Paging Example

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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?

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.

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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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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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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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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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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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Files: Good!

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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)

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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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



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



800 180

769

253

127


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



800

180

769

253

127


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



800 180

769

253

127



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).


800 180

769

253

127



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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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

Documents

Paging Example