Operating Systems Terms and Definitions Chapter Objectives After completing these slides you will You will have a better understanding of the role of the operating system You will have some familiarity with two of the most popular operating systems Windows and Unix What is the Operating System A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner Computer System Structure Computer system can be divided into four components Hardware provides basic computing resources CPU memory I O devices Operating system Controls and coordinates use of hardware among various applications and users Application programs define the ways in which the system resources are used to solve the computing problems of the users Word processors compilers web browsers database systems video games Users People machines other computers Components of the Computer System Operating System Structure The operating system is divided into a number of layers levels each built on top of lower layers The bottom layer layer 0 is the hardware the highest layer N is the user interface With modularity layers are selected such that each uses functions operations and services of only lower level layers Operating System Modules Most modern operating systems implement kernel modules Uses object oriented approach Each core component is separate Each talks to the others over known interfaces Each is loadable as needed within the kernel Overall similar to layers but with more flexibility Layered Operating System Functions of the Operating System Process Management Memory Management Storage Management I O Subsystem Protection and Security Resource Allocation Accounting Examples of Operating Systems MS DOS Windows XP Unix Mac OS X MS DOS MS DOS written to provide the most functionality in the least space Not divided into modules Although MS DOS has some structure its interfaces and levels of functionality are not well separated Structure of MS DOS Unix UNIX The UNIX OS consists of two separable parts Systems programs The kernel Consists of everything below the system call interface and above the physical hardware Provides the file system CPU scheduling memory management and other operating system functions a large number of functions for one level Structure of Unix Mac OS X Process Management An operating system executes a variety of programs Batch system jobs Time shared systems user programs or tasks Process a program in execution process execution must progress in sequential fashion A process includes program counter stack data section Process Control Block PCB Information associated with each process Process state Program counter CPU registers CPU scheduling information Memory management information Accounting information I O status information Process Control Block PCB Context Switch When CPU switches to another process the system must save the state of the old process and load the saved state for the new process Context switch time is overhead the system does no useful work while switching Time dependent on hardware support CPU Switch from Process to Process CPU Scheduling Selects from among the processes in memory that are ready to execute and allocates the CPU to one of them according to the following scheduling criteria CPU utilization keep the CPU as busy as possible Throughput of processes that complete their execution per time unit Turnaround time amount of time to execute a particular process Waiting time amount of time a process has been waiting to be run Response time amount of time it takes from when a request was submitted until the first response is produced Scheduling Algorithms Shortest Job First SJF schedule the process that will take the shortest amount of time Priority Scheduling assign a priority with each process and schedule the one with the highest priority Round Robin Scheduling give each process in turn a fixed amount of CPU time Storage Structure Main memory only large storage media that the CPU can access directly Secondary storage extension of main memory that provides large nonvolatile storage capacity Magnetic disks rigid metal or glass platters covered with magnetic recording material Storage systems organized in hierarchy Speed Cost Volatility Storage Device Hierarchy Memory Management Transfer data to memory during processing Transfer all instructions to memory in order to execute Memory management activities Keeping track of which parts of memory are currently being used and by whom Deciding which processes or parts thereof and data to move into and out of memory Allocating and de allocating memory space as needed Storage Management OS provides uniform logical view of information storage Abstracts physical properties to logical storage unit file Each medium is controlled by device i e disk drive tape drive Varying properties include access speed capacity data transfer rate access method sequential or random File System management Files usually organized into directories Access control on most systems to determine who can access what OS activities include Creating and deleting files and directories Primitives to manipulate files and dirs Mapping files onto secondary storage Backup files onto stable non volatile storage media File Protection File owner creator should be able to control what can be done by whom Types of access read write execute Three classes of users RWX a owner access 7 111 b group access 6 110 c public access 1 001 Windows XP Access Control Mass Storage Management Usually disks used to store data that does not fit in main memory or data that must be kept for a long period of time Proper management is of central importance Entire speed of computer operation hinges on disk subsystem and its algorithms OS activities Free space management Storage allocation Disk scheduling Some storage need not be fast Tertiary storage includes optical storage magnetic tape Still must be managed Varies between WORM write once read many times and RW read write I O Subsystem Management One purpose of OS is to hide peculiarities of hardware devices from the user I O subsystem responsible for Memory management of I O including buffering storing data temporarily while it is being transferred caching storing parts of data in faster storage for performance spooling the overlapping of output of