Unit 5-6 Virtual Memory

Unit 5-6Virtual

MemoryDr Damitha Karunaratna

University of Colombo school of computing

Operating Systems 1

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 2

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.

Operating Systems 3

Virtual Memory

Operating Systems 4

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.

Operating Systems 5

Logical View

Operating Systems 6

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 7

Virtual Memory

Operating Systems 8

Keep track of whether a page is in a physical page frame or not.

Page table

Page table may maintain a bit in each entry to indicate whether the page is in the memory or not.

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.

Operating Systems 9

Using Virtual Memory

Operating Systems 10

http://www.tutorialspoint.com/operating_system/os_virtual_memory.htm

Virtual Addresses


Virtual address to Real Address


Paging Example


ABCd

123

CCEf

4

Virtual Memory

Page 0

Page n

1

0

Page table

row 0

row n

CCEf

ABCD

Physical Memory

Page frame 0

Page frame 3

CPU2 4

MMU 0 4

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.


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


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


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

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


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


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


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.


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)


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.


Unix Directory structure


File allocation Common file allocation techniques

◦ Contiguous◦ Linked◦ Indexed

Typically the allocation techniques allocate storage space on the basis of fixed size addressable units.


File allocation - Contiguous

Allocate disk space as a collection of adjacent/contiguous blocks.

This technique needs to keep track of unused disk space.


File Name Start Block No No of Blocks

file1 00 03

file2 12 06

file3 20 11

Directory

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


File allocation - Linked Inside each block a link is maintained to point to the next block of the file


File Name Start Block No No of Blocks

file1 00 03

Directory

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

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


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.


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.


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.


http://windows.microsoft.com/en-us/windows-vista/comparing-ntfs-and-fat-file-systems

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.


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.


References Operating System Concepts by Avi Silberschat, Peter Baer Galvin and Greg Gagne

◦ http://codex.cs.yale.edu/avi/os-book/OS8/os8c/slide-dir/


Documents

Unit 5-6 Virtual Memory