Typical Computer System Whats an O S and

Typical Computer System

What's an O. S. and What Does it Do? • Software extensions to Hardware • Provides for efficient control of and access to system facilities • Provides a structure for running Programs • Acts as a user interface

Different ways of Operating Single-User Environment • One User at a time • Initially, running only one program at a time • Now, Single-User PC allows several Programs to run at the same time (e. g. , Windows 95, Mac System 7. x, OS/2) • Can have WP and Spreadsheet open at same time and `switch' between them.

Multi-Access/Multi-user Environments • Many users logged in at same time (e. g. , via campus network or Internet) • Same application run several times, or many different application run at the same time • Each user appears to have all systems resources available - The Virtual Machine

Multi-tasking/Multi-processing • A process is an instance of a program in execution - Definition • Processes are run apparently in parallel by sharing system resources • Multi-tasking does not imply Multi-user, nor vice versa

Multi-programming • Multi-tasking + instructions / data from different processes co-resident in memory • Multi-programming implies multi-tasking, but not vice versa

Reasons for different operating requirements Many users interactively using same computer system for many different tasks: • Each user could be running a program interactively or in batch • editing a file • checking system status information • At Lunch! But Logged in • Needs to provide different levels of service • Needs Quick Response • Need to Protect users from each other • System needs to be robust

Many Users running the same program e. g. , Flight reservations: • Needs quick response time • Easy to use • sharing up-to-date information • consistency and protection of data

Few Users, or fully automated system, mainly for checking system status - e. g. , real-time system controlling nuclear reactor: • Very fast - needs to respond to changing environment • No real need for comprehensive user interface • Reliable • Secure

Batch system - e. g. , bureau service • only supervisor interacts with system • all other programs are JOBS, collections of programs identified by a job card. • Little need for extensive user interface • must run jobs according to some known set of criteria, as users may be charged for use of the system. • reliable • secure

General Purpose OS: • allows interaction and batch processing • reasonable to excellent user interface • inherits characteristics of like environments already described.