Note for Operating Systems - OS by Satyasundara Mahapatra

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Notes on Operating System Module 1

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INTRODUCTION What is an Operating System?  A program that acts as an intermediary between a user of a computer and the computer hardware.  Operating system goals: o Execute user programs and make solving user problems easier. o Make the computer system convenient to use.  Use the computer hardware in an efficient manner. Computer System Components  Hardware – provides basic computing resources (CPU, memory, I/O devices).  Operating system – controls and coordinates the use of the hardware among the various application programs for the various users.  Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).  Users (people, machines, other computers). Abstract View of System Components Operating System Definitions  Resource allocator – manages and allocates resources.  Control program – controls the execution of user programs and operations of I/O devices .  Kernel – the one program running at all times (all else being application programs).

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Mainframe Systems  Reduce setup time by batching similar jobs  Automatic job sequencing – automatically transfers control from one job to another. First rudimentary operating system.  Resident monitor o initial control in monitor o control transfers to job o when job completes control transfers pack to monitor Memory Layout for a Simple Batch System Batch operating system is one where programs and data are collected together in a batch before processing starts. A job is predefined sequence of commands, programs and data that are combined in to a single unit called job. Multi-programmed Batch Systems Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.

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Features Needed for Multiprogramming  I/O routine supplied by the system.  Memory management – the system must allocate the memory to several jobs.  CPU scheduling – the system must choose among several jobs ready to run.  Allocation of devices. Time-Sharing Systems  The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).  A job swapped in and out of memory to the disk.  On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next ―control statement‖ from the user‘s keyboard.  On-line system must be available for users to access data and code. Desktop Systems  Personal computers – computer system dedicated to a single user.  I/O devices – keyboards, mice, display screens, small printers.  User convenience and responsiveness.  Can adopt technology developed for larger operating system‘ often individuals have sole use of computer and do not need advanced CPU utilization of protection features.  May run several different types of operating systems (Windows, MacOS, UNIX, Linux) Parallel Systems  Multiprocessor systems with more than on CPU in close communication.  Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.  Advantages of parallel system: o Increased throughput o Economical o Increased reliability