Operating Systems: Concepts and Terminologies
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by Joydip Kanjilal
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Memory Management

This is the module of the Operating System that manages memory in a system by deciding when to allocate and de-allocate memory to a process, how much to allocate and the scheduling of these processes.  The following section lists the major concepts and terminologies that are related to Memory Management.

Contiguous and Non–Contiguous Memory Allocation

In Contiguous Memory allocation strategies, the Operating System allocates memory to a process that is always in a sequence for faster retrieval and less overhead, but this strategy supports static memory allocation only.  This strategy is inconvenient in the sense that there are more instances of internal memory fragmentation than compared to Contiguous Memory Allocation strategies.  The Operating System can also allocate memory dynamically to a process if the memory is not in sequence; i.e. they are placed in non–contiguous memory segments.  Memory is allotted to a process as it is required.  When a process no longer needs to be in memory, it is released from the memory to produce a free region of memory or a memory hole.  These memory holes and the allocated memory to the other processes remain scattered in memory.  The Operating System can compact this memory at a later point in time to ensure that the allocated memory is in a sequence and the memory holes are not scattered.  This strategy has support for dynamic memory allocation and facilitates the usage of Virtual Memory.  In dynamic memory allocation there are no instances of internal fragmentation.


This is the core of the Operating System that is responsible for Memory and Processor Management in the system.  The Kernel in MS DOS Operating System is housed in the msdos.sys file.

Bootstrap Process

This is a process by which an Operating System is loaded from the disk onto the primary memory of the system.  The Bootstrap loader is the module of the Operating System that gets loaded first and is present in the first physical sector of the disk.


In Virtual Memory Systems, a program in execution or a process is divided into equal sized logical blocks called pages that are loaded into frames in the main memory.  The size of a page is always in a power of 2 and is equal to the frame size.  Dividing the process into pages allows non-contiguous allocation in these systems.


Segmentation is a memory management technique that supports Virtual Memory.  The available memory is divided into segments and consists of two components- a base address that denotes the address of the base of that segment and a displacement value that refers to the length of an address location from the base of that segment.  The effective physical address is the sum of the base address value and the length of the displacement value.

Page Fault

A Page Fault occurs when there is a request for a page that is not available in the main memory. The Page Map Table for such a page has its presence bit not set.  When a page fault occurs, the Operating System schedules a disk read operation to retrieve the page from the secondary storage and load the same to the main memory.

Virtual Memory

Virtual Memory refers to the concept whereby a process with a larger size than available memory can be loaded and executed by loading the process in parts.  The program memory is divided into pages and the available physical memory into frames.  The page size is always equal to the frame size.  The page size is generally in a power of 2 to avoid the calculation involved to get the page number and the offset from the CPU generated address.  The virtual address contains a page number and an offset.  This is mapped to the physical address by a technique of address resolution after searching the Page Map Table.

Demand Paging

In Virtual Memory Systems the pages are not loaded in memory until they are "demanded" by a process; therefore the term, demand paging.  Demand paging allows the various parts of a process to be brought into physical memory as the process needs them to execute.


This is a condition that indicates that due to excessive paging a particular process is in the halted state or executing very slowly.  It is a condition in which a multi-programmed environment is equivalent to a mono-programmed environment.  The causes of thrashing can be attributed to one or more of the following.

·         Increase in the degree of multi programming

·         Insufficient memory at a particular point of time

·         The program does not exhibit locality of reference

Thrashing can be reduced by analyzing the CPU utilization and reducing the degree of multi-programming, which in turn is a non-negative integer that indicates how many programs are in the memory at the same point of time in a multi-programmed environment waiting for its turn to get the processor.

Cache Memory

Cache Memory is a high speed memory in the Random Access Memory (RAM).  The processor looks for data first in the Cache Memory and then depending on whether there is a Cache hit or miss, searches for the same in other parts of the primary memory.

A Cache hit indicates that the data searched for in the Cache by the CPU is available.  The reverse is Cache miss.  Typically the size of the Cache in a system is limited and varies depending on the system’s configuration.

Memory Fragmentation

This occurs in dynamic memory allocation when a process is allocated memory blocks that are non-contiguous to support multi-programming.  Memory fragmentation can be of the following two types.

·         Internal Fragmentation

·         External Fragmentation

Internal fragmentation refers to the space that remains unused inside an allocated block.  Internal fragmentation is internal to the allocated memory block and therefore the name.  In dynamic memory allocation systems there are situations when the combined size of the free memory blocks is insufficient to satisfy an incoming request to load a process in the main memory.  This is termed external fragmentation.

Context Switch

This refers to switching the CPU from one process to another by saving the state of the old process in the stack and executing the new process.  The time required to perform this kind of a switch is an overhead as the CPU remains idle at that point of time and it varies from one Operating System to another.


This is an activity of the Operating System that decides the next process to be executed by the CPU.  The module of the Operating System that is responsible for this activity is known as the Scheduler.  There is variety of scheduling algorithms and the algorithm to be used for scheduling depends on the Operating System being used.


A semaphore is the name given to a protected variable that can be assessed by only one process at any point of time.


Synchronization guarantees that only one thread can access the synchronized block of code or synchronized object at any point of time.

Mutual Exclusion

This ensures that only one process performs a certain task to a resource at any point of time.

Critical Section

This is a block of code that can be executed by only one thread at any point of time.  We say that this block is synchronized to ensure thread safety.


A deadlock is a condition that occurs when two threads attempt to access a resource that has already been locked by them.  In such a situation, none of the threads can execute and they are in a halted state.  This situation can be avoided if both threads acquire these locks on the resources in the same order.

Race Conditions

A race condition is one that can occur due to improper thread synchronization when several threads try to access the same resource at the same time.  As a consequence, the resource remains in an undefined state.  A race condition can be avoided by using thread synchronization to ensure thread safety.

Real Time Operating Systems

A Real Time Operating System (RTOS) is one in which the response to an event takes place in real time (in other words, as and when it is required).  A typical example of a RTOS is the operating system used for flight control.  Real Time Operating Systems are typically of two types, Hard Real Time and Soft Real Time.  In the former the critical tasks are completed in time, while in the latter they are executed based on their priority.

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User Comments

Title: OS   
Name: Anil gopal
Date: 2006-12-18 7:44:54 AM
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Title: operating system concepts   
Name: shashi
Date: 2006-11-16 11:40:25 AM
hello sir,
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Title: OS   
Name: Anuj
Date: 2006-10-04 2:53:45 AM
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Title: os   
Name: rajesh
Date: 2006-08-10 10:01:12 AM
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Title: HI   
Name: Hardik
Date: 2006-08-07 8:44:54 AM
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Title: os   
Name: saravanan
Date: 2006-08-07 1:29:43 AM
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