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Operating Systems. Memory Management. Purpose of Memory Management. Provide the memory space to enable several processes to be executed at the same time. Provide a satisfactory level of performance for the system users. Protect each process from one another.

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Operating systems

Operating Systems

Memory Management

Purpose of memory management
Purpose of Memory Management

  • Provide the memory space to enable several processes to be executed at the same time.

  • Provide a satisfactory level of performance for the system users.

  • Protect each process from one another.

  • Enable sharing of memory space between processes.

  • Make the addressing of memory space transparent to programmers.


  • Main memory is the primary area for holding the instructions and data that processes are using.

  • The cache memory is smaller (< 64 kB) and faster than main memory.

  • Cache memory is used to store data and instructions that are currently being used, or are predicted to be used shortly.

  • The processor will look for information in the cache before looking in main memory.


  • Modern computer systems can make use of a cache in three places :-

    • The Level One (L1) Cache is inside the CPU chip.

    • The Level Two (L2) Cache is external to the CPU chip but is between the L1 Cache and the main memory.

    • A Hard Drive Cache is between the Hard Drive and the CPU.

Memory management algorithms
Memory Management Algorithms

  • Single-Process System

    • Process to be executed is loaded into the free space area of the memory.

    • Used by MS-DOS.

  • Fixed Partition Memory

    • Divide memory into a number of fixed areas of different size

    • Each area holds an active process.

    • Each area may contain unused space resulting in internal fragmentation.

    • A process may not be able to run because it cannot find a large enough partition.

    • Used by IBM 360s.

Memory management algorithms1
Memory Management Algorithms

  • Variable Partition Memory

    • A process is allocated the exact amount of memory it requires.

    • Processes are loaded into consecutive areas until memory is full.

    • As a process terminates, the space it occupies is available for the use of a new process.

    • A new process may not need the entire free area, leaving holes, resulting in external fragmentation.

    • A process may not be able to run because there are not enough consecutive free areas.

Memory management algorithms2
Memory Management Algorithms

  • Simple Paging

    • Each process is divided into a number of fixed chunks, called pages.

    • Memory is divided into a set of page frames of the same size.

    • The size of a page is typically 4 kB.

    • A process does not have to be loaded into consecutive page frames.

    • Simple Paging is the most common algorithm in use.

Virtual memory
Virtual Memory

  • Using simple paging a process can be loaded in parts.

  • Only the portions of a process that are being referenced at an instant need to be present in memory.

  • Therefore a process can have available more memory than is physically present.

  • The memory that a process has available to it is referred to as its virtual memory space.

Virtual memory1
Virtual Memory

  • The size of the virtual memory space is determined by the processor’s bit-width.

  • A 32-bit processor, like the Pentium IV, has 232 bytes or 4 GB of virtual memory. A 64-bit processor can have 264 bytes or 16 exabytes of virtual memory.

  • Most desktop computers have 512 MB (229 bytes) of physical memory (RAM) or less.

  • The operating system uses an area on disk called the paging file or swap file to allocate additional memory to processes.

Managing the swap file in windows
Managing the Swap File in Windows

  • Microsoft recommends that the paging file size be at least 1.5 times the size of real memory.

  • On Windows 95/98 there was a choice between letting the OS automatically control the paging file size or to specify a custom size.

  • On Windows 2000, a custom paging file size is created by setting an initial and maximum size.

  • On Windows XP, either a custom, system managed or no paging file can be created.

Memory addressing
Memory Addressing

  • A virtual memory address is in the form (p,o)

    • p is the number of the page containing the memory location

    • o is the offset of the location from the start of the page

  • For a 32-bit address space with a 4 kB page size:

    • o will occupy 12 bits (4 kB = 212)

    • p will occupy 20 bits.

The memory management unit mmu
The Memory Management Unit (MMU)

  • When a page is loaded into real memory, the virtual page number is translated into a physical page number.

  • This translation is done by a module of the CPU called the Memory Management Unit (MMU).

  • The MMU maintains page tables that map how the virtual page translates to a physical page.

  • The page tables also keep track of whether a virtual page is associated with a physical page and when it was last accessed.

Address translation
Address Translation

  • The page number portion of the address is further sub-divided into a page directory index and a page table index.

  • The MMU first locates a table known as the page directory.

  • Each process has its own private page directory, and the address of that directory is stored in its PCB.

  • The MMU uses the page directory index to locate an entry in the page directory table.

