CS 217 - Operating Systems Introduction and Overview Lecture 02 Somia Razzaq NFC-IET Slides adopted from Silbershatz, Galvin and Gagne, VU
Introduction • What Operating Systems Do • Different Kind of Systems and OS History • Computer-System Organization • Computer-System Architecture • Operating-System Structure • Operating-System Operations • Process Management • Memory Management • Storage Management • Protection and Security • Distributed Systems • Special-Purpose Systems • Computing Environments • Open-Source Operating Systems
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
Single User Systems • Personal computers – computer system dedicated to a single user. • Interactive • User convenience and responsiveness. • Individuals usually have sole use of computer and do not need advanced protection features. • May run several different types of operating systems (Windows, MacOS, UNIX, Linux)
Batch Systems • First rudimentary system. • User operator • Reduce setup time by batching similar jobs • Automatic job sequencing – automatically transfers control from one job to another. • Resident monitor : • initial control in monitor • control transfers to job • when job completes control transfers back to monitor
Multiprogrammed Systems Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.
CPU I/O Burst Burst P1 P2 P1 P2 Multiprogrammed Systems Example: Two processes P1 and P2 with CPU and I/O bursts of one time unit each …
OS Features Needed for Multiprogramming • SPOOLing (Simultaneous Peripheral Operation On-Line) • Memory management • CPU scheduling
Time-sharing Systems • An interactive system with multiprogramming • A job is swapped in and out of memory to the disk if needed. • On-line file system must be available for users to access data and code.
Real-time Systems • Well-defined fixed-time constraints. • Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems. • Real-Time systems may be either hard or soft real-time.
Real-time Systems … • Hard real-time systems: • Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM) • No virtual memory—time cannot be “wasted” on translation of logical to physical addresses • OS code structured for efficiency • Plane landing systems, processcontrol in nuclear power plants, respirators, etc • Soft real-time systems • Output should be produced within the given time constraints but if it is not, the result is not life threatening • Useful in applications (multimedia, virtual reality) requiring advanced operating-system features
Fathers of Operating System • There are two people considered the pioneers of Operating Systems • E.W. Dijkstra • P.B. Hansen
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
Operating System Definition • OS is a resource allocator • Manages all resources • Decides between conflicting requests for efficient and fair resource use • OS is a control program • Controls execution of programs to prevent errors and improper use of the computer
Operating System Definition (Cont.) • No universally accepted definition • “Everything a vendor ships when you order an operating system” is good approximation • But varies widely • “The one program running at all times on the computer” is the kernel. Everything else is either a system program (ships with the operating system) or an application program
Computer Startup • bootstrap programis loaded at power-up or reboot • Typically stored in ROM or EPROM, generally known as firmware • Initializes all aspects of system • Loads operating system kernel and starts execution
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
Computer-System Operation • I/O device controllers and the CPU can execute concurrently • Each device controller is in charge of a particular device type • Each device controller has a local buffer • CPU moves data from/to main memory to/from local buffers • I/O is from the device to local buffer of controller • Device controller informs CPU that it has finished its operation by causing an interrupt
Interrupts, Traps and Signals • The occurrence of an event is usually signaled by an interrupt either by h/w or a s/w program • Hardware interrupt may occur at any time by sending a signal to CPU via system bus • Software interrupt may be triggered via a system call • A process can generate a trap, for example, by dividing a number by zero • A user or a process may generate a signal (an interrupt to a process) Answer the Phone
Common Functions of Interrupts • Interrupt transfers control to the interrupt service routine generally, through the interruptvector, which contains the addresses of all the service routines • Interrupt architecture must save the address of the interrupted instruction • Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt • A trap is a software-generated interrupt caused either by an error or a user request • An operating system is interrupt driven
Interrupt Handling • The operating system preserves the state of the CPU by storing registers and the program counter • Determines which type of interrupt has occurred • Separate segments of code determine what action should be taken for each type of interrupt
I/O Structure • Synchronous -- after I/O starts, control returns to user program only upon I/O completion: Wait loop: • Idles the CPU until the next interrupt • Contention for memory access • At most one I/O request is outstanding at a time, no simultaneous I/O processing • Asynchronous -- after I/O starts, control returns to user program without waiting for I/O completion • System call – request to the operating system to allow user to wait for I/O completion • Device-status table contains entry for each I/O device indicating its type, address, and state • Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt
