Explain the single and multiple partition techniques?

There are a number of allocation techniques available and all have different properties and allocate memory based on different principles. One prominent type of allocation is the partitioned allocation. 

- In partitioned allocation the primary or the main memory of the system is divided into a number of contiguous memory blocks which are commonly known as the memory partitions. 

- Each of these partitions consists of all the information that might be required for carrying out a specific task. 

- The task of allocating these memory partitions to various jobs and processes and de-allocating them after use is the duty of the memory management unit.  

- But partitioned allocation cannot be carried out by the help of software alone. 

- It requires some hardware support. 

- This support prevents interference of the various jobs in to each other and with the operating system as well. 

- For example, a lock and key technique was used by the IBM system/ 360. 

- Some other systems made use of the registers called the base and bound registers containing the partition limits and these were also used for flagging if any invalid access was made. 

- Limits register was used by the UNIVAC 1108 having separate base and bound data and instructions. 

- A technique called the memory interleaving was used by this system for placing so called I banks and d banks in different memory modules. 

Partitions are of two types namely:

Ø  Static partitions: These are defined at the boot time or IPL (initial program load) or sometimes by the computer operator. An example of system using static partitions is IBM system/360 operating system multi-programming with MFT (fixed number of tasks).

Ø  Dynamic partitions: These are created automatically for the specified job. An example is of the IBM system/ 360 operating system multi-programming with MVT (variable number of tasks).

- The hardware typed memory such as the base and bound registers (GE – 635, PDP – 10 etc.), Burroughs corporation B5500 etc. is used for relocating the memory partitions. 

- The partitions that can be relocated can be compacted to form larger contiguous memory chunks in the main memory. 

- Some systems allow for swapping out the partitions to the secondary storage and in turn to some additional memory.

The partitioned allocation offers two types of allocation techniques namely:

1. Single partition techniques
2. Multiple partition techniques

- Single partition techniques are the ones that are used for the single time sharing partition for swapping in and out the memory partitions. 

- These techniques are used by the IBM’s TSO (time sharing option). 

- The multiple partition techniques are used in the multiple time sharing partition. 

- In DOS systems when the disk is partitioned, each of the memory partitions act as if it is an individual disk drive. 

- Partitioning is useful for the systems where there are more than one operating system. 

- Partitioning techniques are meant for increasing the efficiency of the disk. 

- Hard and soft partitioning is used on the apple Macintosh computers. 

- The creation, relocation and deletion of the memory partitions can be harmful for the data. 

- That’s why it is good to have back up of the data stored on your system. 

- Several issues have to be considered if you want to install more than one operating system on your computer. 

- Day by day disks are becoming less expensive and bigger. 

- You can go for separate disks for storing data and installing Oss. 

Explain the methods for free space management? – Part 2

- Managing the free space is easy only when the space that has to be managed is divided in to units of fixed size. 

- If this is the case, the list of these fixed size units can be kept. 

- The first entry can be returned if it is requested by the client. 

- Managing free space gets difficult when the space to be managed consists of units of variable sizes. 

- This is the case with the memory allocation library at the user level. 

- This is also the case in the physical memory where the segmentation is used for the implementation of the virtual memory. 

- In such cases, external fragmentation is the main problem. 

- This leads to the splitting up of the disk space in to pieces of variable size. 

- The next coming requests might fail because of unavailability of contiguous free space. 

- For example, the request might fail even if 20 bytes are available and the request requires only 15 bytes because this 20 bytes space is non-contiguous. 

- Thus, the main problem is how the free space should be managed while satisfying the variable sized variables. 

How these strategies can manage free space while at the same time keeping the fragmentation in control.

Low level Mechanisms: 

- Most of the allocator use some common mechanisms as these: coalescing and splitting.

- Here, the free list consists of a set of elements describing about all the free spaces available in the heap. 

- Once a pointer to a space is handed over to the program, the determination of the pointers to this space becomes somewhat difficult. 

- These pointers are stored either in the registers or in the variables at some point of execution. 

- However, this is not the case of garbage collected and strongly typed languages as a measure for enabling compaction for combating fragmentation. - Suppose the program makes a request for a single byte of memory.

- In such a case the action performed by the allocator is called splitting. 

- Here a free memory chunk is searched for and split in to two. 

- The first one is returned to the calling request and the second one stays in the list itself. 

- This approach is used in the allocators where small requests are made requesting space that is of size smaller than the chunk. 