  • The MMU retrieves from the page directory the location of the page table.

Address translation1
Address Translation

  • The MMU uses the page index to locate an entry in the page table.

  • The MMU retrieves from the page table the address of the page frame in physical memory.

  • The MMU uses the page byte offset as an index into the physical page and isolates the data that the process wants to reference.

Address translation2
Address Translation

  • Why a 3-step process?

    • Only a process' page directory must be fully defined.

    • Page tables are defined only as necessary.

    • If the majority of a process' 4 GB address space is unallocated, a significant saving in memory results because page tables are not allocated to define the unused space.

    • Otherwise 4 MB would be required to allocate each process’ page tables in a 32-bit address space.

Translation look aside buffer tlb
Translation Look-Aside Buffer (TLB)

  • The three-step translation process would cause a system's performance to be unbearably poor if the process occurred on every memory access.

  • Instead the processor has a translation look-aside buffer (TLB) which stores the most recent virtual page to physical page translation.

  • When a process makes a memory reference, the MMU takes the virtual page number and simultaneously compares it with the virtual page number of every translation pair stored in the TLB.

  • If there is a match, the MMU can bypass the page directory and page table lookups because it has already obtained the page frame number from the TLB.

Mechanics of virtual memory
Mechanics of Virtual Memory

  • If a process needs an instruction or data from page p1 which is not already in memory, a page fault is generated, the OS then does the following:

    • Find out where the contents of page p1 are stored on disk.

    • Use a page replacement algorithm to choose another page p2 mapped to some frame f of physical memory.

    • Copy the contents of frame f out to disk.

    • Update the page tables to indicate that p2 is no longer associated with a physical page.

    • Copy p1's data from disk to frame f.

    • Update the page tables so that p1 is mapped to frame f.

Locality of reference
Locality of Reference

  • Processes exhibit a characteristic known as locality of reference.

  • Over intervals of time address references made by a process tend to cluster around narrow ranges contained in a few pages.

  • If memory references jumped around virtual space at random, there would be a disk read and write for each new reference and virtual memory would be as slow as a disk.

Page replacement policy
Page Replacement Policy

  • Pages must be removed whenever physical memory is full of in-use pages and a process requires a page not in physical memory.

  • Two characterizations for replacement policies are global and local.

    • In a global replacement policy, the MMU considers all pages of physical memory as replacement candidates.

    • In a local replacement policy, the MMU considers as replacement candidates only pages belonging to the process that is accessing the page to be brought in.

Page replacement policy1
Page Replacement Policy

  • Page Replacement Policies are further characterized by the algorithm used to choose a page to remove.

  • First-in First-Out (FIFO) algorithm

    • Select for removal the page which has been in memory for the longest time.

    • Implement as a linked-list queue, the page at the head of the queue is the oldest.

    • A heavily used page will be periodically removed, thereby resulting in a page fault the next time it is needed.

Page replacement policy2
Page Replacement Policy

  • Clock or Second Chance algorithm

    • Variation of FIFO that uses a circular queue.

    • An entry in the queue has a “used” bit.

    • When a page is first loaded set its used bit to zero.

    • When the page frame is subsequently referenced set its used bit to one.

    • When a page replacement is required go round the list until a used bit of zero is found, change all intermediate used bits from one to zero.

    • Essentially, what second-chance does is, as its name suggests, giving every page a "second-chance" - an old page which has been referenced is probably in use, and should not be swapped out over a new page which has not been referenced

Page replacement policy3
Page Replacement Policy

  • Least Recently Used (LRU) algorithm

    • Select for replacement the page whose time since last reference is greatest.

    • Requires that a time stamp recording is made for a page frame at the time of each reference.

    • The overhead of maintaining a time stamp and finding the oldest value is very high.

Page replacement policy4
Page Replacement Policy

  • Not Recently Used (NRU) algorithm

    • Each page frame has associated with it a “page referenced” bit.

    • At intervals, the OS resets all if these bits to zero.

    • Subsequent reference to a page will set its page referenced bit to one.

    • When a page fault occurs, a page with the bit set to zero is selected for replacement.

    • If a page fault occurs right after the bits are reset, unable to determine if a page was recently used.


  • Thrashing occurs when the total memory requirement of all the processes is significantly greater than physical memory.

  • The CPU spends more time swapping pages than doing productive work.

  • This is usually an indication that the computer requires more physical memory (RAM).