2.2 Operating Systems
Introduction
Computers do not operate only with hardware. They function only with the use of software called an operating system (OS).
2.2.1 Configuration and Objectives of OS
Points
¾ There is a broadly defined OS and more narrowly defined OS.¾ The objective of an OS is the effective use of computers.
The definition of an operating system (OS) is not clear. The basic software is called an OS in a broad sense while the control program is called an OS in a narrow sense.
Configuration of OS
An operating system is the basic software that comprehensively controls and manages the entire operation of hardware and software of a computer system. A program referred to as the basic software and its role are shown below. 45
- Job management - Data management - Operation management - Communications management - Task management
- Memory management - Fault management - Miscellaneous
- Linkage editor - Sort/merge program - System generator - Text editor - Media conversion program - Debugger -Miscellaneous
Basic software
- Compiler - Assembler - Interpreter - Generator - Miscellaneous
Service programs and language processors are sometimes called processing programs, which run on the control program. For this reason, the control program is called an OS in a narrow sense.
45 (Note) Operating systems for personal computers include Windows XP, Mac OS X, and OS/2. Those for workstations include Windows Server 2003, UNIX, and Mac OS X Server. For general-purpose machines, there is also MVS developed by IBM. In addition, there is a free OS program called Linux, which is compatible with UNIX.
Control program (OS in a narrow sense)
Service program
Language processor
Objectives of OS
An OS attempts to improve the productivity of the entire system by eliminating unnecessary operations and waste of various resources surrounding the computer and by operating the computer system efficiently. The objectives of an OS are organized in the following figure.
Effective use of hardware resources: Multiprogramming,46 spooling function,47 etc.
Response to various processing modes: Batch processing, online real-time processing, etc.
Securing reliability and safety: Improvement in RASIS, etc.
Load reduction of application software: Virtual memory,48 library management,49 etc.
Support of computer control and operation: Continuous processing, recording the operation data, etc.
Let us take a look at these individual objectives in detail:
Effective use of hardware resources
Hardware resources include the central processing unit (CPU), memory, I/O units (including channels). etc. The OS controls these resources so that they can be used efficiently.
Response to various processing modes
One computer can handle various processing modes such as batch processing, remote batch processing, online processing, real-time processing, and interactive mode processing. In particular, since online processing has become widespread, the scope of computer applications has been dramatically enlarged.
Securing reliability and safety
Indexes for reliability and safety include RASIS. This is a term coined by taking the initial letters of the words Reliability, Availability, Serviceability, Integrity, and Security.
Load reduction of application software
Application software refers to a program which runs under the control of the OS. The OS provides an environment in which application programs can be efficiently executed.
Objectives of OS
Support of computer control and operation
An OS eliminates human intervention as much as possible, as it processes programs (jobs) continuously and records the operation status (log). The record of the operation status is used to check the circumstances under which a fault occurred.50
Human
intervention Job 1 preparation Idle time Job 1 end Job 2 preparation Idle time OS Resources free Job 1 execution Resources free Job 2 execution
Automated
Job 1 preparation Job 2 preparation Job 1 end Job 2 end
Job 1 execution Job 2 execution
2.2.2 Job Management
Points
¾ Jobs are units of tasks given to the computer, consisting of multiple programs (job steps).
¾ Job management has functions such as scheduling and spooling.
One of the functions of the control program, the “OS" in a narrow sense, is "job management.” In job management, the priorities of jobs are determined, and the jobs are synchronized. In batch processing, the OS analyzes the contents of JCL (job control language) to assign resources51 and schedule jobs. In interactive mode processing, the OS analyzes instructions entered at the terminal, assigns resources, and performs scheduling. In addition, job management has other functions such as spooling and cataloged procedures.
50 (FAQ) Concerning operating systems, many exam questions involve knowledge of terms. Be sure to know terms such as multiprogramming, virtual memory, and spooling function.
51 Resource: A resource is a device/unit of various kinds necessary for the computer to operate. It refers to any device related to memory, input, output, control, and other functions; specifically, these include the CPU, main memory, and files.
Scheduler:
Managing the order of job execution Interface with the operator
Managing reception, selection, start, and finish of jobs Reading jobs
Preparing for the beginning of jobs and programs Clean-up after jobs and programs
Input management for jobs, output management for process results Support for execution of typical jobs
Job management
Scheduler
In job management, jobs are continuously executed under a master scheduler and job scheduler.
The master scheduler plays the role of an interface with the operator via the console panel.52 The job scheduler manages the reception, selection, start, and finish of the jobs.
Reader
This reads the contents of JCL, analyzes them, schedules jobs, and places them in a queue.
Initiator
This selects the programs with high execution priorities among those in the queue and assigns the resources that those programs need.
Terminator
This releases resources that were used by programs just completed. If there is another program following, the terminator starts up the initiator.
Spooling (Spool)
Spooling is the function of the I/O of jobs independent of the programs. Any output results to low-speed units such as a printer are first stored in a spool file. Then, after the program is finished, the output results are printed on the printer from the spool file by the service program of the OS.53 The reason this is done is that, when the I/O unit is slow, directly performing the I/O process would reduce the processing speed of the computer.54