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 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: 8 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:
Logical page number offset within page 8 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. 8 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. 8 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: Windows: 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. Typical file extensions.
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 Random 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 Both the directory structure and the files reside on disk
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. rmdir removes an entire directory (and all sub-directories).
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. File system containing a shared file
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

45. File Control Block

46. Accessing a File

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

48. Directory Structure with Contiguous Allocation of File Blocks

49. 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

50. Linked-list Allocation

51. File Allocation Table

52. Entry 4 bytes. Blocks 1K. 20 Million Entries (not files
Entry 4 bytes. Blocks 1K. 20 Million Entries (not files!) == 80 MB for table.

53. Indexed Allocation

54. File Allocation Table

55. Unix inode

56. Unix Directory Entry
inode number File Name
15 Tester

57. Unix File System: 1 inode for each file/directory.B S
Inode list
Data blocks
Boot area
superblock

58. File Attributes
File Attributes
File Attributes
File Attributes
100
Disk block addresses (NOT inode addresses)
Open file /usr/pmd

59. File Attributes
File Attributes
File Attributes
File Attributes
100
400
800
253
127
180
769
Four inodes in a Unix system
Step 1: Fetch inode for root directory (will be stored in memory).
Open file /usr/pmd

60. File Attributes
File Attributes
File Attributes
800
253
127
180
769
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
Open file /usr/pmd
File Attributes
100
400

61. File Attributes
File Attributes
File Attributes
File Attributes
100
400
800
253
127
180
769
inodes in a Unix system
Directory
Open file /usr/pmd
Bin 1
usr 2
Step 1: Fetch inode for root directory (will be stored in memory).
Step 2. Fetch disk block 100 and search for file usr.
Disk block 100

62. File Attributes
File Attributes
File Attributes
File Attributes
100
400
800
253
127
180
769
inodes in a Unix system
Directory
Open file /usr/pmd
Bin 1
usr 2
Step 1: Fetch inode for root directory (will be stored in memory).
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2.
Disk block 100

63. File Attributes
File Attributes
File Attributes
100
400
800
253
127
inodes in a Unix system
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
File Attributes
180
769
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2. Retrieve disk block 180.

64. File Attributes
File Attributes
File Attributes
File Attributes
100
400
800
180
769
253
127
inodes in a Unix system
Open file /usr/pmd
pmd 3
B_Man 21
Jtm 34
Step 1: Fetch inode for root directory (will be stored in memory).
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2. Retrieve disk block 180.

65. File Attributes
File Attributes
File Attributes
File Attributes
100
400
800
180
769
253
127
inodes in a Unix system
Step 1: Fetch inode for root directory (will be stored in memory).
Open file /usr/pmd
pmd 3
B_Man 21
Jtm 34
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2. Retrieve disk block 180.
Step 4. Retrieve inode 3. This points to my home directory starting at block 253.

66. Miscellaneous
If you lost credit for question 6.4 or 5.8 please see me to get those points back.
Please check your and notify me ASAP if your records disagree with mine!
Some students still have not demoed project 2. Please make arrangements with the TA to do so today.

67. Each will account for 50% of the final grade.
Grading
Test average:
25% Prelim I, 25% Prelim II, and 50% final exam.
Assignment average:
75% BRAIN, 15% programming assignments, and 10% homework.
Each will account for 50% of the final grade.

68. Final Exam New Material
Virtual Memory: Sections 9.1 up to but not including (UT) 9.2.2, 9.4 UT 9.5.2, 9.6 UT 9.7.2, UT
File Systems: 10.3 UT 10.4, 11.1 UT , 11.5 UT Also, there was material not in the Text but in slides: Understand the differences between the Unix file system and Windows file system. This includes differences in directory entries, methods for keeping track of blocks belonging to a file, and approaches to locating and opening a file.

69. Final Exam Earlier Material
Make sure you understand the scheduling algorithms we discussed (FIFO, SJF, SJF-preemptive, RR).
Please understand semaphores and the TestandSet instruction, how they are used and what can be done with them!!

70. File Allocation Table

71. Unix inode

72. Unix Directory Entry
inode number File Name
15 Tester

73. The steps in looking up /usr/ast/mbox
The UNIX File System
The steps in looking up /usr/ast/mbox

74. Disk Space Management (1)
Block size
Dark line (left hand scale) gives data rate of a disk
Dotted line (right hand scale) gives disk space efficiency
All files 2KB

75. Tracking Free Disk Blocks
Bit Vector

76. Linked-list