Can you explain what happens when the operating system cannot find a page in memory and accesses the disk?

When the operating system cannot find the required page in RAM, an event called a "page fault" occurs. In this case, the OS accesses the disk to load the needed page from the swap file or other storage into memory.

The process looks like this:

1. The process attempts to access a page that is not in RAM.
2. An interrupt occurs, and the OS records a page fault.
3. The OS searches for the page on the disk (e.g., in a swap file or paging file).
4. If the page is found, it is loaded into a free block of memory.
5. The page tables are updated to indicate that the page is now in memory.
6. The process resumes work, now with access to the required page.

If there is no free memory, the OS may unload another page to disk to free up space.

This is a virtual memory mechanism that allows programs to work with larger amounts of data than is available in physical memory.