Explain the concept of demand paging.

Demand paging is a memory management technique used by operating systems to efficiently utilize memory resources. It allows the system to load only the necessary portions of a program into memory, rather than loading the entire program at once.

In demand paging, the main memory is divided into fixed-size blocks called pages, and the secondary storage (usually a hard disk) is divided into fixed-size blocks called page frames. When a program is executed, only the required pages are loaded into the main memory, based on the demand from the CPU.

The concept of demand paging is based on the principle of locality of reference, which states that programs tend to access a small portion of their code and data at any given time. This means that not all pages of a program need to be loaded into memory simultaneously.

When a process requests a page that is not currently in memory, a page fault occurs. The operating system then retrieves the required page from the secondary storage and loads it into an available page frame in the main memory. This process is known as page swapping.

Demand paging offers several advantages. Firstly, it reduces the amount of memory required to run programs, as only the necessary pages are loaded. This allows for more efficient memory utilization and enables the system to run multiple programs simultaneously. Additionally, it reduces the time required to load programs into memory, as only the required pages are loaded on demand.

However, demand paging also has some drawbacks. The main disadvantage is the occurrence of page faults, which can cause a delay in program execution. When a page fault occurs, the CPU has to wait for the required page to be loaded from secondary storage, resulting in increased response time. To mitigate this issue, operating systems employ various techniques such as page replacement algorithms to determine which pages to evict from memory when it becomes full.

Overall, demand paging is a memory management technique that allows for efficient utilization of memory resources by loading only the necessary pages into memory on demand. It strikes a balance between memory efficiency and program execution speed, making it a widely used approach in modern operating systems.