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OS & Computer Architecture

OS & Computer Architecture. Modern OS Functionality (brief review) Architecture Basics Hardware Support for OS Features. Modern OS Functionality. CPU management Concurrency (multiple simultaneous activities) How to achieve it? How to schedule? How to avoid conflict? Memory management

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OS & Computer Architecture

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  1. OS & Computer Architecture • Modern OS Functionality (brief review) • Architecture Basics • Hardware Support for OS Features

  2. Modern OS Functionality • CPU management • Concurrency (multiple simultaneous activities) • How to achieve it? How to schedule? How to avoid conflict? • Memory management • Disk management • I/O Device Management • Advanced: • Protection & security • support for distributed systems & networks

  3. Operating Systems = Governments • Infinite RAM, CPU • Protect users from each other: safety • Fair allocation of resources • To each according to his needs • Manage resources efficiently

  4. Canonical System Hardware • CPU: Processor to perform actual computations • I/O devices: terminal, disks, video, printer… • Memory: data & programs • System Bus: Communication channel between above

  5. Services & Hardware Support

  6. Protection • OS protects users from each other • Users cannot read or write other user’s memory • Protects self from users • Safe from errant or malicious users • Code & data protected

  7. Kernel Mode:Privileged Instructions • CPU provides “kernel” mode restricted to OS • Inaccessible to ordinary users • Kernel = core of operating system • Privileged instructions & registers: • Direct access to I/O • Modify page table pointers, TLB • Enable & disable interrupts • Halt the machine, etc. • Indicated by status bit in protected CPU register

  8. Protecting Memory:Base and Limit Registers • Hardware support to protect memory regions • Loaded by OS before starting program • CPU checks each reference • Instruction & data addresses • Ensures reference in range

  9. Hardware Support

  10. Interrupt example I • Interrupt: Let CPU work while doing I/O • I/O is s..l..o..w for CPU • I/O device has own processor • When finished, device sends interrupt on bus • CPU “handles” interrupt

  11. Interrupt example II • Interrupt: for fairness • Suppose one process is long • How can other processes use CPU? • Take turns, interrupts when one’s quota finished • CPU “handles” interrupt

  12. CPU Interrupt Handling • Handling interrupts: relatively expensive • CPU must: • Save hardware state (why?) • Registers, program counter • Disable interrupts (why?) • Invoke via in-memory interrupt vector (like trap vector, soon) • Enable interrupts • Restore hardware state • Continue execution of interrupted process

  13. Hardware Support

  14. Traps • Special conditions detected by architecture • E.g.: page fault, write to read-only page, overflow, system call • On detecting trap, hardware must: • Save process state (PC, stack, etc.) • Transfer control to trap handler (in OS) • CPU indexes trap vector by trap number • Jumps to address • Restore process state and resume

  15. Hardware Support

  16. Memory-Mapped I/O • 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

  17. Hardware Support

  18. Synchronization • How can OS synchronize concurrent processes? • E.g., multiple threads, processes & interrupts, DMA

  19. Synchronization example: bank • CPU must providemechanism for atomicity: Series of instructions that execute as one or not at all • Bank account balance: x, deposit $500, withdraw $500, ending balance? deposit withdraw 1. R1=x 2. R1= R1+500 3. x=R1 1’. R2=x 2’. R2= R2-500 3’. x=R2

  20. Synchronization: How-To • One approach: • Disable interrupts • Perform action • Enable interrupts • Advantages: • Requires no hardware support • Conceptually simple • Disadvantages: • Could cause starvation

  21. Synchronization: How-To, II • Modern approach: atomic instructions • Small set of instructions that cannot be interrupted • Examples: • 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.) • Used to implement locks

  22. Hardware Support

  23. Virtual Memory • Provides illusion of complete access to RAM • All addresses translated from physical addresses into virtual addresses • OS loads pages from disk as needed • Keeps track of which pages are in memory (“in core”) and which are on disk • Many benefits, including: • Allows users to run programs without loading entire program into RAM • May not fit in entirety (think MS Office)

  24. Translation Lookaside Buffer • First virtual memory systems performed address translation in software • On every memory access! (s..l..o..w..) • Modern CPUs contain hardware to do this: the TLB • Hash-based scheme • Maps virtual addresses to physical addresses • Fast, fully-associative cache

  25. Hardware Support

  26. Scheduling & Timers • OS needs timers for: • Time of day • CPU scheduling • Fairness: limited quantum (e.g., 100ms) for each task • When quantum expires, switch processes • Uses interrupt vector

  27. Summary • OS relies on hardware for many services • Protection • Interrupts • Traps • Synchronization • Virtual memory • Timers • Otherwise impossible or impractically slow in software

  28. From Architecture to OS to User • Architectural resources, OS services, user abstractions

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