Operating System-3 "Memory Management" by Adi.pdf

Introduction
to
Operating Systems – 3
Memory Management
PROF. DR. K. ADISESHA

Introduction
to
Memory
Management
Memory Concepts
Memory Allocation
Memory Partitions
Paging/ Segmentation
Virtual Memory
2
Memory Management

Introduction
Prof. Dr. K. Adisesha
3
Memory Hierarchy Design:
In the Computer System Design, Memory Hierarchy is an enhancement to organize the
memory such that it can minimize the access time.
➢This Memory Hierarchy Design is divided into 2 main types:
❖ External Memory or Secondary Memory: Comprising
of Magnetic Disk, Optical Disk, Magnetic Tape i.e.
peripheral storage devices which are accessible by the
processor via I/O Module.
❖ Internal Memory or Primary Memory: Comprising of
Main Memory, Cache Memory & CPU registers. This is
directly accessible by the processor.

Introduction
4
Memory Management:
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.
➢ Main Memory refers to a physical memory that is the internal memory to the computer.
➢ 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.

Introduction
5
Process Address Space:
The process address space is the set of logical addresses that a process references in its code.
➢ The operating system takes care of mapping the logical addresses to physical addresses at the time of
memory allocation to the program.
➢ There are three types of addresses used in a program before and after memory is allocated .
❖Symbolic addresses: The addresses used in a source code. The variable names, constants, and instruction
labels are the basic elements of the symbolic address space.
❖Relative addresses: At the time of compilation, a compiler converts symbolic addresses into relative
addresses.
❖Physical addresses: The loader generates these addresses at the time when a program is loaded into main
memory.

Introduction
6
Memory Partitioning Techniques:
The memory management techniques can be classified into following main categories:.
➢Contiguous memory management schemes
➢Non-Contiguous memory management schemes

Memory Partitioning
7
Contiguous memory management:
In a Contiguous memory management scheme, each program occupies a single
contiguous block of storage locations, i.e., a set of memory locations with consecutive
addresses.
➢ The Single contiguous memory management scheme is the simplest memory
management scheme used in the earliest generation of computer systems.
➢ In this scheme, the main memory is divided into two contiguous areas or partitions.
❖ The operating systems reside permanently in one partition,
❖ The user process is loaded into the other partition.

Memory Partitioning
8
Contiguous memory management:
Advantages of Single contiguous memory management schemes:
❖ Simple to implement.
❖ Easy to manage and design.
❖ In a Single contiguous memory management scheme, once a process is loaded, it is
given full processor's time, and no other processor will interrupt it.
Disadvantages of Single contiguous memory management schemes:
❖ Wastage of memory space due to unused memory as the process is unlikely to use
all the available memory space.
❖ It does not support multiprogramming, i.e., it cannot handle multiple programs
simultaneously.

Memory Partitioning
9
Multiple Partitioning:
The operating system divides the available main memory into multiple parts to load
multiple processes into the main memory. Thus multiple processes can reside in the
main memory simultaneously..
➢The multiple partitioning schemes is of two types:
❖ Fixed (or static) partitioning
❖ Variable (or dynamic) partitioning

Memory Partitioning
10
Fixed (or static) partitioning:
The main memory is divided into several fixed-sized partitions in a fixed partition
memory management scheme or static partitioning.
➢ These partitions can be of the same size or different sizes. Each partition can hold a
single process.
➢ The number of partitions determines the degree of multiprogramming
➢ Advantages:
❖ Simple to implement.
❖ Easy to manage and design.
➢ Disadvantages :
❖ This scheme suffers from internal fragmentation.
❖ The number of partitions is specified at the time of system generation.

Memory Partitioning
11
Dynamic Partitioning:
In a dynamic partitioning scheme, each process occupies only as much memory as they
require when loaded for processing.
➢ Requested processes are allocated memory until the entire physical memory is
exhausted or the remaining space is insufficient to hold the requesting process.
➢ In this scheme the number of partitions is not defined
at the system generation time.
➢ Simple to implement and to manage.
➢ This scheme also suffers from internal fragmentation.
➢ The number of partitions is specified at the time of
system segmentation.

Memory Partitioning
12
Non-Contiguous memory management:
In a Non-Contiguous memory management the program is divided into different blocks
and loaded at different portions of the memory that need not necessarily be adjacent to
one another.
➢ This scheme can be classified depending upon the size of blocks and whether the
blocks reside in the main memory or not.
❖ Paging technique: the main memory is divided into fixed-
size blocks of physical memory called frames.
❖ Segmentation technique: the main memory is divided into
variable-size blocks of physical memory called segments.

