|
Summary of Operating System
|
|
|
|
|
Operating systems have developed over the past thirty years for two main purposes. First, they provide a convenient environment for the development and execution of programs. Second, operating systems attempt to schedule computational activities to ensure good performance of the computing system.
|
|
|
|
Initially, computers were used from the front console. Software such as assemblers, loaders, and compilers improved on the convenience of programming the system, but also required substantial set-up time. To reduce the setup time, operators were hired and similar jobs were batched together.
|
|
|
|
Batch systems allowed automatic job sequencing by a resident monitor and improved the overall utilization of the computer greatly. The computer no longer had to wait for human operation. CPU utilization was still low, however, because of the slow speed of the I/O devices relative to the CPU. Off-line operation of slow devices was tried.
|
|
|
|
Spooling also provides a pool of jobs which have been read and are waiting to be run. This job pool supports the concept of multiprogramming. With multiprogramming, several jobs are kept in memory at one time； the CPU is switched back and forth between them in order to increase CPU utilization and to decrease the total real time needed to execute a job.
|
|
|
|
Multiprogramming, which was developed to improve performance, also allows time sharing. Time-shared operating systems allow many users(from one to several hundred)to use a computer system interactively at the same time.
|
|
|
|
A real-time system is often used as control device in a dedicated application. Sensors bring data to the computer. The computer must analyze the data and possibly adjust controls to modify the sensor inputs. Systems which control scientific experiments, medical computer systems, industrial control systems, and some display systems are real-time systems. A real-time operating system has well-defined fixed time constraints.
|
|
|
|
|
|
在过去30年间操作系统主要朝着两个目标发展：第一，为程序的开发和执行提供一个方便的环境；第二，操作系统试图通过对计算任务的调度以确保计算系统的良好性能。
|
|
|
|
最初，计算机只能通过控制台使用。诸如汇编程序、装入程序及编译程序这样的软件使系统软件编程更方便，但同时也需要大量的启动时间。为减少启动时间，计算机工作就需要操作员，且将相同的一批作业排在一起。
|
|
|
|
批处理系统通过一个驻留内存的监控程序使得作业自动排列，同时也大大提高了计算机的整机利用率，计算机不再需要等待人工操作。但是，由于I/O设备的速度相对于CPU仍然很低，因此CPU的利用率仍很低，于是就采用低速设备的脱机操作。
|
|
|
|
假脱机（SPOOLING）系统同时还提供了一个作业池，其中的作业已读出并等待运行，这个作业池支持多道程序设计的概念。在多道程序下，几个作业可同时保存在内存中，CPU在它们之间来回服务，以便提高CPU的利用率，减少执行一个作业实际所需的总时间。
|
|
|
|
为提高系统性能而开发的多道程序设计技术同样也允许分时。分时操作系统允许多个用户（从一个到几百个）同时交互使用一个计算机系统。其他操作系统类型包括实时系统和多处理机系统。
|
|
|
|
实时系统常用作专用系统中的控制设备。传感器将数据传到计算机系统，计算机应分析数据并尽可能调整控制以修改该传感器的输入。控制科学实验的系统、医学计算机系统、工业控制系统以及一些显示系统都是实时系统。实时系统有严格定义的、固定的时间限制，处理过程应在规定的时间内完成；否则系统失效。
|
|
|