- Most of the allocators use a corollary mechanism called the coalescing of the free space. 

- Suppose a small heap is given and an application calls a function to obtain some memory. 

- This function returns the space available in the middle of the heap. 

- All these strategies are based up on simple policies. 

- An ideal allocator is the one that both minimizes fragmentation and is fast. 

- But since it is possible for the stream of free requests and allocation to be arbitrary, any strategy would go wrong if wrong inputs are given. 

- Thus, the best approach cannot be described. 

There are 4 major approaches in this regard:

1. Best fit: 

- Simplest and first searches the list for free chunks of memory that might be bigger than the size requested. 

- The smallest one in the searched ones is returned and this is known as the best fitting chunk or smallest fitting too. 

- One pass is enough for finding the appropriate block to be returned.

2. Worst fit: 

- This one is just the opposite of the best fit. 

- It looks for the largest chunk and returns the amount requested while keeping the remaining memory space.

3. First fit: 

- This one also looks for the first bigger block and out of it allocates the amount requested by the thread.

4. Next fit: 

- Here, one extra pointer is kept at the location where the last search was done. 

Explain the methods for free space management? – Part 1

- For efficient working of the programs and the entire operating system, it is important that the memory of the system should be managed. 

- When the files and programs are allocated memory space, some free space is left in the storage area. 

- It is required that these free spaces must be managed properly. 

- Since there is a limitation to the disk space, this same space has to be used again and again after deleting and creating new files. 

- A free space list is maintained by the operating system for keeping the track of the available free space. 

- All the free disk spaces are listed in free space list. 

- For the creation of a new file this free space list is searched in order to get the amount of space needed and then if the space is available, it is allocated to the file to be created. 

- In the case of deletion, after deleting the file, its space is added to the list of free spaces.

Methods for Free Space Management

There are 4 methods for the management of free space namely:

- Bit vector

- Linked list

- Grouping

- Counting

What is Bit Vector?

- Quite a many times, the free space list about which we mentioned above, is implemented as the bit vector (also known as the bitmap). 

- Here, 1 bit is used for representing each block. 

- If a particular block has been allocated to some file or program, its representative bit is set to 0 and when the block is available, the bit is set to one.

- Consider an example, suppose the following disk blocks are free and rest are allocated: 1, 2, 4, 5, 6, 7, 9, 10, 12, 13, 14, 18, 19, 21, 26, 27, 28. 

- Then for this allocation we have the following free – space bit map:

01101111011011100011010000111…

- This method of free space management is relatively simple and has good efficiency. 

- This method is known for its efficiency to locate the n consecutive free blocks or the first free block available in the storage area. 

- But this method can be inefficient if it is not kept in the main memory of the system. 

- Also, when required occasionally for the recovery needs, this map can also be written to the disk. 

- Keeping such maps in the physical memory is an easy thing if the system has a small memory but this is not always possible in the case of the systems with larger memories.

What is a Linked list?

- In this method, all the free spaces are linked together and the first block in this linked list is assigned a pointer which is stored in the cache memory. 

- Similarly, the pointer to the second block is stored in the first block.

What is Grouping?

- This method is a modified version of the free list approach and it stores the addresses of the all the free blocks in the first block that is free. 

- Here, actually the first n- 1 blocks only are free and the address of another n free blocks is stored in the last block. 

- This lets the system to find the addresses of a number of free blocks which is not possible in the case where the approach being used is the linked list approach.

What is Counting?

- This approach takes advantage of the simultaneous allocation or freeing of the contiguous blocks through clustering or by using contiguous allocation algorithm. 

- Thus, it requires only to keep the address of the first free block and the rest of the blocks follow it. 

Explain the various Disk Allocation methods? – Part 2

In this article we discuss about the non-contiguous disk allocation methods i.e., the linked allocation and the indexed allocation. 

What is a Linked Allocation?

- In linked allocation a single file might be stored all over the disk and these scattered parts are linked to each other just like a linked list. 

- Few bytes in the memory block are used for storing the address of the following linked block.

- This type of allocation has two major advantages mentioned below:

1. Simplicity and
2. Non – requirement of disk compaction

- Since the nature of the allocation method is non-contiguous, it does not lead to any external fragmentation of the disk space.

- And since all the memory blocks have been linked to each other, a memory block available anywhere in the memory can be used for satisfying the requests made by the processes. 

- Declaring the file size for the linked allocation is not required during its creation. 

- There are no issues even if the file continues to grow as long as free blocks are available and since the blocks can always be linked up. 