Direct Memory Access Structure • Used for high-speed I/O devices able to transmit information at close to memory speeds • Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention • Only one interrupt is generated per block to tell the device driver of operation completion, rather than the one interrupt per byte • While the device controller performs these operations, the CPU is available for other jobs
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 CPU
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
Caching • Important principle, performed at many levels in a computer (in hardware, operating system, software) • Information in use copied from slower to faster storage temporarily • Faster storage (cache) checked first to determine if information is there • If it is, information used directly from the cache (fast) • If not, data copied to cache and used there • Cache smaller than storage being cached • Cache management important design problem • Cache size and replacement policy
Computer-System Architecture • Most systems use a single general-purpose processor (PDAs through mainframes) • Most systems have special-purpose processors as well • Multiprocessors systems growing in use and importance • Also known as parallel systems, tightly-coupled systems • Advantages include • Increased throughput • Economy of scale • Increased reliability – graceful degradation or fault tolerance • Two types • Asymmetric Multiprocessing – Master-Slave architecture • Symmetric Multiprocessing – All processors work as peers
Asymmetric Multiprocessing • Master-Slave relationship • Each processor is assigned a specific task • A master processor works as the controller – schedules and allocates work to other processors • Other processor (slaves) either look to the master for instructions or have predefined tasks
Symmetric Multiprocessing • Peer to peer relationship • Each processor performs all tasks • Each processor has its own registers and local cache • All share memory • N processes can run simultaneously if there are N CPUs • I/O needs a careful control for data to reach appropriate processor • Load balancing and performance tuning needs to be handled
Difference between Symmetric and Asymmetric multiprocessing may result from either hardware or software. • Special hardware may be used or • Special software may be written to allow only one master and multiple slaves • Example: • Sun’s OS ver 4 (SunOS) allows Asymmetric • Sun’s OS ver 5 (Solaris) allows Symmetric multiprocessing on same hardware
A Dual-Core Design • More efficient: on chip vs. between chips communication • Requires less power • More suited for database and web-servers • Other designs might use a shared cache or a combination of local and shared caches • Multi-core CPUs appear to the OS as N standard processors
Clustered Systems • Like multiprocessor systems, but multiple systems working together • Objective: High availability service • Usually sharing storage via a storage-area network (SAN) • Closely linked usually via a LAN • Provides a high-availability service which survives failures • Asymmetric clustering has one machine in hot-standby mode • Symmetric clustering has multiple nodes running applications, monitoring each other • Some clusters are for high-performance computing (HPC) • Applications must be written to use parallelization • Distributed Lock Manager: a function to ensure no conflicts on shared data access
Operating System Structure • Operating System provides an environment within which programs are executed • Multiprogramming needed for efficiency • Single user cannot keep CPU and I/O devices busy at all times • Multiprogramming organizes jobs (code and data) so CPU always has one to execute • A subset of total jobs in system is kept in memory • One job selected and run via job scheduling • When it has to wait (for I/O for example), OS switches to another job • Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing • Response time should be < 1 second • Each user has at least one program executing in memory process • If several jobs ready to run at the same time CPU scheduling • If processes don’t fit in memory, swapping moves them in and out to run • Virtual memory allows execution of processes not completely in memory
Memory Layout for Multiprogrammed System OS keeps several jobs in memory Job Pool: contain all processes awaiting allocation of main memory
Hardware Protection • Dual-Mode Operation • I/O Protection • Memory Protection • CPU Protection
Dual-Mode Operation • Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly. • Provide hardware support to differentiate between at least two modes of operations. • User mode – execution done on behalf of a user • Monitor mode (also kernel mode or system mode) – execution done on behalf of operating system
Transition from User to Kernel Mode • Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1). • When an interrupt or fault occurs hardware switches to monitor mode • Privileged instructions can be issued only in monitor mode
I/O Protection • All I/O instructions are privileged instructions • Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part of its execution, stores a new address in the interrupt vector).
Memory Protection • Must provide memory protection outside the address space of a process. • In order to have memory protection, add two registers that determine the range of legal addresses a program may access: • Base register – holds the smallest legal physical memory address. • Limit register – contains the size of the range • Memory outside the defined range is protected.