1. Dr Damitha Karunaratna University of Colombo school of computing
Unit 5-6 Virtual Memory
Dr Damitha Karunaratna
University of Colombo school of computing
Operating Systems

2. Virtual memory
Is it necessary to load an entire program to the memory for execution?
If the size of the program is larger than the available memory how can it be execute?
Operating Systems

3. Virtual Memory - Goals
Allow applications larger than physical memory to execute.
Run partially loaded programs – Entire program need not to be in memory all the time.
Multiprogramming: Many programs simultaneously reside in memory.
Allow re-locatable programs – anywhere, anytime
Application Portability:
Applications should not have to manage memory resources
Write machine independent code – program should not depend on memory architecture.
Permit sharing of memory segments or regions.
For example, read-only code segments should be shared between program instances.
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4. Virtual Memory
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5. Virtual memory Virtual memory is partitioned in to equal size pages.
Main memory is also partitions into equal size page frames.
Size of a page = size of a page frame
Programs are also partitioned into pages at the time of loading.
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6. Logical View
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7. Virtual Memory
Process runs on a virtual machine as defined by the underlying hardware.
Focus is on Hardware support for a virtual address space
virtual addresses independent of physical memory
Key hardware component is the Memory Management Unit (MMU)
address translation: virtual to physical memory
ensures virtual address space protection
Operating Systems

8. Virtual Memory
Page table
Keep track of whether a page is in a physical page frame or not.
Page table may maintain a bit in each entry to indicate whether the page is in the memory or not.
Operating Systems

9. Handling missing pages
When a page fault occurs, the Operating System:
moves the current process to the blocked/waiting state (since it must wait for a page to be made resident).
finds an empty frame or, if necessary, makes a frame empty in main memory by swapping out a page in the main memory.
determines the location of the requested page on the paging device.
schedules a disk read operation to load the page into the selected main memory frame (via a “pagein”)
later handles page fetch completion (which is recognized via an I/O completion interrupt) and then moves the blocked process back to the ready state.
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10. Using Virtual Memory
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11. Virtual Addresses
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12. Virtual address to Real Address
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13. Paging Example MMU CPU Physical Memory Virtual Memory Page table 2 4 4d 1 2 3 E f 4 1 C E f A B D
row 0
Page frame 0
row n
Page frame 3
Page n 2 4
CPU
MMU 4
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14. Using Virtual Memory
Check an internal table for this process, to determine whether the reference was a valid or it was an invalid memory access.
If the reference was invalid, terminate the process. If it was valid, but page have not yet brought in, page in the latter.
Find a free frame.
Schedule a disk operation to read the desired page into the newly allocated frame.
When the disk read is complete, modify the internal table kept with the process and the page table to indicate that the page is now in memory.
Restart the instruction that was interrupted by the illegal address trap. The process can now access the page as though it had always been in memory. Therefore, the operating system reads the desired page into memory and restarts the process as though the page had always been in memory.
Operating Systems

15. Virtual Memory - Disadvantages
Space: Translation tables and other data used by VM system reduce available memory to programs
Time: Address translation time is added to the cost (execution time) of each instruction.
Overhead: Memory management operations have been measured to consume up to 10% of the CPU time on a busy system.
Efficiency: Allocating memory in pages may result in fragmentation
Operating Systems

16. Virtual Memory
Allows programmers to address memory from a logical point of view
Another layer of indirection
Allow the illusion of operating with a larger memory space than what is available in reality
By storing some of the information on the file system
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17. Device Drivers Device driver is a software.
The computer communicates with peripheral devices through device drivers.
A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without knowing the precise hardware details.
Device drivers depends on both the hardware and the operating system loaded in to the computer
Operating Systems

