Explain virtual memory in operating system. Discuss the advantages and disadvantages of non contiguous storage allocation.

Introduction to Virtual Memory

Virtual memory is a memory management technique that creates an illusion to users of having a large main memory. It is a feature of an operating system (OS) that allows a computer to compensate for a lack of physical memory by using disk space to simulate additional memory. The main purpose of virtual memory is to extend the available memory space by using a portion of the hard disk as an extension of RAM.

The concept of virtual memory comes into play in a system where memory management is important. The Memory Management Unit (MMU) in the system is responsible for translating virtual addresses to physical addresses. This is done through a process known as paging, where the MMU maintains a page table to keep track of where each page of memory is stored.

The physical address is calculated from a virtual address using the formula:

Physical Address = Base Address + (Page Number * Page Size) + Offset

A diagram illustrating the concept of virtual memory would be beneficial here. However, due to the text-based nature of this platform, a diagram cannot be provided.

Advantages of Virtual Memory

Virtual memory provides several advantages:

1. Memory Abstraction: It allows programs to be designed and implemented as though there is a large, contiguous address space available, regardless of the actual amount of available memory.

2. Efficient Use of Memory: It allows the system to run larger applications or multiple applications simultaneously by swapping out pages of memory to disk when they are not in use.

3. Protection and Isolation of Processes: Each process runs in its own virtual address space, preventing it from interfering with another process's memory.

4. Larger Address Spaces: It allows for larger address spaces than physical memory, thus enabling the execution of larger programs or more programs at the same time.

Introduction to Non-Contiguous Storage Allocation

Non-contiguous storage allocation is a memory management scheme where a process is divided into blocks and these blocks can be scattered throughout the memory. The blocks do not need to be in contiguous locations.

This method works by using a concept known as fragmentation. When a process is removed from memory, it leaves a hole that may not be big enough for other processes to use. Over time, these holes can add up to a significant amount of memory, a situation known as external fragmentation. Non-contiguous storage allocation helps to reduce this fragmentation by allowing processes to be allocated memory in non-contiguous blocks.

A diagram illustrating the concept of non-contiguous storage allocation would be beneficial here. However, due to the text-based nature of this platform, a diagram cannot be provided.

Advantages of Non-Contiguous Storage Allocation

Non-contiguous storage allocation has several advantages:

1. Reduces External Fragmentation: By allowing processes to be allocated in non-contiguous blocks, it reduces the amount of wasted memory due to external fragmentation.

2. Dynamic Memory Allocation: It allows for dynamic memory allocation, which means that memory can be allocated and deallocated as needed, leading to more efficient use of memory.

3. More Efficient Use of Memory: It allows for more efficient use of memory as it can fit smaller blocks into the holes left by previously removed processes.

Disadvantages of Non-Contiguous Storage Allocation

Despite its advantages, non-contiguous storage allocation also has some disadvantages:

1. Overhead: There is an overhead involved in managing non-contiguous storage allocation. The system needs to keep track of all the blocks of a process and this can be time-consuming and require additional memory.

2. Potential for Internal Fragmentation: While it reduces external fragmentation, it can lead to internal fragmentation if the blocks allocated to a process are larger than required.

3. Complexity: Memory management with non-contiguous storage allocation is more complex than with contiguous allocation. This can lead to increased complexity in the design and implementation of the operating system.

Conclusion

In conclusion, virtual memory and non-contiguous storage allocation are important concepts in operating systems. They allow for more efficient use of memory and enable the execution of larger programs or more programs simultaneously. However, non-contiguous storage allocation comes with its own set of challenges, including overhead, potential for internal fragmentation, and increased complexity. Therefore, the choice of memory management scheme depends on the specific requirements and constraints of the system.

Summary

Virtual memory is a memory management technique that creates an illusion of having a large main memory by using disk space to simulate additional memory. It provides memory abstraction, efficient use of memory, protection and isolation of processes, and larger address spaces. Non-contiguous storage allocation is a memory management scheme where a process is divided into blocks that can be scattered throughout the memory. It reduces external fragmentation, allows for dynamic memory allocation, and enables more efficient use of memory. However, it has overhead, potential for internal fragmentation, and increased complexity.

Analogy

Virtual memory is like having a virtual library where you can access books from a large collection, even if you don't have enough physical space to store all the books. Non-contiguous storage allocation is like organizing your files in different folders on your computer, instead of storing them all in one folder.