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Systems Operating CMPSC 473 Introduction and Overview August 29 2008 - Lecture 3 Instructor: Bhuvan Urgaonkar Why not provide this functionality in hardware? Why this separate piece of software? Why do we need an OS? A Historical Perspective A Little Bit of History The Dark Ages (1940s 60s) Hardware: expensive; humans: cheap Evolution of functionality Single user Batch processing Overlap of I/O and computation Multi-programming 1. Single User One user at a time on console Computer executes one function at a time User must be at console to debug No overlap: computation & I/O Multiple users => inefficient use of machine 2. Batch Processing Execute multiple jobs in batch: Load program Run Print results, dump machine state Repeat Users submit jobs (on cards or tape) Human schedules jobs A program loads & runs jobs The Operating System More efficient use of machine, complicates debugging Before: machine waits for I/O to complete New approach: more work by the OS Allow CPU to execute while waiting Add buffering and interrupt handling I/O events trigger a signal 3. Overlap I/O and Computation Data fills buffer and then output Allow several programs to run at same time OS manages interaction between programs: Run one job until I/O Run another job, etc. 4. Multiprogramming Which jobs to start Protects program s memory from others Two kinds of functionality Sharing CPU between tasks Memory management Load programs, run them So what does an OS do? Management of computer resources Certain services to users OS Complexity Increased functionality & complexity First OS failures Multics (GE & MIT): announced 1963, released 1969 OS/360 released with 1000 known bugs Need to treat OS design scientifically Managing complexity becomes key to The Renaissance (1970s) Hardware: cheap; humans: expensive Users share system via terminals The UNIX era Multics: UNIX: army of programmers, six years three guys, two years Shell : composable commands No distinction between programs & data The Industrial Revolution (1980s) Hardware very cheap; humans expensive Widespread use of PCs IBM PC: 1981, Macintosh: 1984 No multiprogramming, concurrency, memory protection, virtual memory, Later: networking, filesharing, remote printing GUI added to OS Simple OS (DOS, MacOS) Hardware cheap; processing demands increasing Real operating systems on PCs Different modalities: NT (1991); Mac OS X; Linux The Modern Era (1990snow) Parallel: Multiple processors, one machine Distributed: Multiple networked processors Real-time: Strict or loose deadlines Sensor networks: Many small computers OSes Today Active research area (so I like to believe!) Top CS conferences, several distinguished researchers/groups Intersects with theory/algos., software engg., architecture, distrib. comp. Microsoft, Apple, HP, Major market presence New environments, new devices New challenges Web, P2P systems, Internet apps, sensor networks, mobile devices, multimedia, Distributed systems, heterogeneous devices, ubiquitous computing, utility computing, mobile computing, autonomic computing Some resources for computing history buffs MIT s Science Museum Computing History Museum, San Francisco http://www.computerhistory.org Basics of Computer Architecture CPU: Processor to perform computations Memory: Programs and data I/O Devices: Disk, monitor, printer, System Bus: Communication channel between the above Canonical System Hardware CPU CPU Clock Semiconductor device, digital logic (combinational and sequential) Can be viewed as a combination of many circuits Synchronizes constituent circuits Registers CPU s scratchpads; very fast; loads/stores Most CPUs designed so that a register can store a memory address n-bit architecture Cache Fast memory close to CPU Faster than main memory, more expensive Not seen by the OS Memory/RAM Semiconductor device DIMMs mounted on PCBs Random access: RAM DRAM: Volatile, need to refresh OS sees and manages memory Capacitors lose contents within few tens of msecs Memory controller: Chip that implements the logic for Programs/data need to be brought to RAM Reading/Writing to RAM (Mux/Demux) I/O Devices Large variety, varying speeds Each has a controller Disk, tape, monitor, mouse, keyboard, NIC Serial vs parallel Hides low-level details from OS Manages data flow between device and CPU/memory Hard Disk Secondary storage Mechanically operated Cheap => Abundant Very