Memory Loading
13
Memory Loading:
All the programs are loaded in the main memory for execution. Sometimes complete
program is loaded into the memory, but some times a certain part or routine of the
program is loaded into the main memory only when it is called by the program.
➢ There are two types of Loading techniques:
❖ Static Loading
❖ Dynamic Loading

Memory Loading
14
Static Loading:
Static loading is the process of loading the complete program into the main memory
before it is executed
➢ Loading the entire program into the main memory before the start of the program
execution is called static loading.
➢ If static loading is used then accordingly static linking is applied.
➢ Linking is a process of collecting and maintaining pieces of code and data into a single
file. Linker also links a particular module into the system library.
➢ A statically linked program takes constant load time every time it is loaded into the
memory for execution.

Memory Loading
15
Dynamic Loading:
In dynamic loading, a routine is not loaded until it is invoked. All of the routines are
stored on disk in a reloadable load format.
➢ Dynamic routines of the library are stored on a disk in relocatable form and are loaded
into memory only when they are needed by the program.
➢ Loading the program into the main memory on demand
is called dynamic loading.
➢ If dynamic loading is used then accordingly dynamic
linking is applied.
➢ Dynamic loading is utilized to ensure optimal memory
consumption

Memory Loading
16
Overlays in Memory Management:
Overlaying is defined as "the process of inserting a block of computer code or other data
into internal memory, replacing what is already there."
➢ It is a method that permits applications to be larger than the primary memory.
➢ Advantages of using overlays include:
❖ Increased memory utilization: Overlays allow multiple programs to
share the same physical memory space.
❖ Reduced load time: Only the necessary parts of a program are loaded
into memory, reducing load time and increasing performance.
❖ Improved reliability: Overlays reduce the risk of memory overflow,
which can cause crashes or data loss.
❖ Reduce memory requirement and reduce time requirement.

Memory Loading
17
Swapping:
Swapping is the process of bringing in each process in main memory, running it for a
while and then putting it back to the disk.
➢ Swapping is also known as a technique for memory
compaction.
➢ Sometimes there is not enough main memory to hold all
the currently active processes in a timesharing system.
➢ The total time taken by swapping process includes the
time it takes to move the entire process to a secondary
disk and then to copy the process back to memory, as well
as the time the process takes to regain main memory.

Memory Allocation
18
Memory Allocation:
In the operating system, the following are four common memory management
techniques.
➢ Single contiguous allocation: Simplest allocation method used by MS-DOS. All
memory (except some reserved for OS) is available to a process.
➢ Partitioned allocation: Memory is divided into different blocks or partitions. Each
process is allocated according to the requirement.
➢ Paged memory management: Memory is divided into fixed-sized units called page
frames, used in a virtual memory environment.
➢ Segmented memory management: Memory is divided into different segments. In this
management, allocated memory doesn’t have to be contiguous.

Memory Allocation
19
Partition Allocation :
In Partition Allocation, when there is more than one partition freely available to
accommodate a process’s request, a partition must be selected.
➢To choose a particular partition, a partition allocation method is needed.
➢When it is time to load a process into the main memory and if there is more than one
free block of memory of sufficient size then the OS decides which free block to allocate.
➢There are different Placement Algorithm:
❖First Fit: The first hole that is big enough is allocated to program.
❖Best Fit: The smallest hole that is big enough is allocated to program.
❖Worst Fit: The largest hole that is big enough is allocated to program.

Memory Allocation
20
First Fit:
In the First Fit, the first available free hole fulfil the requirement of the process
allocated.
➢ Here, in this diagram, a 40 KB memory
block is the first available free hole that can
store process A (size of 25 KB), because the
first two blocks did not have sufficient
memory space.

Memory Allocation
21
Best Fit:
In the Best Fit, allocate the smallest hole that is big enough to process requirements.
For this, we search the entire list, unless the list is ordered by size.
➢ Here in this example, first, we traverse the
complete list and find the last hole 25KB is
the best suitable hole for Process A(size
25KB). In this method, memory utilization is
maximum as compared to other memory
allocation techniques.

