Operating system 3

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

Introduction
to
Operating System
Memory Concepts
Memory Allocation
Memory Partitions
Paging
Virtual Memory
2
Memory Management

Introduction
Prof. K. Adisesha (Ph. D)
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.
➢The Memory Hierarchy was developed based on a program behavior known as locality of
references.
➢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.

Memory Loading
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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: The absolute program (and data) is loaded into memory in order
for execution to start.
❖ Dynamic Loading: 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.

Memory Loading
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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
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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
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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
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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
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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
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Fixed / Contiguous Memory Partitions :
In this type of allocation, main memory is divided into a number of fixed-sized
partitions where each partition should contain only one process.
➢ In this technique, the main memory is divided into partitions of equal or different sizes.
➢ The operating system always resides in the first partition while the other partitions can be used
to store user processes.
➢ The memory is assigned to the processes in contiguous way.
➢ The partitions cannot overlap.
➢ A process must be contiguously present in a partition for the execution

Memory Allocation
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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
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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
15
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
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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.

Memory Allocation
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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
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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
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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.

Virtual memory
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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
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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
22
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
23
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
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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
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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
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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.

Operating System - Properties
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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.

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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.

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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
30
Queries ?
Prof. K. Adisesha
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