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Transcript of 1 Paging Example Assume a page size of 1K and a 15-bit logical address space. How many pages are in...
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Paging Example
Assume a page size of 1K and a 15-bit logical address space.
How many pages are in the system?
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Answer: 2^5 = 32. Assuming a 15-bit address space with 8 logical
pages. How large are the pages?
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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.
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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
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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?
8
0
2
Step 1. Convert to binary:
000010000000001
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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?
8
0
2
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
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Consider logical address 1025 and the following
page table for some process P0. What is the physical address?
8
0
2
Step 3. Use logical page number as an index into the page table.
00001 0000000001
00001
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Consider logical address 1025 and the following
page table for some process P0. What is the physical address?
8
0
2
Take physical page number from the page table and concatenate the offset.
So the physical address is byte 1.
000000000000001
00001
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Long-term Information Storage
1. Must store large amounts of data
2. Information stored must survive the termination of the process using it
3. Multiple processes must be able to access the information concurrently. In short:
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Long-term Information Storage
Files: Good!
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Long-term Information Storage
Files: Good! No Files: Bad!
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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!!).
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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.
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File Naming
Typical file extensions.
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File Structure None - sequence of words, bytes Simple record structure
Lines Fixed length Variable length
Complex Structures Formatted document Relocatable load file
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File Structure
Three kinds of files byte sequence (i.e., no structure). record sequence Tree (e.g., data base)
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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.
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File Types: Text and Binary
An executable file (Unix)
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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
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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
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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).
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File Operations Create Write Read Reposition within file Delete Truncate
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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)
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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
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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).
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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
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Directory A collection of data structures containing information about files
F 1 F 2F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on diskBackups of these two structures are kept on tapes
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Operations Performed on Directory
Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system
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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, …)
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Single-Level Directory
Naming problem
Grouping problem
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Two-Level Directory Separate directory for each user
Path name
Can have the same file name for different user
Efficient searching
No grouping capability
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Tree-Structured Directories
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Tree-Structured Directories (Cont)
Efficient searching
Grouping Capability
In Unix, a directory is a file that contains meta-data about the files it contains.
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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)
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35A UNIX directory tree
Path Names
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To Open dict the absolute path name is: /usr/lib/dict.
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Assume Current Directory is /usr/jim. Then .. is /usr, . is /usr/jimTo access dict: ../lib/dict.
Relative Path Name
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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 calledrm –r * don’t do it!!
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Shared Files (1)
File system containing a shared file
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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
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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?
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Operations Performed on Directory
Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system
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Implementing Directories (1)
(a) A simple directory fixed size entriesdisk addresses and attributes in directory entry
(b) Directory in which each entry just refers to an i-node
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File Control Block
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Accessing a File
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Allocation of File Blocks
Contiguous allocation Linked-list allocation FAT Indexed (inodes).
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Directory Structure with Contiguous Allocation of File Blocks
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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
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Linked-list Allocation
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File Allocation Table
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Entry 4 bytes. Blocks 1K. 20 Million Entries (not files!) == 80 MB for table.
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Indexed Allocation
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File Allocation Table
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Unix inode
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Unix Directory Entry
15 Tester
inode number File Name
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Unix File System
Unix File System: 1 inode for each file/directory.
B S Inode list Data blocks
Boot area superblock
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File Attributes
Disk block addresses (NOT inode addresses)
File Attributes
File Attributes
File Attributes
0 1 2 3
100
Open file /usr/pmd
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
100
400
Open file /usr/pmdStep 1: Fetch inode for root directory (will be stored in memory).
Four inodes in a Unix system
800 180
769
253
127
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
100
400
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800 180
769
253
127
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
Bin 1
usr 2
Disk block 100
100
400
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800 180
769
253
127
Step 2. Fetch disk block 100 and search for file usr.
Directory
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
Bin 1
usr 2
Disk block 100
100
400
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800 180
769
253
127
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2.
Directory
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
100
400
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800
180
769
253
127
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2. Retrieve disk block 180.
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
100
400
Open file /usr/pmd
Step 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800 180
769
253
127
Step 2. Fetch disk block 100 and search for file usr.
Step 3. Fetch inode 2. Retrieve disk block 180.
pmd 3
B_Man 21
Jtm 34
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File Attributes
File Attributes
File Attributes
File Attributes
0 1 2 3
100
400
Open file /usr/pmdStep 1: Fetch inode for root directory (will be stored in memory).
inodes in a Unix system
800 180
769
253
127
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.
pmd 3
B_Man 21
Jtm 34
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Miscellaneous
If you lost credit for question 6.4 or 5.8 please see me to get those points back.
Please check your email 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.
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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.
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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, 9.9.1 UT 9.9.4. File Systems: 10.3 UT 10.4, 11.1 UT 11.4.4, 11.5 UT
11.6. 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.
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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!!
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File Allocation Table
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Unix inode
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Unix Directory Entry
15 Tester
inode number File Name
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The UNIX File System
The steps in looking up /usr/ast/mbox
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Disk Space Management (1)
Dark line (left hand scale) gives data rate of a disk Dotted line (right hand scale) gives disk space efficiency All files 2KB
Block size
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Tracking Free Disk Blocks
Bit Vector
000111000000000111111000011
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Linked-list