OS & Computer Architecture

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