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Discussion Week 4

Discussion Week 4. TA: Kyle Dewey. Overview. Project #1 debriefing System calls Project #2. Task 1 Bugs/Ugliness. About that Project. I’m not actually grading anything (probably) Clarity is a wonderful thing I apologize for any incorrect information

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Discussion Week 4

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  1. Discussion Week 4 • TA: Kyle Dewey

  2. Overview • Project #1 debriefing • System calls • Project #2

  3. Task 1 Bugs/Ugliness

  4. About that Project... • I’m not actually grading anything (probably) • Clarity is a wonderful thing • I apologize for any incorrect information • Output is not the same with semaphores • Please output information in laundromat

  5. System Calls

  6. Why Operating Systems? • Multitasking • Resource management • CPU • Memory • Disks • Printers...

  7. Abstraction • “I just want to print!” • Lots of different, very similar hardware • Unify with a common interface

  8. Isolation • Each process “thinks” it is the only one on the system • Each has access to resources

  9. Total Resource Access • Process A prints “Hello world!” • Process B prints “Goodbye cruel world!” Hello woGoodbye crld! ruel world!

  10. Mediated Access • Gain access through another entity • The entity makes sure everything is isolated

  11. Mediated Access • Process A prints “Hello world!” • Process B prints “Goodbye cruel world!” Hello world! Goodbye cruel world!

  12. “Entity” • The entity is the OS • The pathway for mediation is a system call • System calls allow processes to communicate with the OS

  13. Syscall Frequency • Any I/O (network, disk, ...) • Any process manipulation • Interprocess communication • Shared library access • Essentially access to any shared resource

  14. Tools • strace: Linux tool for intercepting syscalls • truss: Solaris/BSD tool for intercepting syscalls • Usage: strace ./a.out

  15. “Useless” C Program

  16. C Hello World

  17. Java Hello World (Note -F -f was needed)

  18. Python Hello World

  19. The Point • Syscalls are made all over the place • Rarely, if ever, directly called in actual code • Unwieldy • Layer of abstraction

  20. System Call Execution read() read() User Space read() readFromDisk() Kernel Space

  21. Why Kernel Space? • Kernel executes call “on behalf” of a process • If a normal process could do it, then there is no isolation • Possible to have more than just “kernel level” and “user level”

  22. Something in Between - Microkernels • Goal: put as much of the kernel as possible in user space • Turns out a lot can be done in user space

  23. Microkernel

  24. Project #2 Part 1

  25. Basic Idea • Implement system calls for basic process and file manipulation • Note that “basic” means the base of everything - not simple!

  26. Syscall Naming in NACHOS • Names shared with threads implementation • These are very different, though you may need the corresponding thread operations

  27. Fork( func ) • Copies address space of caller • Creates a new process in this copied address space • Executes the given function, with this new process in the new address space • Note the dissimilarity to UNIX’s fork()

  28. Fork() Example

  29. Yield() • Temporary yields the calling process to any other processes available to run

  30. Exit( int ) • Terminates the calling thread • The parameter is the exit status (ignored for this project)

  31. Exec( filename ) • Spawns a new process • Executes the code specified in filename using the new process • Note this does not clobber the calling process, as it does with UNIX

  32. Join( SpaceId ) • Waits for the process with the given SpaceId • The calling process blocks until the process backing SpaceId returns

  33. Project #2 Part 2

  34. NACHOS Filesystem • Under Linux, it’s simply a single big file • Under NACHOS, it contains NACHOS’ directory hierarchy • Unless otherwise mentioned, the slides refer to the NACHOS hierarchy

  35. UNIX Similarity • NACHOS is modeled after UNIX • Some files are actually device interfaces

  36. Filesystem Stubs • Some stubs are provided that may help • Extremely basic and limited • May need to scrap entirely

  37. Create( name ) • Create a new, empty file with the given name • Note that you need to be able to extend file lengths - FileSystem::Create indicates an issue with this

  38. Open( name ) • Opens the file with the given name • Returns NULL if it does not exist

  39. Close( OpenFileId ) • Closes the file denoted by the given open file ID

  40. ReadAt( buffer, size, pos ) • Reads size bytes starting at pos into buffer • Note that this is a method on the OpenFile object

  41. WriteAt( buffer, size, pos ) • Write size bytes from buffer, starting at pos • Note that this is a method on the OpenFile object

  42. Project #2 Notes • Far more provided detail • Well-defined outputs • Due November 8 at midnight • Much more difficult than project #1 (worth twice as much, too)

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