Memory Allocation
22
Worst Fit:
In the Worst Fit, allocate the largest available hole to process. This method produces
the largest leftover hole.
➢ Here in this example, Process A (Size 25 KB)
is allocated to the largest available memory
block which is 60KB. Inefficient memory
utilization is a major issue in the worst fit.

Memory Allocation
23
First Fit:
In the First Fit, the first available free hole fulfil the requirement of the process
allocated.
➢Here, in this diagram, a 40 KB memory block is the first available free hole that can
store process A (size of 25 KB), because the first two blocks did not have sufficient
memory space.
❖First Fit: The first hole that is big enough is allocated to program.
❖Best Fit: The smallest hole that is big enough is allocated to program.
❖Worst Fit: The largest hole that is big enough is allocated to program.

Memory Allocation
24
Memory Partitions :
Memory allocation is a process by which computer programs are assigned memory or
space.
➢Main memory usually has two partitions −
❖Low Memory − Operating system resides in this memory.
❖High Memory − User processes are held in high memory.
➢Operating system uses the following memory allocation mechanism.
➢Memory Partitioning types are:
❖Fixed / Static-partition allocation
❖Dynamic/ Multiple-partition allocation

Memory Allocation
25
Dynamic Memory Partitioning:
In this technique, the partition size is not declared initially. It is declared at the time of
process loading
➢ The first partition is reserved for the operating system.
➢ The remaining space is divided into parts.
➢ The size of each partition will be equal to the size of the
process.
➢ The partition size varies according to the need of the
process so that the internal fragmentation can be avoided

Memory Allocation
26
Paging:
Pages of the process are brought into the main memory only when they are required
otherwise they reside in the secondary storage.
➢ Considering the fact that the pages are mapped to the frames in Paging, page size needs
to be as same as frame size.
➢ Different operating system defines different frame sizes.
➢ The pages belonging to a certain process are loaded into available.
➢ The sizes of each frame must be equal.
➢ Pages size is power of 2, between 512 bytes and 8192 bytes.
➢ The size of the process is measured in the number of pages..

Memory Allocation
27
Paging:
Address Translation.
➢ Page address is called logical address and represented by page number and the offset.
❖ Logical Address = Page number + page offset
➢ Frame address is called physical address and
represented by a frame number and the offset.
❖ Physical Address = Frame number + page offset
➢ 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.

Memory Allocation
28
Paging Faults:
A page fault occurs when a program attempts to access a block of memory that is not
stored in the physical memory, or RAM.
➢ The fault notifies the operating system that it must
locate the data in virtual memory, then transfer it
from the storage device to the system RAM.
➢ Page fault arise the exception, that specifies the
O/S, which access the memory slots from virtual
memory for running the program in continue
nature.

Memory Allocation
29
Advantages and Disadvantages of Paging:
Advantages of Paging
➢ Paging offers simplified memory management so that the programs need not worry about the
physical memory addresses.
➢ It allows efficient memory usage.
➢ Provides memory protection by preventing unauthorized access.
➢ The mapping between virtual and physical addresses is quite simple.
Disadvantages of Paging
➢ Since the pages are of fixed size, there is a possibility for internal fragmentation.
➢ Page table overhead arises in systems that require large memory.
➢ An additional layer of memory access is created by paging.
➢ Too many pages in a physical memory at the same time lead to thrashing.

Memory Allocation
30
Segmentation:
Segmentation is a memory management technique in which the memory is divided into
the variable size parts. Each part is known as a segment which can be allocated to a
process.
➢ The details about each segment are stored in a table
called a segment table.
➢ Segment table contains mainly two information
about segment:
❖ Base: It is the base address of the segment
❖ Limit: It is the length of the segment.

Memory Allocation
31
Segmentation:
Translation of Logical address into physical address by segment table.
➢ Suppose a 16 bit address is used with 4 bits for the segment number and 12 bits for the
segment offset so the maximum segment size is 4096 and the maximum number of
segments that can be refereed is 16.
➢ CPU generates a logical address
which contains two parts:
❖ Segment Number
❖ Offset

Memory Allocation
32
Segmentation:
Advantages & Disadvantages of Segmentation.
➢ Advantages of Segmentation.
❖ Average Segment Size is larger than the actual page size.
❖ Less overhead
❖ It is easier to relocate segments than entire address space.
❖ The segment table is of lesser size as compared to the page table in paging.
➢ Disadvantages of Segmentation.
❖ It can have external fragmentation.
❖ it is difficult to allocate contiguous memory to variable sized partition.
❖ Costly memory management algorithms.

