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 Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner 1 2 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 1 3 Four Components of a Computer System 1 4 Computer Startup bootstrap program is loaded at power up or reboot Typically stored in ROM or EPROM generally known as firmware Initializates all aspects of system Loads operating system kernel and starts execution 1 5 Computer System Organization Computer system operation One or more CPUs device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles 1 6 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 Disk surface is logically divided into tracks which are subdivided into sectors The disk controller determines the logical interaction between the device and the computer 1 7 Storage Hierarchy Storage systems organized in hierarchy Speed Cost Volatility Caching copying information into faster storage system main memory can be viewed as a last cache for secondary storage 1 8 Storage Device Hierarchy 1 9 Performance of Various Levels of Storage Movement between levels of storage hierarchy can be explicit or implicit 1 10 Operating System Operations Interrupt driven by hardware Software error or request creates exception or trap Division by zero request for operating system service Other process problems include infinite loop processes modifying each other or the operating system Dual mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged only executable in kernel mode System call changes mode to kernel return from call resets it to user 1 11 Process Management A process is a program in execution It is a unit of work within the system Program is a passive entity process is an active entity Process needs resources to accomplish its task CPU memory I O files Initialization data Process termination requires reclaim of any reusable resources Single threaded process has one program counter specifying location of next instruction to execute Process executes instructions sequentially one at a time until completion Multi threaded process has one program counter per thread Typically system has many processes some user some operating system running concurrently on one or more CPUs Concurrency by multiplexing the CPUs among the processes threads 1 12 Process Management Activities The operating system is responsible for the following activities in connection with process management Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Providing mechanisms for deadlock handling 1 13 Memory Management All data in memory before and after processing All instructions in memory in order to execute Memory management determines what is in memory when Optimizing CPU utilization and computer response to users 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 deallocating memory space as needed 1 14 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 datatransfer 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 1 15 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 1 16 I O Subsystem 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 one job with input of other jobs General device driver interface Drivers for specific hardware devices 1 17 Protection and Security Protection any mechanism for controlling access of processes or users to resources defined by the OS Security defense of the system against internal and external attacks Huge range including denial of service worms viruses identity theft theft of service Systems generally first distinguish among users to determine who can do what User identities user IDs security IDs include name and associated number one per user User ID then associated with all files processes of that user to determine access control Group identifier group ID allows set of users to be defined and controls managed then also associated with each process file Privilege escalation allows user to change to
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