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Paging Example

Paging Example. Assume a page size of 1K and a 15-bit logical address space. How many pages are in the system?. Answer: 2^5 = 32. Assuming a 15-bit address space with 8 logical pages. How large are the pages?. Answer: 2^5 = 32.

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Paging Example

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  1. Paging Example Assume a page size of 1K and a 15-bit logical address space. How many pages are in the system?

  2. Answer: 2^5 = 32. • Assuming a 15-bit address space with 8 logical pages. How large are the pages?

  3. Answer: 2^5 = 32. • Assuming a 15-bit address space with 8 logical pages. How large are the pages? • Answer: 2^12 = 4K. It takes 3 bits to reference 8 logical pages (2^3 = 8). This leaves 12 bits for the page size thus pages are 2^12.

  4. Consider logical address 1025 and the following page table for some process P0. Assume a 15-bit address space with a page size of 1K. What is the physical address to which logical address 1025 will be mapped? 8 0 2

  5. Consider logical address 1025 and the following page table for some process P0. Assume a 15-bit address space with a page size of 1K. What is the physical address to which logical address 1025 maps? Step 1. Convert to binary: 000010000000001 8 0 2

  6. Consider logical address 1025 and the following page table for some process P0. Assume a 15-bit address space with a page size of 1K. What is the physical address to which logical address 1025 maps? Step2. Determine the logical page number: Since there are 5-bits allocated to the logical page, the address is broken up as follows: 00001 0000000001 Logical page number offset within page 8 0 2

  7. Consider logical address 1025 and the following page table for some process P0. What is the physical address? Step 3. Use logical page number as an index into the page table. 00001 0000000001 8 0 2 00001

  8. Consider logical address 1025 and the following page table for some process P0. What is the physical address? Take physical page number from the page table and concatenate the offset. So the physical address is byte 1. 000000000000001 8 0 2 00001

  9. Long-term Information Storage • Must store large amounts of data • Information stored must survive the termination of the process using it • Multiple processes must be able to access the information concurrently. In short:

  10. Long-term Information Storage Files: Good!

  11. Long-term Information Storage Files: Good! No Files: Bad!

  12. File System • Operating system determines how files are: • Structured • Named • Accessed • Used • Protected • Implemented • Most important aspect to users is how files appear to them: naming convention, available operations, protection, etc. (Not implementation!!).

  13. File Naming • Unix: • Case sensitive. • Allows, but does not require, extensions (e.g., prog.c). • Assigns no meaning to extensions. • Add as many extensions as desired (e.g., prog.back.stupid.c). • Does not allow spaces in name (unless “\ “) ; • Windows: • Not case sensitive. • Allows 1-3 character extensions. • Extensions have meaning (to other application codes, not to the OS) • Allows spaces in file name.

  14. File Naming Typical file extensions.

  15. File Structure • None - sequence of words, bytes • Simple record structure • Lines • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file

  16. File Structure • Three kinds of files • byte sequence (i.e., no structure). • record sequence • Tree (e.g., data base)

  17. File Structure • Can simulate last two with first method by inserting appropriate control characters • Who decides: • Operating system • Program (i.e., programs can support any model they want) • Unix and Windows support only the sequence of bytes functionality.

  18. File Types: Text and Binary An executable file (Unix)

  19. File Access • Sequential access • read all bytes/records from the beginning • cannot jump around, could rewind or back up • convenient when medium was mag tape • Random access • bytes/records read in any order • essential for data base systems • read can be … • move file marker (seek), then read or … • read and then move file marker

  20. File Attributes • Name – only information kept in human-readable form • Identifier (file descriptor) – unique tag (number) identifies file within file system • Type – needed for systems that support different types • Location – pointer to file location on device • Size – current file size

  21. File Attributes • Protection – controls who can do reading, writing, executing • Time, date, and user identification – data for protection, security, and usage monitoring • Information about files are kept in the directory structure, which is maintained on the disk (although generally cached).

  22. File Operations • Create • Write • Read • Reposition within file • Delete • Truncate

  23. File Operations in Unix • int fd = open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory • fd is a file descriptor (integer). • close (fd) – move the content of entry Fi in memory to directory structure on disk • seek() // change pointer to current location in file. • read(fd, buf, num_bytes)

  24. Open Files • Several pieces of data are needed to manage open files: • File pointer: pointer to last read/write location, per process that has the file open • File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it • Disk location of the file: cache of data access information • Access rights: per-process access mode information

  25. Open Files • Unix maintains an open-file table for each process and for the whole system. • File descriptor is used as an index into the process open-file table. Entries are items that have to do with that particular process (e.g., file pointer, access rights, etc.). • A pointer to the system-wide open-file table is also in the process open-file table. • System-wide open-file table holds process-independent information (e.g., location on disk, last access time, file size, count of the number of processes using the file).

  26. Open File Locking • Provided by some operating systems and file systems • Mediates access to a file • Mandatory or advisory: • Mandatory – access is denied depending on locks held and requested • Advisory – processes can find status of locks and decide what to do

  27. Directory • A collection of data structures containing information about files Directory Files F 1 F 2 F 3 F 4 F n Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

  28. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

  29. Organize the Directory (Logically) to Obtain • Efficiency – locating a file quickly • Naming – convenient to users • Two users can have same name for different files • The same file can have several different names • Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

  30. Single-Level Directory Naming problem Grouping problem

  31. Two-Level Directory • Separate directory for each user • Path name • Can have the same file name for different user • Efficient searching • No grouping capability

  32. Tree-Structured Directories

  33. Tree-Structured Directories (Cont) • Efficient searching • Grouping Capability • In Unix, a directory is a file that contains meta-data about the files it contains.

  34. Tree-Structured Directories (Cont) • Most OS support absolute and relative path names. • Unix has two pre-defined relative path names: • . Represents current directory • .. Represents parent directory • Current directory (working directory) • cd /spell/mail/prog or • cd .. (relative to CD)

  35. Path Names A UNIX directory tree

  36. To Open dict the absolute path name is: /usr/lib/dict.

  37. Relative Path Name Assume Current Directory is /usr/jim. Then .. is /usr, . is /usr/jim To access dict: ../lib/dict.

  38. Unix: mkdir creates a new sub-directory below the current working directory. • rmdir removes an entire directory (and all sub-directories). • rm deletes a file • If someone suggests that you try out a cool command called rm –r * don’t do it!!

  39. Shared Files (1) File system containing a shared file

  40. Links This is termed a hard link. Both directory entry pointing to the same inode. (a) Situation prior to linking (b) After the link is created (c) After the original owner removes the file

  41. Symbolic Links • Provide the path name of the target file in the linked file. • Other processes do not have access to the inode (i.e., directory structure). • What happens when file deleted by owner?

  42. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

  43. Implementing Directories (1) (a) A simple directory fixed size entries disk addresses and attributes in directory entry (b) Directory in which each entry just refers to an i-node

  44. File Control Block

  45. Accessing a File

  46. Allocation of File Blocks • Contiguous allocation • Linked-list allocation • FAT • Indexed (inodes).

  47. Directory Structure with Contiguous Allocation of File Blocks

  48. Implementing Files: Contiguous Allocation (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and E have been removed

  49. Linked-list Allocation

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