18. Secondary Storage Management
Secondary storage is the non-volatile repository for both user and system data and programs.
Secondary storage is typically used to store
Source program
Executable programs
Data for the program
Temporaty data
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19. Files
A file is a named collection of related information, usually a sequence of bytes
A file can be viewed in two different ways.
• Logical (programmer’s) view: how the users see the file.
Liners collection of records.
Image File – rows of intensity values
Linear sequence of bytes.
• Physical (operating system) view: how the file is stored on secondary storage.
Many possibilities, not necessarily contiguous
Operating Systems

20. File Attributes
Each file has an associated collection of information(attributes)
file name
Owner
type (e.g., source, data, executable)
location(s) on the secondary storage.
organization (e.g. sequential, indexed, random)
access permissions – who is permitted to read/write/delete data in the file.
time and date of creation, modification, last access
file size
Operating Systems

21. File Types
File can be classified into various types based on the content.
Executable
Text
Source
Library
Compressed
Word Processor
Spread sheet.
One of the possible implementation technique of file type is to include the type as an extension to the file name.
Operating Systems

22. File Access Methods
File access methods describe how the data stored in a file can be accessed
Sequential: access in order, one record after another.
Direct (random): access in any order, skipping over uninteresting records
Indexed : access in any order, but based on key value(s)
Operating Systems

23. Directories
Directories are used to organize file to logical categories.
A directory is a file that can be searched for information about other files.
Entries in the directory file are created, deleted and modified when the files they describe are create, deleted and modified.
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24. Unix Directory structure
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25. File allocation Common file allocation techniques Contiguous Linked
Indexed
Typically the allocation techniques allocate storage space on the basis of fixed size addressable units.
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26. File allocation - Contiguous
Allocate disk space as a collection of adjacent/contiguous blocks.
This technique needs to keep track of unused disk space.
Directory
File Name
Start Block No
No of Blocks
file1 00 03
file2 12 06
file3 20 11
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27. File allocation - Contiguous
Advantages:
Simple easy access.
Easy Access.
Disadvantage
File size is not known at the time of creation.
Extending file size is difficult
External fragmentation
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28. File allocation - Linked
Inside each block a link is maintained to point to the next block of the file
Directory
File Name
Start Block No
No of Blocks
file1 00 03
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29. File allocation - Linked
Advantages:
No external fragmentation.
Files can grow easily.
Disadvantage
Many seek are required to access file data
Example :
MSDOS FAT file system
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30. File allocation - Indexed
Creates a table of pointers(index) at the time of the file creation. This table is modified as new blocks are allocated for the file or removed from the file.
The index table is also saved in a block/s.
Example : UNIX file system
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31. Formatting a storage device
Hard disks, need to be formatted before using.
Formatting a disk configures the disk with a file system so that OS can store information on the disk.
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32. File Allocation Table(FAT)
FAT is the file systems used by Windows NT operating system.
FAT uses a file allocation table (FAT) to keep track of files in the storage devices
FAT and the root directory reside at a fixed location of the volume so that the system's boot files can be correctly located.
To protect a volume, two copies of the FAT are kept.
Operating Systems

33. New Technology File System
NTFS (New Technology File System) is a proprietary file system developed by Microsoft. This is improvement of FAT. This improvements includes
The capability to recover from some disk-related errors automatically, which FAT cannot.
Improved support for larger hard disks.
Better security because you can use permissions and encryption to restrict access to specific files to approved users.
NTFS is a recoverable file system which keeps track of transactions against the file system in a log.
Operating Systems

34. Memory Fragmentation
Fragmentation is the unintentional division of memory into many small free areas that cannot be used effectively.
External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous.
Internal Fragmentation – allocated memory may be slightly larger than requested memory
This size difference is in memory internal to a partition, hence cannot be used for any other process.
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35. Memory Fragmentation ….
Compaction is a technique that can be used to deal with (external) fragmentation by moving partitions to bring many small free areas together to form a single large free area.
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36. References
Operating System Concepts by Avi Silberschat, Peter Baer Galvin and Greg Gagne
Operating Systems