- As a consequence of all this, the need for disk compaction is eliminated. 

Disadvantages of Linked Allocation

But there are disadvantages of using linked allocation. They are:

Direct access to the disk blocks becomes slow: 

For finding a particular block of the file, the search has to begin at the starting point of the file and the successive pointers have to be followed till the destination block is reached.

Space required by the pointers: 

- Suppose out of 512 words of a memory block, 2 are required for storing the pointers, then we have 39 percent of the total disk being used by the pointers rather than for data. 

- This adds to the space requirement of the file blocks.

Reliability: 

- Because of all the blocks being linked via the pointers, even if one pointer gets damaged or wrong, the successive blocks can become inaccessible. 

- This problem is quite common and thus most of the operating systems avoid this problem by keeping redundant copies of these pointers in a special file. 

- The basic idea here is to keep the list of the pointers in the physical memory of the system. 

- This also allows for the faster access to the disk blocks.

What is Indexed allocation?

- The linked allocation method does not provide support for the direct access and this problem is solved by the indexed allocation method. 

- In this method, all the pointers are placed over an index. 

- Thus, these pointers together form the index block. 

- The address of this address block is then stored in the directory. 

- The pointer at the nth number in the index block points to the nth block of the associated file. 

- The purpose of the index blocks is somewhat similar to that of the page map tables; however both of these are implemented in a different way.
- First level index is used for searching the index of second level and the second one is used for searching the third one and the process may continue till the fourth level. 

- But in most of the cases the indexes of the first two levels are sufficient. 

- With this method, second level index blocks (128 x 128) can be easily addressed and files of size of up to 8mb can be supported. 

- Assuming the same thing, files of size up to 1 gb can be addressed.

Advantages and Disadvantage of Indexed Allocation

- The major advantage of this allocation technique is that it does not give rise to external fragmentation and offers a high level of efficiency in random accessing. 

- Also, using this technique mapping can be done around the disk blocks that are known to be bad. 

-Bit mapping can be used for indexing the free space. 

The biggest disadvantage of this allocation technique is the large number disk accesses required for the retrieval of the address of the destination block in the memory. 

Explain the various Disk Allocation methods? – Part 1

Management of space of the secondary data storage devices is one of the most necessary functions of the file system. This includes tracking which blocks of memory or disk are being allocated to the files and which blocks are free to be allocated. 

There are two main problems faced by the system during allocation of the space to different files. Firstly, the access to the files has to be fast and secondly, the disk space has to be effectively utilized. Both of these combine to form the big problem of disk management. These two problems are more common with the physical memory of the system. 

However, the secondary storage of the system also introduces 2 additional problems which are long disk access time and blocks of larger size to deal with. In spite of all these problems, there are considerations that are common for both the storage such as the non – contiguous and contiguous space allocation. 

Types of Disk Allocation Methods

The following are the 3 widely used allocation techniques:

Ø  Contiguous

Ø  Linked

Ø  Indexed

- The linked and the indexed allocation techniques fall under the category of the non-contiguous space allocation. 

- All the methods have their own pros and cons.

- It is in the term of blocks that all the input and output operations are carried out on the disk. 

- Software is responsible for converting from the logical record to physical storage blocks.

Contiguous Allocation: 

- This allocation technique assigns only the contiguous memory blocks to the files. 

- The size of the memory required is mentioned by the user in advance for the holding the file. 

- The file is then created only if that much amount of memory is available otherwise not. 

- It is the advantage of the contiguous memory allocation technique that all the successive file records are saved adjacent to each other physically. 

- This causes an increase on the disk access time of the files. 

- This can be concluded from the fact that if the files are scattered all about the disk, then it takes a lot more time to access them. 

- Accessing files when they have been organized in a proper order is quite easy.

- To access the files sequentially, the system uses the address of the last memory block and if required moves to the next one. 

- Both direct and sequential accesses are supported by the contiguous memory allocation technique. 

- The problem with this allocation method is that every time a new contiguous space cannot be found for the new files if a majority of the memory is already in use and if the file size is larger than the available memory.  

Non–Contiguous Memory Allocation: 

- It happens a lot of time that the files grow or shrink in size with usage and time. 

- Therefore, it becomes difficult to determine the exact space required for the files since the users do not have any advance knowledge about the growing size of their files. 

- This is why the systems using the contiguous memory allocation techniques are being replaced by the ones with the non-contiguous storage allocation which is more practical and dynamic in nature. 

