Department of Computer Science
DCS
COMSATS Institute of
Information Technology
Assistant Professor
COMSATS Lahore
Pakistan
Operating System Concepts

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Primary memory (real memory, physical memory) of computer system has been a major influence on OS design.
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Main memory is still expansive compared to secondary storage.
Department of Computer Science

Operating System Release date
Windows 1.0
Windows 2.03
Windows 3.0
Windows 3.1
Windows 95
Windows NT 4.0
Windows 98
Windows ME
Windows 2000
Professional
Windows XP
Home
Windows XP prof
Windows Vista
Windows 7
Nov 1985
Nov 1987
Mar 1990
Apr 1992
Aug 1995
Aug 1996
Jun 1998
Sep 2000
Feb 2000
Oct 2001
Oct 2001
Jan 2007
Oct 2009
Minimum memory req
256 KB
320 KB
896 KB
2.6 MB
8 MB
32 MB
24 MB
32 MB
64 MB
Recomm.
Memory
I MB
4 MB
16 MB
96 MB
64 MB
128 MB
128 MB
64 MB
128 MB
512 MB
1 GB
128 MB
256 MB
1 GB
2 GB
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It is the component of the OS concerned with the system’s memory organization scheme and memory management strategies.
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It determines, how available memory space is allocated to processes and how to respond to changes in a memory process’s memory usage.
It also interact with special purpose memory management Hardware to improve performance.
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Memory
Management
Strategies
Fetch strategies
Placement strategies
Replacement strategies
Demand fetch
Anticipatory fetch
First fit
Best fit
Worst fit
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Fetch Strategy
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It determines when to move the next piece of a program or data to main memory from the secondary memory.
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Demand Fetch
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In the conventional methods, the system places the next piece of the code or data in main memory when a running program references it.
That is, programs are loaded into the RAM area when they are needed.
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Anticipatory strategy
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The program or data which is loaded on the basis of prediction that it will be reference in the near future.
This will increase the performance but if the loaded pages are not used, then there is an inefficient memory utilization.
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Placement strategy
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It determines where in main memory the system should place incoming program or data pieces.
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First Fit Strategy
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Place the incoming page into the first available hole in the memory
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It is faster but leads to memory waste.
It allows the system to make a placement decision quickly.
Best Fit Strategy
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Place the incoming page into the hole in which it fits tight.
Best use of memory space.
It is slower in making allocation.
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Worst Fit Strategy
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Worst-fit memory allocation is opposite to best-fit.
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It allocates free available block to the new job and it is not the best choice for an actual system.
Place the incoming job in the largest available whole
Problem  Memory fragmentation  Memory waste.
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Replacement strategy
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When memory is too full to accommodate a new program, the system must remove some of program or data currently resides in memory.
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The system’s replacement strategy determines which piece to remove for making room for incoming jobs.
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To execute a program in the early systems, the
OS had to find enough contiguous main memory to accommodate the entire program.
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If the program size is larger than the available memory, the program could not execute it.
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This is done in contiguous memory allocation, in which all the program pages are loaded into the adjacent memory allocations.
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Non Contiguous Memory Allocation
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In noncontiguous memory allocation, a program is divided into blocks or segments, that the system may place in non adjacent slots in main memory
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This allows making use of holes (unused gaps) in memory that would be too small to hold whole programs.
The OS thereby incurs more overhead as for as using these unused holes for incoming pages.
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When is noncontiguous preferable to contiguous memory allocation???
ANSWER!
When the available memory contain no area large enough to hold the incoming programs in one Contiguous space, but sufficient smaller pieces of memory are available, that in total are large enough.
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Contiguous memory allocation limited the size of programs that could execute on the system
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One way in which software designer could overcome the memory limitation was to create overlays, which allowed the system to execute programs larger than main memory.
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Programmer divides the program into logical sections.
When the program does not need the memory for one section, the system can replace some or all of its with the memory for a needed section.
Overlays enables a programmer to extend main memory.
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0
Operating System a b
Portion of user code and data than must remain in main memory for duration of execution
Overlay Area
A
B
C c b
User portion with memory requirements larger than available portion of main memory
Initialization phase b
Processing phase
A
Load initialization phase at b and run
B
C
Load Processing phase at b and run
Load Output phase at b and run b
Output phase
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Managed overlay requires careful and time consuming planning and the programmer often must have detailed knowledge of the system memory organization.
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Later on virtual memory system obviate the need for programmer controlled overlays.
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Protection in single user contiguous memory allocation systems can be implemented with a single
processor.
built into the
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The boundary register contains the memory address at which the user’s program begins.