Memory Allocation
33
Memory Partitions :
Segmentation
Paging

Memory Allocation
34
Non-Contiguous memory allocation:
Paging Segmentation
Paging divides program into fixed size pages. Segmentation divides program into variable size
segments.
OS is responsible Compiler is responsible.
Paging is faster than segmentation Segmentation is slower than paging
Paging is closer to Operating System Segmentation is closer to User
It suffers from internal fragmentation It suffers from external fragmentation
Logical address is divided into page number and
page offset
Logical address is divided into segment number and
segment offset
Page table is used to maintain the page
information.
Segment Table maintains the segment information

Memory Allocation
35
Fragmentation:
Fragmentation is an unwanted problem where the memory blocks cannot be allocated
to the processes due to their small size and the blocks remain unused.
➢ The process with the size greater than the size of the largest partition could not be
executed due to the lack of sufficient contiguous memory blocks cannot be allocated to
new upcoming processes and results in inefficient use of memory.
➢ Basically, there are two types of fragmentation:
❖Internal Fragmentation
❖External Fragmentation

Memory Allocation
36
Internal Fragmentation:
Memory block assigned to process is bigger. Some portion of memory is left unused, as
it cannot be used by another process.
➢ In this fragmentation, the process is allocated a memory block of size more than the size of
that process.
➢ Due to this some part of the memory is left unused and this cause internal fragmentation.

Memory Allocation
37
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.
➢ In this fragmentation, although we have total space
available that is needed by a process still we are not
able to put that process in the memory because that
space is not contiguous.
➢ After some time P1 and P3 got completed and their
assigned space is freed, but they cannot be used to
load a 2 MB process in the memory since they are
not contiguously located

Memory Allocation
38
Compaction:
Compaction technique can be used to create more free memory out of fragmented
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.

Virtual memory
39
Definition:
Virtual memory is a feature of an operating system that enables a computer to be able to
compensate shortages of physical memory by transferring pages of data from random
access memory to disk storage.
➢ Virtual memory is a memory management technique where secondary memory can be
used as if it were a part of the main memory
➢ Virtual memory serves two purposes.
❖ It allows us to extend the use of physical memory.
❖ It allows us to have memory protection, because each
virtual address is translated to a physical address.

Virtual memory
40
Virtual memory:
Virtual memory allows a computer to treat secondary memory as though it were the main
memory.
➢ Virtual memory uses hardware and software to allow a computer to compensate for
physical memory shortages.
➢ Memory manager is in charge of keeping track of the shifts between physical and virtual
memory.
➢ Types of virtual memory are:
❖ Demand Paging
❖ Segmentation

Virtual memory
41
Demand Paging:
A demand paging system is quite similar to a paging system with swapping where
processes reside in secondary memory and pages are loaded only on demand, not in
advance.
➢ Demand paging is used to implement virtual memory, an essential component of
operating systems.
➢ The important jobs of virtual memory in Operating Systems are two.:
❖ Frame Allocation
❖ Page Replacement

Virtual memory
42
Frame Allocation in Virtual Memory:
In Demand paging a physical Address is required by the Central Processing Unit (CPU)
for the frame creation and the physical Addressing provides the actual address to each
frame created.
➢ Frame Allocation Constraints:
❖ The Frames that can be allocated cannot be greater than total number of frames.
❖ Each process should be given a set minimum amount of frames.
❖ When fewer frames are allocated then the page fault ration increases and the
process execution becomes less efficient
❖ There ought to be sufficient frames to accommodate all the many pages that a single
instruction may refer to

Virtual memory
43
Frame Allocation in Virtual Memory:
Frame Allocation Algorithms.
➢ There are three types of Frame Allocation Algorithms in Operating Systems:
❖ Equal Frame Allocation Algorithms: We take number of frames and number of
processes at once. We divide the number of frames by number of processes.
❖ Proportionate Frame Allocation Algorithms: We take number of frames based on
the process size. For big process more number of frames & for small processes less
number of frames is allocated by the operating system.
❖ Priority Frame Allocation Algorithms: According to the quantity of frame
allocations and the processes, priority frame allocation distributes frames.
Processes with lower priorities are then later executed in future and first only high
priority processes are executed first.

