Operating Systems CMPSCI 377 Lecture 2: OS & Architecture

1. Operating SystemsCMPSCI 377Lecture 2: OS & Architecture Emery Berger University of Massachusetts, Amherst

2. Last Class: Introduction • Operating system =interface between user & architecture • OS history:Change is only constant User-level Applications virtual machine interface Operating System physical machine interface Hardware

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

4. Modern OS Functionality • Provides support for: • Concurrency • I/O Device Management • Memory Management • Files • Distributed Systems & Networks

5. Concurrency • Supports multiple activities, apparently occurring simultaneously • Multiple users: • Several users work as if each has private machine • Multiple processes/threads: • One on CPU at a time • Multiple active concurrently

6. Modern OS Functionality, Cont. • I/O • CPU works while waiting on slow I/O devices • Memory Management • Coordinates allocation of RAM • Moves data between disk & main memory • Files • Coordinates how disk space is used • Distributed Systems & Networks • Lets group of PC’s to work together on distributed h/w

7. Operating Systems = Governments • Close analogy of operating systemsto utopian political systems • Libertarianism • Socialism • Communism

8. OS as Utopian Governments • Libertarianism: • Infinite RAM, CPU • Protects users from each other

9. OS as Utopian Governments • Socialism: • Safe & secure communication • Protects everyone • Fair allocation of resources

10. OS as Utopian Governments • Communism: • To each according to his needs • Centralized control • Allocates resources efficiently (in theory…) Impossible or too slow without hardware support

11. 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

12. Services & Hardware Support

13. Protection • OS protects users from each other • Users cannot read or write other user’s memory • Name OS’s that do and don’t do this! • Protects self from users • Safe from errant or malicious users • Privileged instructions (e.g., halt machine) • Code & data protected

14. 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

15. 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

16. Hardware Support

17. 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 • CPU “handles” interrupt

18. CPU Interrupt Handling • Handling interrupts: relatively expensive • CPU must: • Save hardware state • 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

19. Hardware Support

20. 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

21. Memory Traps • Special case: trigger trap on write to protected memory area • Widely used in operating systems: • Debugging • Distributed virtual memory • Approximating LRU • Garbage collection • Copy-on-write • Pay performance penalty only when needed

22. Hardware Support

23. 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

24. Hardware Support

25. Synchronization • How can OS synchronize concurrent processes? • E.g., multiple threads, processes & interrupts, DMA • CPU must providemechanism for atomicity • Series of instructions that execute as one or not at all

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

27. 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

28. Hardware Support

29. 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)

30. 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 • Today’s workloads are often “TLB-miss dominated”

31. Hardware Support

32. 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

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