slow of Orders magnitude Sequential access Interconnects A bus is an interconnect for flow of data and information System Bus PCI Bus Wires, protocol Data arbitration Connects CPU-memory subsystem to Fast devices Expansion bus that connects slow devices SCSI, IDE, USB, Will return to these later Functionality Expected from a Modern OS Theory vs. Practice Performance Isolation Efficient and fair resource allocation, illusion of unlimited resources Protect everyone from each other and from the OS Secure communication How to do this efficiently? Hardware support Architectural Support Expected by Modern OSes Protection: Kernel/User mode, Protected Instructions, Base & Limit Registers Scheduling: Timer Interrupts: Interrupt Vectors System Calls: Trap Instructions Efficient I/O: Interrupts, Memorymapping Synchronization: Atomic Instructions Virtual Memory: Translation Lookaside Buffer (TLB) Services & Hardware Support Kernel/User Mode A modern CPU has at least two modes Indicated by status bit in protected CPU register OS runs in privileged mode Also called kernel or supervisor mode Applications run in normal mode Pentium processor has 4 modes Events that need the OS to run switch the processor to privileged mode OS can switch the processor to E.g., division by zero Protected Instructions Privileged instructions & registers: Direct access to I/O Modify page table pointers, TLB Enable & disable interrupts Halt the machine, etc. Hardware support to protect memory regions CPU checks each reference Loaded by OS before starting program Instruction & data addresses Base and Limit Registers Ensures reference in range Interrupts Polling = are we there yet? no! (repeat ) Inefficient use of resources Annoys the CPU Interrupt = silence, then: we re there I/O device has own processor When finished, device sends interrupt on bus Handling interrupts: relatively expensive CPU must: Save hardware state Registers, program counter CPU Interrupt Handling Disable interrupts (why?) Invoke via in-memory interrupt vector (like trap vector, soon) Enable interrupts Restore hardware state Continue execution of interrupted process Traps Special conditions detected by architecture E.g.: page fault, write to read-only page, overflow, system call Save process state (PC, stack, etc.) Transfer control to trap handler (in OS) Restore process state and resume On detecting trap, hardware must: CPU indexes trap vector by trap number Jumps to address Synchronization How can OS synchronize concurrent processes? E.g., multiple threads, processes & interrupts, DMA CPU must provide mechanism for atomicity Series of instructions that execute as one or not at all One approach: Synchronization: How-To Advantages: Disable interrupts Perform action Enable interrupts Requires no hardware support Conceptually simple Could cause starvation Disadvantages: Modern approach: atomic instructions Small set of instructions that cannot be interrupted Examples: Synchronization: HowTo, II Used to implement locks Test-and-set ( TST ) if word contains given value, set to new value Compare-and-swap ( CAS ) if word equals value, swap old value with new Intel: LOCK prefix (XCHG, ADD, DEC, etc.) Timer OS needs timers for Interrupt vector for timer Time of day CPU scheduling Memory-mapping Direct access to I/O controller through memory Reserve area of memory for communication with device ( DMA ) Video RAM: CPU writes frame buffer Video card displays it Fast and convenient Virtual Memory Provide the illusion of infinite memory OS loads pages from disk as needed Many benefits Page: Fixed sized block of data Allows the execution of programs that may not fit entirely in memory (think MS Office) OS needs to maintain mapping between physical and virtual Translation Lookaside Buffer (TLB) Initial virtual memory systems used to do translation in software Meaning the OS did it An additional memory access for each memory access! S.l.o.w.!!! Modern CPUs contain hardware to do this: the TLB Fast cache Modern workloads are TLB-miss dominated Good things often come in small sizes We have see other instances of this Summary Modern architectures provide lots of features to help the OS do its job TLB Base and Limit registers Multiple modes Interrupts Timers Atomic instructions Otherwise impossible or impractically slow in software Which of these are essential? Which are useful but not essential?

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