configured or generated for each specific computer site, a process sometimes known as system generation (SYSGEN). The operating system is normally distributed on disk or CD-ROM. To generate a system, we use a special program. The SYSGEN program reads from a given file, or asks the operator of the system for information concerning the specific configuration of the hardware system, or probes the hardware directly to determine what components are there. view more..
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Ans: A question that arises in discussing operating systems involves what to call all the CPU activities. A batch system executes jobs, whereas a time-shared system has user programs, or tasks. Even on a single-user system such as Microsoft Windows, a user may be able to run several programs at one time: a word processor, a web browser, and an e-mail package. Even if the user can execute only one program at a time, the operating system may need to support its own internal programmed activities, such as memory management. In many respects, all these activities are similar, so we call all of them processes. The terms job and process are used almost interchangeably in this text. Although we personally prefer the term process, much of operating-system theory and terminology was developed during a time when the major activity of operating systems was job processing. It would be misleading to avoid the use of commonly accepted terms that include the word job (such as job scheduling) simply because process has superseded job. view more..
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Ans: An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs. view more..
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Ans: The processes in most systems can execute concurrently, and they may be created and deleted dynamically. Thus, these systems must provide a mechanism for process creation and termination. we explore the mechanisms involved in creating processes and illustrate process creation on UNIX and Windows systems view more..




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