Lecture 5 memory management in operating systems.pptx

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IT105
Operating Systems
Memory Management
Harris Chikunya

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Introduction
• Memory management is the functionality of an operating system
which handles or manages primary memory and moves processes
back and forth between main memory and disk during execution.
• Memory management keeps track of each and every memory
location, regardless of either it is allocated to some process or it is
free.
• It checks how much memory is to be allocated to processes.
• It decides which process will get memory at what time.
• It tracks whenever some memory gets freed or unallocated and
correspondingly it updates the status.

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Objectives
• By the end of this unit, you should be able to:
 Describe the various activities handled by the operating system while
performing the memory management function;
 Identify the logical and physical memory organisation;
 Discuss the memory protection against unauthorised access and
sharing;
 Manage swapping between main memory and disk in case main
storage is small to hold all processes;
 Summarise the principles of memory management as applied to paging
and segmentation;

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Logical & Physical Address Space
• Logical Address
 Logical address is the address at which a memory location appears to reside from the
perspective of an executing application program
 It is the address generated by the CPU.
 The set of all logical addresses generated by a program is a logical-address space
• Physical Address
 A physical address, also real address, or binary address, is the memory address,
that is electronically (binary number) presented on the computer address bus
circuitry in order to enable the data bus to access a particular storage cell of main
memory.
 Is the address as seen by the MMU.
 The set of all physical addresses corresponding to the logical addresses is a
physical-address space.
• Logical and physical addresses are the same in compile-time and load-time
address-binding schemes;
• logical (virtual) and physical addresses differ in execution-time address-binding
scheme

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Memory management Unit (MMU)
• Hardware device that maps virtual address to physical address.
• In MMU scheme, the value in the relocation register is added to
every address generated by a user process at the time it is sent to
memory
• The user program deals with logical addresses; it never sees the real
physical addresses

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Swapping
• Swapping is the act of moving processes between memory and a
backing store.
• This is done to free up available memory.
• Swapping is necessary when there are more processes than
available memory.
• To move a program from fast-access memory to a slow-access
memory is known as “swap out”, and the reverse operation is known
as “swap in”.

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Benefits of using swapping
1. Allows higher of multiprogramming;
2. Allows dynamic relocation. i.e., if address binding at execution
time is being used we can swap in different location else in case of
compile and load time bindings processes have to be moved to
same location only;
3. Better memory utilisation;
4. Less wastage of CPU time on compaction; and
5. Can easily be applied on priority-based scheduling algorithms to
improve performance

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Memory Protection
• Memory protection is required
 To protect Operating System from the user processes and
 To protect user processes from one another
• This protection is done by the Relocation-register & Limit-register Scheme
• The relocation register contains the value of smallest physical address i.e.
base value e.g. 30004
• Limit register contains range of logical addresses e.g 12090 – each logical
address must be less than the limit register

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Memory Protection (cont.)
• Hardware Support: The relocation-register scheme used to protect
user processes from each other, and from changing operating
system code and data.
• If a logical address is greater than the limit register, then there is an
addressing error and it is trapped.
• The limit register hence offers memory protection.

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Memory Allocation
• The main memory must accommodate both the operating system
and the various user processes.
• We need to allocate different parts of the main memory in the most
efficient way possible.
• Main memory usually has two partitions:
 Low Memory -- Operating system resides in this memory.
 High Memory -- User processes are held in high memory.
• There are two ways to allocate memory for user processes:
1. Contiguous memory allocation
2. Non contiguous memory allocation

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Memory Allocation hierarchy

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Contiguous memory allocation
• Each process is contained in a single contiguous section of memory
• There are two methods namely:
 Fixed-Sized Partition memory allocation
 Variable-Sized Partition memory allocation

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Fixed-Sized Partition
• Divide memory into fixed size partitions, where each partition has
exactly one process
• any process whose size is less than or equal to the partition size can
be loaded into any available partition.
• The block of available memory is known as a Hole.

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Example
• Fixed-size partition scheme
• Any program, no matter how small, occupies entire partition.
• In our example, process B takes 150K of partition2 (200K) size).
• We are left with 50K sized hole. This phenomenon, in which there is
wasted space internal to a partition, is known as internal
fragmentation.

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Fixed-Sized Partition
• Advantages:
 Simple to implement and little operating system overhead.
• Disadvantage:
 Inefficient use of memory due to internal fragmentation i.e memory
space unused within a partition is wasted (e.g. when process size <
partition size
 Maximum number of active processes is fixed.

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Variable-Sized Partition
• Divide memory into variable size partitions, depending upon the size
of the incoming process.
• When the process terminates, the partition becomes available for
another process.
• As processes complete and leave they create holes in the main
memory
 Hole – block of available memory; holes of various size are scattered
throughout memory.

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Variable-Sized Partition
• Advantages:
 No internal fragmentation and more efficient use of main memory.
• Disadvantages:
 Inefficient use of processor due to the need for compaction to counter
external fragmentation.

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Fragmentation
• As processes are loaded and removed from memory, the free
memory space is broken into little pieces.
• It happens after sometimes that processes cannot be allocated to
memory blocks considering their small size and memory blocks
remains unused.
• This problem is known as Fragmentation.
• Fragmentation is of two types:
 External fragmentation Total memory space is enough to satisfy a
request or to reside a process in it, but it is not contiguous, so it cannot
be used.
 Internal fragmentation Memory block assigned to process is bigger.
Some portion of memory is left unused, as it cannot be used by
another process.

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Fragmentation (Cont.)
• The following diagram shows how fragmentation can cause waste of
memory
• External fragmentation can be reduced by compaction or shuffle memory
contents to place all free memory together in one large block. To make
compaction feasible, relocation should be dynamic.
• The internal fragmentation can be reduced by effectively assigning the
smallest partition but large enough for the process.

