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A short class with Sarah and her students. The other ways are under More.
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Today, we will discuss what happens during a page fault. Can someone explain what a page fault is?
It's when the system cannot find the required data in memory.
Exactly! And what does it mean if the valid bit in the page table entry is 0?
It means that the corresponding physical page isn't mapped.
Perfect! Now, if we detect a page fault, what’s the first thing we need to check?
We check if the virtual address is valid or not.
Great job! If it’s an invalid reference, what action does the OS take?
The OS aborts the process immediately.
Exactly! So, let’s summarize: a page fault indicates data is missing, and a 0 valid bit means it’s not mapped. We first check the validity of the address.
Once a page fault is confirmed, what is the next step taken by the OS?
It finds a free physical page frame to load the required data.
Good! And if no free frame is available?
It might need to replace an existing page.
Correct! After getting a page frame, how does the OS retrieve the necessary data?
It looks up the page table to find where on the disk the page is stored.
Exactly! And what happens after retrieving the data from the disk?
The OS updates the page table to mark the page as valid and indicates its physical frame number.
Right again! Let’s recap these key steps: find a frame, retrieve data from disk, update the table.
After the page has been brought into memory, what does the OS do next?
It restarts the instruction that caused the page fault.
Exactly! Why is this important?
Because the program needs to continue execution without errors.
Great! So to summarize this process: if a page fault occurs, we check validity, locate a frame, retrieve data, update the table, and restart the instruction.
An event indicating required data is not in memory; leads to OS handling.
Indicates if a page is present in physical memory.
Memory location allocated for a page during a page fault.
Maps virtual addresses to physical addresses, essential for handling faults.
Include seeking and retrieving pages from secondary memory.
In a scenario where a process tries to access a variable stored on a page not currently loaded into RAM, a page fault will occur. The OS must retrieve the page from the disk.
Imagine a program that requires an array but only portions of it are loaded in memory. If the program tries to access an unloaded array segment, the OS must handle the page fault by loading that specific portion from the disk.
If a page is not in sight, a fault will bring new light.
Picture a librarian who must fetch books from a storage room when students ask for titles not on the shelves. Each time they must confirm the title is valid, return to the storage room, and ensure it’s noted as available for future access.
P-F-R-T: Page Fault Requires Team action: check Validity, find Frame, Retrieve, Update table, Then restart the instruction.
V-R-F-U-R: Validity check, Retrieve page, Find Frame, Update table, Restart the instruction.
Term
Page Fault
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Term
Valid Bit
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Term
Physical Frame
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Term
Page Table
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An event that occurs when a program attempts to access a page not currently mapped to physical memory.
A flag in a page table entry that indicates whether the corresponding page is present in physical memory.
A block of physical memory that can hold a page of data.
A data structure used by the OS to maintain the mapping between virtual addresses and physical memory.
Non-volatile storage such as disk drives where pages are stored when they are not in physical memory.
This section explores the process that occurs when a page fault happens, including how the operating system handles it by retrieving the necessary data from secondary memory.
When a page fault occurs, it indicates that the required data is not in memory. The operating system must first determine whether the reference is valid. If it is valid, the OS locates a physical page frame to load the data from the disk, updates the page table, and then restarts the instruction that caused the fault.
In this section, we delve deeply into the mechanics of page faults, a critical aspect of memory management in computer systems. A page fault occurs when the data required by a process is not available in memory, indicated by a valid bit of 0 in the page table entry. The process involves several steps: determining if the reference is valid, finding an appropriate physical page frame, retrieving the required page from the disk or secondary memory, and then updating necessary data structures.
The section also examines practical architectures, like the Intrinsity FastMATH, illustrating how the TLB (Translation Lookaside Buffer) operates in conjunction with page faults and various addressing schemes.