Virtual memory
44
Page Replacement Methods :
In an operating system that uses paging for memory management, a page replacement
algorithm is needed to decide which page needs to be replaced when a new page comes
in.
➢ Page Fault: A page fault happens when a
running program accesses a memory page
that is mapped into the virtual address space
but not loaded in physical memory.

Virtual memory
45
Demand Paging:
A demand paging system is quite similar to a paging system with swapping where
processes reside in secondary memory and pages are loaded only on demand, not in
advance.
➢ Types of Page Replacement Methods are:
❖ FIFO
❖ Optimal Algorithm
❖ LRU Page Replacement

Virtual memory
46
FIFO Page Replacement:
FIFO (First-in-first-out) is a simple implementation method, the process selects the page
for a replacement that has been in the virtual address of the memory for the longest time.
➢ Features of FIFO Page Replacement Methods are:
❖ Whenever a new page loaded, the page recently comes in the memory is removed.
❖ The oldest page in the main memory is one that should be selected for replacement
first

Virtual memory
47
Optimal Algorithm:
The optimal page replacement method selects that page for a replacement for which the
time to the next reference is the longest.
➢ Features of Optimal algorithm are:
❖ Optimal algorithm results in the fewest number of page faults. This algorithm is
difficult to implement.
❖ Replace the page which unlike to use for a longer period of time. It only uses the
time when a page needs to be used.

Virtual memory
48
LRU Page Replacement:
Least Recently Used (LRU) page, this method helps OS to find page usage over a short
period of time.
➢ This algorithm should be implemented by associating a counter with an even- page.
➢ Features of LRU Page Replacement are:
❖ The LRU replacement method has the highest count. This counter is also called
aging registers, which specify their age and how much their associated pages should
also be referenced.
❖ The page which hasn't been used for the longest time in the main memory is the one
that should be selected for replacement.
❖ It also keeps a list and replaces pages by looking back into time

Virtual memory
49
Virtual memory:
Virtual memory is important for improving system performance, multitasking, using
large programs and flexibility.
➢ Benefits of using virtual memory
❖ Frees applications from managing shared memory and saves users from having to
add memory modules when RAM space runs out.
❖ Increased security because of memory isolation.
❖ Multiple larger applications can be run simultaneously.
❖ Doesn't need external fragmentation.
❖ Effective CPU use.
❖ Data can be moved automatically.

Virtual memory
50
Thrashing :
Thrashing is when the page fault and swapping happens very frequently at a higher rate,
and then the operating system has to spend more time swapping these pages..
➢ Thrashing occurs when a computer's virtual memory resources are overused, leading to a
constant state of paging and page faults, inhibiting most application-level processing.
➢ Causes of Thrashing
❖ CPU utilization is plotted against the degree of multiprogramming.
❖ As the degree of multiprogramming increases, CPU utilization also increases.
❖ If the degree of multiprogramming is increased further, thrashing sets in, and CPU
utilization drops sharply.
❖ So, at this point, to increase CPU utilization and to stop thrashing, we must decrease
the degree of multiprogramming.

Operating System - Properties
51
Distributed Environment:
A distributed environment refers to multiple independent CPUs or processors in a
computer system.
➢ An operating system does the following activities related to distributed environment:
❖ The OS distributes computation logics among several physical processors.
❖ The processors do not share memory or a clock. Instead, each processor has its own local
memory.
❖ The OS manages the communications between the processors.
❖ They communicate with each other through various communication lines.

52
Spooling:
Spooling is an acronym for simultaneous peripheral operations on line.
➢ Spooling refers to putting data of various I/O jobs in a buffer.
➢ This buffer is a special area in memory or hard disk which is accessible to I/O devices.
➢ Advantages
❖ The spooling operation uses a disk as a very large
buffer.
❖ Spooling is capable of overlapping I/O operation for
one job with processor operations for another job.

53
Spooling:
Spooling is an acronym for simultaneous peripheral operations on line.
➢ Spooling refers to putting data of various I/O jobs in a buffer.
➢ An operating system does the following activities related to distributed environment .
❖ Handles I/O device data spooling as devices have different data access rates.
❖ Maintains the spooling buffer which provides a waiting station where data can rest while the
slower device catches up.
❖ Maintains parallel computation because of spooling process as a computer can perform I/O
in parallel fashion.
❖ It becomes possible to have the computer read data from a tape, write data to disk and to
write out to a tape printer while it is doing its computing task

Discussion
54
Queries ?
Prof. K. Adisesha
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