Handling Page Faults

Summary

This video explains the process of handling page faults in virtual memory systems. It covers the steps taken when a page is not in physical memory, including checking the page table, identifying invalid memory access, allocating a free frame, and reading the page from the backing store (disk). Once the page is loaded into memory, the page table and process control block are updated, and execution resumes. This process ensures that a program can continue running seamlessly even when pages are swapped in and out of memory.

Takeaways

• 😀 When the invalid bit in the page table is set, it indicates that the page is not currently in physical memory.
• 😀 The first step in handling a page fault is to check the internal table in the process control block (PCB) to validate the memory reference.
• 😀 If the page reference is invalid and the page exists on the backing store, a page fault trap is generated by the OS.
• 😀 A free frame is located by searching the free frame list to allocate physical memory for the incoming page.
• 😀 A disk operation is initiated to load the desired page from the backing store into the allocated memory frame.
• 😀 Once the disk operation is complete, the page table and the internal process table are updated to reflect the page's presence in memory.
• 😀 The instruction that was interrupted by the page fault is restarted after the page has been loaded into memory.
• 😀 The process can now access the page as if it had always been in memory, allowing for seamless operation.
• 😀 Handling page faults requires coordination between the OS, memory management, and storage devices.
• 😀 The procedure ensures that even if a page is not initially in memory, it can still be accessed once it is loaded from the backing store.

Q & A

What does the invalid bit in the page table indicate?

The invalid bit in the page table indicates that the referenced page is not currently in physical memory.

What is the first step when a page fault occurs due to an invalid reference?

The first step is to check an internal table, usually kept with the process control block, to determine if the reference was valid or invalid.

What happens when the memory reference is found to be invalid because the page is on the backing store?

A page fault trap is generated, signaling the operating system that the page needs to be loaded from the backing store into physical memory.

How does the operating system handle the situation when a page is on the backing store?

The OS schedules a disk operation to read the page from the backing store and load it into a free frame in physical memory.

What is the role of the free frame list in handling a page fault?

The free frame list is used by the OS to find an available frame in physical memory where the requested page can be loaded.

What happens after the disk operation to load the page is complete?

Once the disk operation is complete, the OS updates the page table and the internal table to mark the page as being in physical memory.

What happens to the process after the page is successfully loaded into memory?

The process can then access the page as though it had always been in memory, and the instruction that caused the page fault is restarted.

Why is it necessary to restart the instruction after handling a page fault?

The instruction is restarted because the process needs to complete the memory reference that caused the page fault, now that the page is loaded into memory.

What does the term 'backing store' refer to in the context of a page fault?

The backing store refers to the secondary storage (usually a hard drive or SSD) where pages are kept when they are not in physical memory.

How does the page table help in managing page faults?

The page table tracks which pages are in physical memory and which are not, using an invalid bit to indicate when a page is not in memory, triggering a page fault when accessed.