Restarting Instructions after Page Fault
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Understanding Page Faults
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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.
The Role of the OS in Page Faults
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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.
Restarting Instructions
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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.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
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.
Detailed
Restarting Instructions after Page 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.
Key Steps in Handling a Page Fault:
- Invalid Reference Check: Firstly, the OS checks if the virtual address that caused the page fault is valid. If it isn't part of the process's address space, the OS aborts the process.
- Physical Frame Allocation: The OS locates a physical page frame to hold the required data. If no free frame exists, it may need to replace an existing one.
- Retrieving Data: The OS identifies where the required page is stored on the disk and schedules a disk operation to load it into memory.
- Updating Page Table: Once the data is loaded, the page table is updated to set the valid bit to 1 and record the new physical frame number.
- Instruction Restart: Finally, the instruction that caused the fault is restarted, enabling the process to continue execution.
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.
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Understanding Page Faults
Chapter 1 of 7
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Chapter Content
So, during a page fault when I do not have the required data in memory, I incur a page fault. At that time, the page table entry corresponding to the page I want to access shows that it is invalid.
Detailed Explanation
A page fault occurs when the system tries to access data that is not loaded into physical memory. This happens because the page table entry for that page indicates it is invalid, essentially telling the system that it must retrieve the data from another source, such as a disk.
Examples & Analogies
Imagine a librarian looking for a book that is not on the shelf. The librarian checks the catalog and finds that the book is marked as 'not available.' Just like the librarian needs to fetch the book from another location, the computer needs to retrieve the data from the disk.
Valid Reference Check
Chapter 2 of 7
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Chapter Content
I need to decide whether this translation that I want to do is for an invalid reference or for a valid reference. If it is an invalid reference, it means this virtual address is not present in my virtual address space itself.
Detailed Explanation
The operating system must determine if the requested memory address is valid. If the address doesn't belong to the process’s allocated memory space, it is considered an invalid reference, and the operating system will abort the operation.
Examples & Analogies
Think of a student trying to access a classroom (memory address) that doesn’t exist in their school (address space). The school will simply inform the student that this classroom is invalid, and they'll need to choose a different one.
Handling Valid References
Chapter 3 of 7
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If the valid bit is 0, then the page is just not in memory. Therefore, it has to be brought from the disk or secondary memory.
Detailed Explanation
When the valid bit is 0, it indicates that although the requested address is valid, the data page is not in RAM. The operating system then must fetch this page from the disk, which is slower than accessing RAM.
Examples & Analogies
Imagine needing a book for your research that's stored in a rarely visited archive (disk). Instead of grabbing it off your desk (RAM), you’ll have to walk to the archive and retrieve it, which takes time.
Operating System Intervention
Chapter 4 of 7
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I will have to trap the OS; I will have to trap the operating system to indicate that this is a page fault.
Detailed Explanation
When there’s a page fault, the CPU sends a signal to the operating system. This is called trapping. The OS then takes over to resolve the page fault by checking what to do next, typically locating the physical page frame available in memory.
Examples & Analogies
If a student encounters an issue with their ID access to a library, they signal a librarian (the OS) for help. The librarian will check available resources and help the student access their needed materials.
Fetching the Required Page
Chapter 5 of 7
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The OS will find a physical page frame in the physical memory, and then it will swap the page into this frame via a scheduled disk operation.
Detailed Explanation
Once the OS determines a free space in physical memory, it retrieves the necessary data from the disk. This operation may take time due to the nature of disk accesses, which involve reading from storage.
Examples & Analogies
This is akin to a librarian retrieving a book from the storage at the back of the library. They might have to search through shelves before they bring the book back to the front desk for you.
Updating the Page Table
Chapter 6 of 7
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After I have brought in this page, I have to reset the tables to indicate that the page is now in memory.
Detailed Explanation
Once the page has been successfully fetched into RAM, the OS updates the page table to reflect that the page is now valid and indicates its physical location in memory.
Examples & Analogies
Think of updating the library catalog once a book is returned. The librarian records that the book is back on the shelf, reflecting its current location for future reference.
Restarting the Instruction
Chapter 7 of 7
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Chapter Content
Then I will restart the instruction that caused the page fault.
Detailed Explanation
After the OS has handled the page fault and the correct page is now in memory, the CPU can retry the instruction that initially triggered the page fault, expecting it to complete successfully this time.
Examples & Analogies
Imagine a student who couldn’t complete a quiz because they didn’t have the right materials. Once the materials are acquired, the student can return to the quiz and continue from where they left off.
Key Concepts
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Page Fault: An event indicating required data is not in memory; leads to OS handling.
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Valid Bit: Indicates if a page is present in physical memory.
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Physical Page Frame: Memory location allocated for a page during a page fault.
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Page Table: Maps virtual addresses to physical addresses, essential for handling faults.
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Disk Operations: Include seeking and retrieving pages from secondary memory.
Examples & Applications
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.
Memory Aids
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Rhymes
If a page is not in sight, a fault will bring new light.
Stories
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.
Memory Tools
P-F-R-T: Page Fault Requires Team action: check Validity, find Frame, Retrieve, Update table, Then restart the instruction.
Acronyms
V-R-F-U-R
Validity check
Retrieve page
Find Frame
Update table
Restart the instruction.
Flash Cards
Glossary
- Page Fault
An event that occurs when a program attempts to access a page not currently mapped to physical memory.
- Valid Bit
A flag in a page table entry that indicates whether the corresponding page is present in physical memory.
- Physical Frame
A block of physical memory that can hold a page of data.
- Page Table
A data structure used by the OS to maintain the mapping between virtual addresses and physical memory.
- Secondary Memory
Non-volatile storage such as disk drives where pages are stored when they are not in physical memory.
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