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Fragmentation Solution
1. Coalescing:
 process of merging existing hole adjacent to a process that will
terminate and free its allocated space.
 Thus, new adjacent holes and existing holes can be viewed as a single
large hole and can be efficiently utilised.
2. Storage Compaction
 For utilising scattered holes, shuffle all occupied areas of memory to
one end and leave all free memory space as a single large block, which
can further be utilised.
3. Permit the logical address space of a process to be non-
contiguous. This is achieved through paging and segmentation.

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Non-contiguous memory allocation
• Processes are stored in non-contiguous memory locations
• There are different techniques used to load processes into memory,
as follows:
 Paging
 Segmentation
 Virtual memory paging(Demand paging)

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Paging
• In a paged system, logical memory is divided into a number of fixed
sizes chunks’ called pages.
• The physical memory is also predivided into same fixed sized blocks
(as is the size of pages) called page frames.
• The pages sizes (also the frame sizes) are always powers of 2, and
vary between 512 bytes to 8192 bytes per page.
• the size of a frame is kept the same as that of a page to have
optimum utilization of the main memory and to avoid external
fragmentation

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Principle of operation
• Each process page is loaded to some memory frame.
• These pages can be loaded into contiguous frames in memory or
into non-contiguous frames also as shown in the Figure
• The external fragmentation is alleviated since processes are loaded
into separate holes.

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Page Allocation
• In variable sized partitioning of memory every time when a process of sizes
n is to be loaded, it is important to know the best location from the list of
available/free holes.
• This dynamic storage allocation is necessary to increase efficiency and
throughput of system.
• Most commonly used strategies to make such selection are:
1. Best-fit Policy
• Allocating the hole in which the process fits most “tightly” i.e., the
difference between the hole size and the process size is minimum.
2. First-fit Policy
• Allocating the hole first available hole (according to memory order),
which is big enough to accommodate the new process.
3. Worst-fit Policy
• Allocating the largest hole that will leave maximum amount of unused
space i.e., leftover space is maximum after allocation.

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Address Translation Scheme
• The page address is called local address is divided into 2 parts:
1. A page number (p) in logical address space; and
2. The displacement (d) (or offset) within the page table
• The frame address is called physical address and is divided into:
1. Frame number (f) in physical address space; and
2. Page offset (d)
• A data structure called page map table is used to keep track of the relation
between a page of a process to a frame in physical memory.

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Hardware Support for Paging
• This can be done in the following ways:
1. Page-table base register (PTBR)
2. Translation Look-aside Buffer (TLB)

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Page-table base register (PTBR)
• Aka Paging Address Translation by Direct mapping
• This is the case of direct mapping as page table sends directly to
physical memory page
• Disadvantage
 Decreased speed of translation because page table is kept in primary
storage and its size can be considerably large which increases
instruction execution time

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Translation Look-aside Buffer (TLB)
• A.K.A Paging Address Translation with Associative Mapping
• Each entry in the TLB consists of two parts:
 a key (or tag) and
 a value.
• The TLB is used with page tables in the following way.
 The TLB contains only a few of the page-table entries.
 When a logical address is generated by the CPU, its page number is
presented to the TLB.
 If the page number is found (known as a TLB Hit), its frame number is
immediately available and is used to access memory.
 It takes only one memory access.
 If the page number is not in the TLB (known as a TLB miss), a memory
reference to the page table must be made.

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Translation Look-aside Buffer (TLB)
 When the frame number is obtained, we can use it to access memory.
 It takes two memory accesses.
 In addition, it stores the page number and frame number to the TLB,
so that they will be found quickly on the next reference.
 If the TLB is already full of entries, the operating system must select
one for replacement by using page replacement algorithms.

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Memory protection in Paged Environment
1. read-write or read-only bits
 These bits are kept in the page table.
 One bit can define a page to be read-write or read-only. This
protection bit can be checked to verify that no writes are being made
to a read-only page.
 An attempt to write to a read-only page causes a hardware trap to the
operating system (or memory-protection violation).
2. valid-invalid bit.
 When this bit is set to "valid," this value indicates that the associated
page is in the process' logical address space, and is a legal (or valid)
page.
 If the bit is set to "invalid," this value indicates that the page is not in
the process' logical-address space. Illegal addresses are trapped by
using the valid-invalid bit.

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Research
• Structure of Page Tables
 Hierarchical Page table
 Hashed Page Tables
 Inverted Page Table
• Shared Pages
 Shared code
 Private code and data

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Segmentation
• This scheme divides the logical address space into variable length
chunks, called segments, with no proper ordering among them.
• Each segment has a name and a length.
• Thus, the logical addresses are expressed as a pair of segment
number and offset within segment. <segment-number, offset>
• It allows a program to be broken down into logical parts according to
the user view of the memory, which is then mapped into physical
memory.
• A program is a collection of segments.
• A segment is a logical unit such as: main program, procedure,
function, method, object, local variables, global variables, common
block, stack, symbol table, arrays etc.

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Segmentation Hardware
• Segment table maps two-dimensional physical addresses and each
entry in the table has:
 base – contains the starting physical address where the segments
reside in memory.
 limit – specifies the length of the segment.
• Segment-table base register (STBR) points to the segment table’s
location in memory.
• Segment-table length register (STLR) indicates number of segments
used by a program.

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Segmentation Hardware
• The segment number is used as an index into the segment table.
• The offset d of the logical address must be between 0 and the segment
limit. If it is not, we trap to the operating
• system that logical addressing attempt beyond end of segment.
• If this offset is legal, it is added to the segment base to produce the
address in physical memory of the desired byte.

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