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Each time a process reference a memory address the system determines if the request is for an address greater than or equal to that stored in the boundary register.
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If yes, system service the memory request.
Else program is trying to access OS, request is terminated with an error message.
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0 a
Operating System Processor b c
User Area
Boundary register a
Unused Area
Note :
Operating System prevents the user from accessing the addresses less than a.
Memory protection with single user contiguous memory allocation
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Even with batch processing OSs, a single user systems still wasted a considerable amount of the computing resource.
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A processor wait until IO finished. Because IO speeds were extremely slow compared with processor speed, the CPU was severely underutilized.
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Designer saw that they could further increase CPU utilization by implementing multiprogramming systems, in which several users simultaneously compete for the system resources.
The process currently waiting for IO yields the processor if another process is ready to do calculation.
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Thus IO and processing operation done simultaneously.
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Greatly increases CPU utilization and system throughput.
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To take advantage of Multiprogramming several processes must reside in the computer’s main memory at the same time.
When one process requests IO, the CPU may switch to another process and continue without any delay associated with loading programs from secondary storage.
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The earlier multiprogramming systems used fixed partition multiprogramming.
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In which, system divides main memory into a number of fixed size partitions.
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Each partition holds a single job, and the system switches the processor rapidly between jobs to create the illusion of simultaneously.
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In the early multiprogramming systems, the programmer translated a job using an absolute assembler or compiler (comp. , linking, and loading),
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A job had its precise location in memory determined before it was launched and could run only in a specific partition.
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This restriction led to wasted memory because the job had to wait for its designated partition (if occupied), even if other partitions were available.
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Job Queue for partition 1
These Jobs run only in partition 1
These Jobs run only in partition 2
Job Queue for partition 2
Job Queue for partition 3
These Jobs run only in partition 3
0 a b c
Operating system
Partition 1
Partition 2
Partition 3 d
Fixed partition multiprogramming with absolute translation and loading
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0
Operating system
Job Queue for partition 1 a
No jobs waiting for partition 1
Partition 1
Job Queue for partition 2 b
No jobs waiting for partition 2
Partition 2
Job Queue for partition 3 c
Partition 3
(in use) d
Memory waste under Fixed partition multiprogramming with absolute translation and loading
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jobs
0 a
Operating system
Partition 1
Job Queue b
Partition 2 c
Partition 3 d
Fixed partition multiprogramming with relocateable translation and loading
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The system can delimit each partition with two boundary registers,
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Low and
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High
Also called the base and limit register
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If a process issues a request for memory, the system checks whether the requested is greater than or equal to the process low boundary register value and less then the high register value.
If so, the system honors the request. Else terminate the request with error.
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0 a b
Operating system
Partition 1
Processor
2 b
Currently active partition
Low Boundary c High Boundary Partition 2 c
Partition 3 d
Memory protection in contiguous allocation multiprogramming systems
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One problem prevalent in all memory organizations is that of fragmentation (loss of memory).
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This states that certain area of the available RAM that system cannot use.
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Fixed partition multiprogramming suffers from internal fragmentation , which occurs when the size of the process’s memory and data is smaller than that of the partition in which the process execute.
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0 a
Operating system
Partition 1
Used Memory b
Partition 2 Unused Memory c
Partition 3 d
Internal Fragmentation in a Fixed Partition Multiprogramming System
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Explain the need for relocating compilers, assemblers and loaders???
Answer
Before such tools, programmer manually specified the partition into which
Their program had to be loaded, which potentially wasted memory and
Processor utilization and reduce application portability.
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Explain the benefits and drawbacks of large and small partition sizes???
Answer
Larger partition allows large program to run, but result in internal fragmentation For small programs. Small partition reduce the amount of internal fragmentation And increase the level of multiprogramming by allowing more programs to reside In memory at once, but limit program size.
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This scheme allows a process to occupy only as much space as needed (up to the amount of available main memory).
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This scheme is called variable partition multiprogramming.
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The figure on next slide shows the contiguous allocation schemes where a process must occupy adjacent memory allocations.
The queue at the top contains available jobs and information about their memory requirements.
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Variable partitioning does not suffer from internal fragmentation, because the process’s partition is exactly the size of the process.
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Holes can be created upon process execution / completion.
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These holes cannot accommodate the incoming processes which are larger than these available holes.
These holes are called external fragmentation.
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There are two techniques to overcome the problems of external fragmentation created in variable partitioning system.
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The techniques are,
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Coalescing (Merging adjacent locations)
Compaction
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