In operating systems, which technique is widely used to implement virtual memory by dividing processes and physical memory into fixed size blocks?

Introduction / Context:
Modern operating systems allow programs to use more memory than is physically available in main RAM by implementing virtual memory. This concept gives each process its own logical address space, while the operating system maps parts of that space into physical memory as needed. There are different ways to organise this mapping, but one method based on fixed size blocks has become standard in many systems. This question asks for the name of that technique.

Given Data / Assumptions:
- The topic is virtual memory in operating systems.
- Processes and physical memory can be divided into fixed size blocks.
- Several related terms are offered as options, including fragmentation, segmentation, paging, and defragmentation utility.
- We assume a general purpose operating system that uses widely taught textbook methods.

Concept / Approach:
Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory. In paging, the virtual address space of a process is divided into fixed size units called pages, and physical memory is divided into frames of the same size. The operating system maintains page tables that map each page to a frame or mark it as stored on disk. When a process accesses a page that is not in RAM, a page fault occurs, and the operating system loads that page from secondary storage. Fragmentation and defragmentation are side effects and maintenance operations, not the primary technique for implementing virtual memory. Segmentation uses variable sized segments, which is conceptually different from fixed size paging.

Step-by-Step Solution:
Step 1: Recall that paging uses fixed size pages in virtual memory and matching frames in physical memory. Step 2: Recognise that the question explicitly mentions fixed size blocks, which matches the definition of pages and frames. Step 3: Consider segmentation, where segments have variable lengths based on the logical structure of programs, such as code, data, and stack segments. Step 4: Understand that fragmentation is a problem that can arise when using variable sized allocations, not a method for implementing virtual memory itself. Step 5: Note that a defragmentation utility is a tool used to reorganise data on disk, not a memory management technique. Step 6: Conclude that the correct technique described in the question is paging.

Verification / Alternative check:
Operating systems textbooks describe virtual memory systems in detail. They almost always present paging as the primary mechanism for mapping virtual address spaces to physical memory, especially in mainstream desktop and server systems. They show diagrams of page tables and explain how fixed size pages simplify allocation and reduce external fragmentation. Segmentation is discussed as an alternative or an additional layer, but it relies on variable sized segments. Fragmentation and defragmentation are discussed in separate sections dealing with performance and storage layout. This consistent organisation of topics confirms that paging is the technique referred to by the question.

Why Other Options Are Wrong:
Fragmentation: A condition where free memory is broken into small pieces, not a method for implementing virtual memory.
Segmentation: A memory management approach with variable sized segments, not fixed size blocks as described in the question.
Defragmentation utility: A maintenance tool for rearranging disk data, not a virtual memory technique used by the operating system to manage pages and frames.

Common Pitfalls:
Students may confuse fragmentation, which is an issue that can occur in memory allocation, with paging, which is a solution. Others might mix up segmentation and paging because both involve dividing memory, but they differ in whether block sizes are fixed or variable. The key to avoiding this confusion is to remember that paging uses equal sized units called pages and frames, whereas segmentation follows program structure and uses variable sizes. The reference to fixed size blocks in the question is a clear signal that paging is intended.

Final Answer:
The virtual memory technique that uses fixed size blocks of virtual and physical memory is Paging.