- The linked allocation technique mentioned above in the article, is a disk – based implementation of the linked list. 

- Each file is considered to be a list of the disk blocks linked to each other. 

- It does not matter whether the blocks are scattered about the disk. 

- In each block, few bytes are reserved for storing the address of the next block in chain.

- The pointer to the first and the last block of the file is stored in the directory. 

Explain the various File Access methods?

One of the most important functions of the mainframe operating system is the access methods that make it possible for you to access the data from external devices such as the tape or disk. 

What are access methods?

- Access methods are very useful in providing an API for transferring the data from one device to another.

- Another best thing about this API was that it worked as the device driver for the operating systems on non-mainframe computers. 

- There have been a lot of reasons behind the introduction of the access methods. 

- A special program had to be written for the I/O channel and there has to be a processor entirely dedicated to controlling the access to the peripheral storage device as well as data transfer from and to the physical memory. 

- Special instructions constitute these channel programs and are known as the CCWs or the channel command words.

- To write such programs, very detailed knowledge is required regarding the characteristics of the hardware. 

Benefits of File Access Methods

There are 3 major benefits of the file access methods:

Ø  Ease of programming: The programmer does not have to deal with the procedures of the specific devices, recovery tactics and error detection. A program designed to process a particular thing will do it no matter where the data has been stored.

Ø  Ease of hardware replacement: A program cannot be altered by the programmer during the migration of data from older to newer model of the storage device provided the same access methods are supported by the new model.

Ø  Ease in sharing the data set access: The access methods can be trusted for managing the multiple accesses to the same file. At the same it ensures the security of the system and data integrity.

Some File/Storage Access Methods

Ø  Basic direct access method (BDAM)

Ø  Basic sequential access method (BSAM)

Ø  Queued sequential access method (QSAM)

Ø  Basic partitioned access method (BPAM)

Ø  Indexed sequential access method (ISAM)

Ø  Virtual storage access method (VSAM)

Ø  OAM (object access method)

- For dealing with the records of a data set both the types of access i.e., the queued and the basic are suitable. 

- The queued access methods are an improvement of the basic file access methods. 

- Read ahead scheme and internal blocking of data is well supported by these methods. 

- This allowed combining the multiple records in to one unit, thus increasing the performance. 

- In sequential methods, it is assumed that there’s only a sequential way for processing the records which is just the opposite of the direct access methods. 

- There are devices like the magnetic tape that only enforce the sequential access 

- Sequential access can be used for writing a data set and then later the direct manner can be used for processing it.

Today we have access methods that are network-oriented such as the following:

Ø  Basic telecommunications access method or BTAM

Ø  Queued tele – processing access method or QTAM

Ø  Telecommunications access method or TCAM

Ø  Virtual telecommunications access method or VTAM

- The term access method was used by the IMS or the IBM information management system for referring to the methods for manipulation of the database records. 

- The access methods used by them are:

Ø  GSAM or generalized sequential access method

Ø  HDAM or hierarchical direct access method

Ø  HIDAM or hierarchical indexed direct access method

Ø  HISAM or hierarchical indexed sequential access method

Ø  HSAM or hierarchical sequential access method

Ø  PHDAM or partitioned hierarchical direct access method

Ø  PHIDAM or partitioned hierarchical indexed direct access

What are advantages and disadvantages of artificial neural networks?

The artificial neural networks, since they can simulate the biological nervous system are used in many real life applications which are also their one of the biggest advantages. 

They have made it easy for carrying out complex processes such as:

Ø  Function approximation

Ø  Regression analysis

Ø  Time series prediction

Ø  Fitness approximation

Ø  Modeling

- With the artificial neuron networks, the classification based on sequence and pattern recognition along with other difficult things such as the sequential decision making and the novelty detection is possible. 

- A number of operations falling under the data processing category such as clustering, filtering, compression and blind source separation etc. are also carried out with the help of artificial neural networks. 

- Artificial neural networks can be considered as the backbone of the robotics engineering field. 

- They are used in computer numerical control and in directing the manipulators.

- It offers advantages in the following fields of:

1. System control (this including process control, vehicle control and natural resources management),
2. System identification,
3. Game – playing,
4. Quantum chemistry
5. Decision making (in games such as poker, chess, backgammon and so on.)
6. Pattern recognition (including face identification, radar systems, object recognition and so on.)
7. Sequence recognition (in handwritten text recognition, speech, gesture etc.)
8. Medical diagnosis
9. Financial applications i.e., in automated trading systems
10. Data mining
11. Visualization
12. E – mail spam filtering

- Today several types of cancers can be diagnosed using the artificial neural networks. 

