Operating System 简明教程

Operating System - Processes

Process

进程基本上是一个正在执行的程序。进程的执行必须按顺序进行。

A process is basically a program in execution. The execution of a process must progress in a sequential fashion.

简单地说，我们以文本文件形式编写计算机程序，当我们执行该程序时，它将变成一个进程，执行程序中提到的所有任务。

To put it in simple terms, we write our computer programs in a text file and when we execute this program, it becomes a process which performs all the tasks mentioned in the program.

当一个程序被加载到内存中并成为一个进程时，它可以被划分为四个部分 ─ 栈、堆、文本和数据。下图显示了主内存中一个进程的简化布局 −

When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap, text and data. The following image shows a simplified layout of a process inside main memory −

S.N.	Component & Description
1	Stack The process Stack contains the temporary data such as method/function parameters, return address and local variables.
2	Heap This is dynamically allocated memory to a process during its run time.
3	Text This includes the current activity represented by the value of Program Counter and the contents of the processor’s registers.
4	Data This section contains the global and static variables.

Program

程序是一段代码，可以是一行或数百万行代码。计算机程序通常由计算机程序员使用编程语言编写。例如，下面是一个用 C 编程语言编写的简单程序 −

A program is a piece of code which may be a single line or millions of lines. A computer program is usually written by a computer programmer in a programming language. For example, here is a simple program written in C programming language −

#include <stdio.h>

int main() {
   printf("Hello, World! \n");
   return 0;
}

计算机程序是一组指令，执行时执行特定任务。如果将程序与进程进行比较，我们可以得出结论：进程是一个计算机程序的动态实例。

A computer program is a collection of instructions that performs a specific task when executed by a computer. When we compare a program with a process, we can conclude that a process is a dynamic instance of a computer program.

计算机程序中执行明确定义的任务的部分称为 algorithm 。计算机程序、库和相关数据的集合称为 software 。

A part of a computer program that performs a well-defined task is known as an algorithm. A collection of computer programs, libraries and related data are referred to as a software.

Process Life Cycle

在进程执行期间会经历不同的状态。这些状态在不同的操作系统中可能有所不同，且这些状态的名称也没有进行标准化。

When a process executes, it passes through different states. These stages may differ in different operating systems, and the names of these states are also not standardized.

一般来说，一个进程每次可以处于以下五个状态之一。

In general, a process can have one of the following five states at a time.

S.N.	State & Description
1	Start This is the initial state when a process is first started/created.
2	Ready The process is waiting to be assigned to a processor. Ready processes are waiting to have the processor allocated to them by the operating system so that they can run. Process may come into this state after Start state or while running it by but interrupted by the scheduler to assign CPU to some other process.
3	Running Once the process has been assigned to a processor by the OS scheduler, the process state is set to running and the processor executes its instructions.
4	Waiting Process moves into the waiting state if it needs to wait for a resource, such as waiting for user input, or waiting for a file to become available.
5	Terminated or Exit Once the process finishes its execution, or it is terminated by the operating system, it is moved to the terminated state where it waits to be removed from main memory.

Process Control Block (PCB)

进程控制块是由操作系统为每个进程维护的一个数据结构。进程控制块由整型进程 ID (PID) 标识。进程控制块保留所有所需的信息以跟踪一个进程，如下表所列 −

A Process Control Block is a data structure maintained by the Operating System for every process. The PCB is identified by an integer process ID (PID). A PCB keeps all the information needed to keep track of a process as listed below in the table −

S.N.	Information & Description
1	Process State The current state of the process i.e., whether it is ready, running, waiting, or whatever.
2	Process privileges This is required to allow/disallow access to system resources.
3	Process ID Unique identification for each of the process in the operating system.
4	Pointer A pointer to parent process.
5	Program Counter Program Counter is a pointer to the address of the next instruction to be executed for this process.
6	CPU registers Various CPU registers where process need to be stored for execution for running state.
7	CPU Scheduling Information Process priority and other scheduling information which is required to schedule the process.
8	Memory management information This includes the information of page table, memory limits, Segment table depending on memory used by the operating system.
9	Accounting information This includes the amount of CPU used for process execution, time limits, execution ID etc.
10	IO status information This includes a list of I/O devices allocated to the process.

PCB 的架构完全依赖于操作系统，并且在不同的操作系统中可能包含不同的信息。这是一个简化的 PCB 图示 −

The architecture of a PCB is completely dependent on Operating System and may contain different information in different operating systems. Here is a simplified diagram of a PCB −

PCB 在整个生命周期中都维护着一个进程，并在进程终止时删除。

The PCB is maintained for a process throughout its lifetime, and is deleted once the process terminates.