Operating Systems CMPSC 473

1. Operating SystemsCMPSC 473 Processes August 26, 2010 - Lecture 2 Instructor: Bhuvan Urgaonkar

2. Last class • Definition of an operating system • Software that virtualizes physical resources for applications • CPU, memory, IO devices virtualized to process, virtual memory, and files • OS requires a variety of support from hardware • For protecting processes from each other and to control their resource usage -> User/Kernel Modes

3. Announcements • A brief tutorial on using gdb in the next class • Slides up on Angel

4. Services & Hardware Support • Protection: Kernel/User mode, Protected Instructions, Base & Limit Registers • Scheduling: Timer • Interrupts: Interrupt Vectors • System Calls: Trap Instructions • Efficient I/O: Interrupts, Memory-mapping • Synchronization: Atomic Instructions • Virtual Memory: Translation Lookaside Buffer (TLB)

5. 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

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

7. 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

8. Timer • OS needs timers for • Time of day • CPU scheduling • Interrupt vector for timer

9. Processes

10. Process: Definition • For us: Instance of a program in execution • For OS: The collection of data structures that fully describes how far the execution of the program has progressed • From kernel’s point of view, an entity to which system resources (CPU time, memory, …) are allocated

11. Overview of Process-related Topics • How a process is born • Parent/child relationship • fork, clone, … • How it leads its life • Loaded: Later in the course • Executed • CPU scheduling • Where a process “lives”: Address space • OS maintains some info. for each process: PCB • Process = Address Space + PCB • How processes request services from the OS • System calls • How processes communicate • A variant of processes: threads • How processes die

12. Process Address Space • Text section • Stack • Temporary data • Function params, return addresses, local variables • Data • Global variables • Heap • Used for dynamically allocating memory

13. Process Life-cycle • New: Being created • Running: Instructions are being executed • Only one process can be in this state at any given time per-CPU • Waiting: Waiting for some event to occur (e.g., I/O completion or reception of a signal) • Ready: Waiting to be assigned the CPU • Terminated: Finished execution