- HLND is an ANN based hybrid system for detection of lung cancer. 

- The diagnosis carried out with this is more accurate plus the speed of radiology is more. 

- These diagnoses are then used for making some models based up on information of the patient. 

- Its following theoretical properties are nothing but an advantage to the industry:

1. Computational power: It provides a universal function approximator i.e., the multilayer perceptron or MLP.
2. Capacity: This property indicates about the ability of ANN to model almost any given function. It has a relation with both the notion of complexity and information contained in a network.
3. Convergence: This property is dependent on a number of factors such as:

Ø  Number of existing local minima which in turn depends up on model and the cost function.

Ø  Optimization method used

Ø  Impracticality of few methods for a large amount of parameters.

4. Generalization and statistics: Over training is quite a prominent problem in the applications where it is required to create a system that is capable of generalizing in unseen examples. This in turn leads to problem of the over specified or the convoluted systems along with the network exceeding the limit of the parameters. There are two solutions offered by ANN for this problem:

-   Croos – validation and

-   Regularization

Disadvantages of Artificial Neural Networks

1. It requires a lot of diverse training for making the artificial neural networks ready for the real world operations which is a drawback more prominent in the robotics industry.

2. Many storage and processing resources are required for implementing large software neural networks using ANNs.

3. The human has the ability to process the signals via a graph of neurons. A similar simulation of even a very small problem can call for excessive HD and RAM requirements.

4. Time and money cost for building ANNs is very large. 

5. Furthermore, simulation of the signal transmission through all the connections and associated neurons is required.     

Define the Virtual Memory technique?

Modern operating systems come with multitasking kernels. These multitasking kernels often run in to the problems related to memory management. Physical memory does not suffice for them to execute the tasks assigned to them because of being fragmented. So they have to take some additional from the secondary memory. But they cannot use this memory directly. Virtual memory offers a solution to this problem. 

What is Virtual Memory technique?

- Using this technique makes the fragmented main memory available to the kernels as a contiguous main memory. 

- Since it is really not the main memory but just appears to be, it has been named as the virtual memory and this technique is called the virtual memory technique. 

- Since, it helps in managing the memory, it is essential a memory management technique. 

- The main storage gets fragmented because of many programming and processing problems. 

- The main memory available to the processes and the tasks is virtualized by the virtual memory technique and then it appears to the process as a contiguous memory location. 

- This memory is a global address space. 

- Virtual address spaces such as these are managed by the operating system. 

- The real memory is assigned to the virtual memory by the operating system itself. 

- The virtual addresses of the allocated virtual address spaces are translated in to the physical addresses automatically by the CPU. 

- It achieves this with the help of some memory management hardware specially designed for this purpose. 

- The processes continue to execute uninterrupted as long as this hardware properly translates the virtual addresses in to real memory addresses properly. 

- If it fails in doing so at any point of time, the execution comes to a halt and the control is transferred to the operating system. 

- The duty of the operating system now is to move the requested memory page to the main memory from the backing store. 

- Once done with this, it then returns the control again to the process that was interrupted. 

- It greatly simplifies the whole execution process. 

- Even if the application would require more data or code that would fit in real memory, it does not have to be moved to and fro between the backing store and the real memory. 

- Furthermore, this technique also offers protection to the processes that are provided distinct address spaces by the isolation of the memory allocate to them from other tasks.

- Application programming has been made a lot easier with the help of the virtual memory technique since it hides the fragmentation defects of the real memory. 

- The burden of memory hierarchy management is delegated to the kernel which eliminates the need for the explicit handling of the overlays by the program. 

- Thus each process can execute in an address space that is dedicated to it. 

- The need for relocating the code of the program is obviated along with using relative addressing for accessing the memory. 

- The concept of virtual memory was generalized and eventually named as memory virtualization. 

- Gradually, the virtual memory has become an inseparable part of the architecture of the modern computers. 

- For implementing it, dedicated hardware support is absolutely necessary. 

- This hardware is built in to the CPU in some sort of memory management hardware. - If required for boosting the performance of the virtual memory, some virtual machines and emulators may employ some additional hardware support. 

- The older mainframe computers did not have any support for the virtual memory concept. 

- In virtual memory technique, each program can solely access the virtual memory.