CSE 380 Computer Operating SystemsWhat is an Operating System?Resource Abstraction and SharingPentium ArchitectureAbstractions in OSSharing of MemoryTimesharingChallenges in OSHow does OS work?Inside a CPUDifferent types of MemorySample Scenario 1Sample Scenario 1 (continued)Sample Scenario 2Sample Scenario 2 (continued)Brief History of Operating Systems1940's -- First Computers1950's -- Batch ProcessingTypical Batch System1960's -- Multiprogramming (timesharing)1970's - Minicomputers & Microprocessors1980's - Networking1990's and BeyondOperating System StructureOperating System Structure (1)Operating System Structure (2)Operating System Structure (3)Operating System Structure (4)Operating System Structure (5)1CSE 380Computer Operating SystemsInstructor: Insup LeeUniversity of PennsylvaniaFall 2003Lecture Note 1: Introduction2What is an Operating System?Operating systems provides an interface between hardware and user programs, and makes hardware usable3Resource Abstraction and SharingIt is an extended machine providing abstraction of the hardwareHides the messy details which must be performedPresents user with a virtual machine, easier to useIt is a resource managerTime on CPU is shared among multiple users/programsSpace in memory and on disks is shared among multiple users/programs4Pentium Architecture5Abstractions in OSHardwareDisksMemoryProcessorsNetworkMonitorKeyboardMouseOS abstractionFilesProgramsThreads / ProcessesCommunicationWindows and GUIInputLocator6Sharing of MemoryIssuesAllocation schemesProtection from each otherProtecting OS codeTranslating logical addresses to physicalSwapping programsWhat if physical memory is small: Virtual memoryProgram 2O S Program 3Free spaceProgram 17TimesharingAt any point, only one program can run on CPUContext switch: changing the program that has CPUWhen to switch (goal: to optimize the CPU usage)When a program terminatesWhen a program has run “long enough”When a program executes a system call or waits for I/OWhen an external interrupt arrives (e.g. mouse click)OS must do all the book-keeping necessary for context switch, with minimum number of instructionsP1 P1P2 P3OS OS OS OS8Challenges in OSPerformance is criticalHow to reduce the memory and time overhead due to OSSynchronization and deadlocks due to shared resourcesScheduling of multiple programsFairness, response time, real-time applicationsMemory management Virtual memory, paging, segmentationSecurity and ProtectionAuthorization, authentication, virusesInterrupt management and error handlingMarketability and backward compatibilityWhy can’t Microsoft still get rid of all bugs in Windows ?9How does OS work?OS gets control of the CPU repeatedlyLet’s look at two typical scenarios to get a glimpse of how things work (we will get a more accurate and detailed understanding as the course progresses)Basic knowledge about computer architecture is essential ! (Read Sec 1.4 to review CSE 240)10Inside a CPUState of a running programRegistersProgram counter (PC)Stack pointerProgram status word (PSW)Key distinction in PSW: user mode vs kernel (OS) modeKey instruction for OS calls: TRAP (switch to kernel mode)Many operations (such as accessing I/O devices) are possible only in the kernel mode11Different types of Memory Use of disks unavoidable (permanence and size)Access time is significantly slower for disks12Sample Scenario 1Consider a statement to read from a file in a user program PUser program stores parameters such as file-id, memory-address, number-of-bytes, and system-call number of read, and executes TRAP instruction to invoke OSHardware saves the state of current program, sets the mode-bit in PSW register in CPU to 1, and transfers control to a fixed location in OS codeOS maintains an internal file table that stores relevant information about all open files13Sample Scenario 1 (continued)OS read routine examines the parameters, checks for errors (e.g. file must be open), consults its file table, and determines the disk address from where data is to be retrieved then it sets up registers to initiate transfer by the disk controllerWhile disk controller is transferring data from disk to memory, OS can suspend current program, and switch to a different programWhen OS routine finishes the job, it stores the status code, and returns control to the user program P (hardware resets mode-bit)Note: Disk controller is accessed only by OS code (this is ensured by hardware protection)14Sample Scenario 2Consider an assignment x:=y in a program PCompiler assigns logical addresses, say Add1 and Add2, for program variables in P’s data spaceWhen P is loaded in memory, OS assigns a physical base address to store P and its dataCompiled code looks likeLoad (R, Add1); Store (R, Add2)While executing Load instruction the hardware translates the logical address Add1 to a physical memory location (this is done by Memory Management Unit MMU)15Sample Scenario 2 (continued)However, OS may not keep all of P in memory all the timeOS maintains an internal table, called page table, that keeps track of which blocks of P are in memoryIf Add1 is not in memory, MMU generates a page fault, and transfers control to OSOS examines the cause, and initiates a disk transfer to load in the relevant block of POS needs to decide memory allocation for the block to be fetched (page replacement algorithms)While this block is being fetched, P may be suspended using a context switch16Brief History of Operating Systems1940's -- First Computers1950's -- Batch Processing1960's -- Multiprogramming (timesharing)1970's -- Minicomputers & Microprocessors1980's -- Networking, Distributed Systems, Parallel (multiprocessor) Systems1990's and Beyond -- PCs, WWW, Mobile Systems, embedded systems171940's -- First ComputersComputer dedicated to one user/programmer at a time. Program loaded manually by programmer, using console switches. Debugging using console lights. Advantages:Interactive (user gets immediate response)Disadvantages:Expensive machine idle most of time, because people are slow.Programming & debugging are tedious.Each program must include code to operate peripherals -- error prone, device dependencies.Libraries of subroutines to drive peripherals are