Notes on Operating System Module 1
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).
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.
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