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Introduction to Operating Systems

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Introduction to Operating Systems Introduction to Operating Systems Jakub Olejnik These slides are a much inexact translation of the material available at http://wazniak.mimuw.edu.pl. The authors of the original slides were kind enough to share their files with me thus help me in translation. Literature Lecture • A. Silberschatz „Operating Systems Concepts” • A. Tanenbaum „Modern Operating Systems” • webpage (in polish) – the OS section http://wazniak.mimuw.edu.pl Laboratory classes • C. Newham, B. Rosenblatt „Learning the bash shell” • Unix/Linux manual pages Basic Concepts of Operating Systems Lecture 1 What’s in this lecture? • Definition of an Operating System, its place and its role in computer system. • What kind of Operating Systems are there? • Operating Systems Concepts – how do the OS’s work? What is an Operating System? A definition An Operating System is a set of computer programs which operate the computer’s devices directly and whose task is to present to a user an unified environment for writing and running programs in an convenient and efficient way. thus… An OS is a proxy between the user programs and the actual hardware. User and his programs virtual machine API Operating System drivers Actual Hardware Structure of an Operating System There are basicaly three types of OS’ components i.e.: graphical or text-based shells The operating system system programs: `CD’ commands, adm consoles Kernel modules hardware The Tasks of Operating System • Defining the user interface (shell) • Providing a file system • Providing environment to run user programs – mechanisms of actual code execution, safety. – mechanisms of process synchronization, communication • Managing the I/O devices • Providing procedures for handling basic errors Resource Management Resource Management Resource – any software or hardware component essential to processing • Resource allocation – assigning resources to processes • Resource allocation planning – choosing optimal allocation strategy • Access authorization and resource protection – restricting resource access in accordance with permissions granted • Resource recycling – managing reusable resources • Accounting – maintaining access register Resources managed by Operating System • Processor – each program gets a chance to be executed • Memory – allocation of memory space to processes – memory protection and address transformation • I/O devices – providing access to keyboards, mice, mass storage devices, printers, scanners, plotters etc. • Information (file system) – organizing information on disks, providing access – protection and access authorization Basic classification of Operating Systems OS’s classified with respect to processing • Interactive Systems (on-line systems) – users directly interact with operating system – task execution starts immediately after submission • Common OS’s serve as examples • Batch Systems (off-line systems) – task execution can be delayed – users can not interfere with task execution after their task has been submitted • Not many examples of pure off-line systems • Partial examples include: mainframes, job scheduling, print spooling, bulk transaction processing eg. ELIXIR Batch System – a historical example Early batch system – Programmers bring cards to 1401 – Operator reads cards onto tape – puts tape on 7094 which does computing – puts tape on 1401 which prints output OS’s classified with respect to number of programs executed • Single-tasking systems – it is not possible to start a new user program until previous task has finished – Examples: the well-known MS-DOS and many earlier OS’s • Multi-tasking/Multiprogramming systems – Multiple can be executed `at the same time’ – OS’s provides mechanism of context switching to time- share the CPU – Basic strategies: pre-emptive, co-operative multitasking, real-time systems OS’s classified with respect to number of users supported • Single-user systems – all of computer’s resources are accessed by only one user thus mechanisms of protection and authorization are not necessary – Examples: MS-DOS, AmigaOS, Win95/98, WinXP home ed. – in a way. • Multi-user systems – Many users can access computer’s resources thus mechanisms of protection and authorization are vital – Examples: UNIX, Linux, Windows server families Other types of OS’s • Real-time operating systems • Distributed (network) systems • Server, Mainframe systems • PC systems • Multiprocessor systems • Embedded systems • Smart card systems How does an Operating System work? Von Neumann architecture • Instruction codes as well as operands (args to instr.) are stored in the memory • Instructions are executed in the order in which they are located in memory • Memory location of next command is stored in PC/IP • The order of instruction execution can be changed by flow control instructions (jumps, cond. jumps by PC:=PC + label) • After instruction execution the instruction *(PC) is loaded, decoded and PC:=PC + length(*PC) Instruction cycle in detail – 1 • Instruction cycle (FDX-cycle) – the sequence of actions that the processor (CPU) performs to execute each machine code instruction in a program. • Instruction cycle consists of phases. • Typical phases of FDX-cycle are – instruction loading phase – memory is read – operand loading phase – memory is read – instruction execution phase – operand storing phase – memory is written Instruction cycle in detail – 2 1. Fetch the instruction – PC is presented on address bus, instruction code is read from data bus to CIR 2. Instruction is decoded by decoding unit 3. Fetch operands – Operand's effective address is fetched from memory (optionally), the operand itself is fetched 4. Execution 5. Store results – Operand is stored in the memory 6. Check for interrupts if an interrupt has been raised its number (source) is identified and the context is switched to appropriate interrupt handle routine if no interrupt has been raised go to 1. Modern CPUs Nowadays, processors implement mechanisms to speed up the instruction cycle: • Pipelines – when one instruction is being decoded why not fetch the next one • Superscalar execution – why not have more execution units • Multithreading – multiple threads parallelism • Multi-core CPUs – multithreading without sharing computing unit, CPU cashe, TLB Interrupts An interrupt is a state of computer system in reaction to an asynchronous event (an event which happens independently of processing and its precise moment of occurrence cannot be predicted) . It consists in the processor automatically saving its state of execution (context switch), and starting execution of an interrupt handler. Sources of Interrupts • External interrupts – raised by external devices • Software interrupts – raised by executed program with special instruction (interrupt call) • Diagnostic interrupts (exception handling) – raised to handle an exceptional state of processing: – traps – software errors e.g..: division by 0, memory protection violation, incorrect instruction; OS’s kernel removes the faulty process – hardware errors e.g..: segmentation fault Basics of OS’s operation • The OS’s kernel is called by the use interrupts, in this respect the kernel is a set of int. handlers and related procs. • Hardware memory protection • At least two CPU modes: (user mode, kernel mode) • A set of privileged instructions is available only in the kernel mode • the I/O instructions are privileged • a clock generates clock interrupts at constant intervals of time, so that periodically the kernel regains control Interrupts in the layer cake user software program interrupt external interrupt Operating System machine code diagnostic interrupt microarchitecture Hardware memory protection example • Executed program address makes reference to a specific location in memory • Memory chunks >= BASE which are available to the program are described by two fault registers: BASE and LIMIT < + LIMIT memory is accessed Scheduling by clock interrupts • Kernel regains control by clock interrupts • A clock interrupt (typically once per 10ms) runs a kernel procedure which chooses a program to be executed in turn • In single-tasking systems user programs (or viruses) sometimes modified the clock interrupt handler to emulate ‘double-tasking’. .
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