Early computer systems allowed only one program to be executed at a time. This program had complete control of the system and had access to all the system's resources. In contrast, current-day computer systems allow multiple programs to be loaded into memory and executed concurrently. This evolution required firmer control and more compartmentalization of the various programs; and these needs resulted in the notion of a process, which is a program in execution. A process is the unit of work in a modern time-sharing system.
Operating-system processes executing system code and user processes executing user code.
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.
Text section, Program counter, Stack, Data section, Heap
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 method 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.
Prgram and Process
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, whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.
One program separate processes
Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. For instance, several users may be running different copies of the mail program, or the same user may invoke many copies of the editor program. Each of these is a separate process
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.
(Figure 4.1 Diagram of process state.)
It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and waiting, however.
Process Control Block
Each process is represented in the operating system by a process control block (PCB)—also called a task control block.
It contains many pieces of information associated with a specific process, including these.
(Figure 4.2 Process control block ((PCB).)
(Figure 4.3)Diagram showing CPU switch from process to process.
Threads
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.
Process Scheduling
Process scheduler selects an available process (possibly from a set of several available processes) for program execution on the CPU. For a uniprocessor system, there will never be more than one running process. If there are more processes, the rest will have to wait until the CPU is free and can be rescheduled.
Scheduling Queues
As processes enter the system, they are put into a job queue, which consists of all processes in the system. The processes that are residing in main memory and are ready and waiting to execute are kept on a list called the ready queue. This queue is generally stored as a linked list. A ready-queue header contains pointers to the first and final PCBs in the list. Each PCB includes a pointer field that points to the next PCB in the ready queue.
The system also includes other queues. When a process is allocated the CPU, it executes for a while and eventually quits, is interrupted, or waits for the occurrence of a particular event, such as the completion of an I/O request. Suppose the process makes an I/O request to a shared device, such as a disk. Since there are many processes in the system, the disk may be busy with the I/O request of some other process. The process therefore may have to wait for the disk. The list of processes waiting for a particular I/O device is called a device queue. Each device has its own.
device queue (Figure 4.4).
Operating-system processes executing system code and user processes executing user code.
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.
Text section, Program counter, Stack, Data section, Heap
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 method 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.
Prgram and Process
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, whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.
One program separate processes
Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. For instance, several users may be running different copies of the mail program, or the same user may invoke many copies of the editor program. Each of these is a separate process
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.
(Figure 4.1 Diagram of process state.)
- 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.
It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and waiting, however.
Process Control Block
Each process is represented in the operating system by a process control block (PCB)—also called a task control block.
It contains many pieces of information associated with a specific process, including these.
(Figure 4.2 Process control block ((PCB).)
- 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 afterwards.
- 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 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.
(Figure 4.3)Diagram showing CPU switch from process to process.
Threads
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.
Process Scheduling
Process scheduler selects an available process (possibly from a set of several available processes) for program execution on the CPU. For a uniprocessor system, there will never be more than one running process. If there are more processes, the rest will have to wait until the CPU is free and can be rescheduled.
Scheduling Queues
As processes enter the system, they are put into a job queue, which consists of all processes in the system. The processes that are residing in main memory and are ready and waiting to execute are kept on a list called the ready queue. This queue is generally stored as a linked list. A ready-queue header contains pointers to the first and final PCBs in the list. Each PCB includes a pointer field that points to the next PCB in the ready queue.
The system also includes other queues. When a process is allocated the CPU, it executes for a while and eventually quits, is interrupted, or waits for the occurrence of a particular event, such as the completion of an I/O request. Suppose the process makes an I/O request to a shared device, such as a disk. Since there are many processes in the system, the disk may be busy with the I/O request of some other process. The process therefore may have to wait for the disk. The list of processes waiting for a particular I/O device is called a device queue. Each device has its own.
device queue (Figure 4.4).
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