Possible and Impossible Cases
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Understanding Page Faults
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Today, we're focusing on page faults, which occur when a program tries to access data that isn’t currently in memory. Can anyone tell me what happens during a page fault?
Does the operating system get involved when a page fault happens?
Exactly! The OS must first check if the reference is valid. Can anyone explain what a valid reference means?
A valid reference occurs when the page exists within the virtual address space of the process.
Correct! If the reference is invalid, the system aborts the operation. Now, what do we do if the page is valid but not in memory?
Then the OS has to bring the page in from secondary memory.
Right! This leads to a disk operation to fetch the page. Remember, we can think of this process with the acronym **RESUME**: Retrieve, Examine, Swap, Update, Memory access, Execute.
That’s a useful way to remember the steps! So it manages memory dynamically, right?
Great question! Yes, it manages memory dynamically, adjusting as needed for efficient access.
Valid versus Invalid References
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Next, let’s discuss valid versus invalid references further. How do we confirm if a reference is valid?
The page table entry helps determine if the address is valid or not.
That’s right! What happens to the virtual address if it’s marked as valid but the physical page is missing?
The OS will it retrieve from disk to resolve that issue.
Perfect! Can you give an example where an invalid reference might arise?
If a program tries to access memory outside its allocated space.
Exactly! This results in a program aborting. Keep in mind the mnemonic **FALL**: Fault, Abort, Legitimacy, Load.
The Role of the Operating System
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Let’s elaborate on the OS role during page faults. What’s the first step the OS takes when it receives a page fault signal?
It checks the type of trap it received.
Correct! Once it knows it's a page fault, what does the OS do next?
It finds a physical page frame in memory.
Exactly. After locating a frame, what’s the next step?
The information is fetched from secondary memory.
Great! Then it updates the page table. Can anyone describe what it updates?
It updates the physical page number and sets the valid bit to '1'.
Nicely done! Remember the acronym **PHASE**: Physical frame, Hit the disk, Acquire, Set valid, Execute to recall these steps.
Introduction & Overview
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Quick Overview
Standard
In this section, we investigate the mechanics of page faults. The discussion includes the conditions triggering a page fault, the distinction between valid and invalid references, and the necessary actions the operating system takes to resolve these faults, including page retrieval processes from secondary memory.
Detailed
Detailed Summary
In this section, we take a focused look at page faults, which occur when a program attempts to access data not currently loaded in physical memory. The initial step upon encountering a page fault is to check the validity of the reference by examining the page table entry for the virtual address. If the valid bit of the page is 0, it indicates that the page is not currently in memory.
To process this, the operating system must first determine whether the faulting reference is a valid one. If it’s deemed invalid (the virtual address isn’t part of the process's address space), the system will abort the operation. Conversely, if the page is simply absent (valid bit is 0), the system will initiate steps to retrieve the page from secondary storage.
The OS finds a free physical page frame in memory to swap in the required page via a scheduled disk operation. This includes querying the page table, which holds both the physical memory mapping and the necessary disk location where the page resides, allowing it to retrieve the correct data.
Once the data is secured and brought into the physical memory, the operating system updates the page table to mark the page as valid and restarts the instruction that triggered the fault. The section concludes with a discussion of real architectural implementations of the translation lookaside buffer (TLB) and cache interactions, including the conditions under which TLB hits and misses can occur.
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Understanding Page Faults
Chapter 1 of 5
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Chapter Content
During a page fault when I do not have the required data in memory, I incur a page fault. The page table entry corresponding to the page I want to access shows that it is invalid. This means the valid bit is 0, indicating that the corresponding physical page number is not mapped to the page table entry.
Detailed Explanation
A page fault occurs when the operating system accesses a part of memory that is not currently in RAM. The page table, which tracks where data is stored, indicates that the desired data isn't available by showing a valid bit of 0. This indicates that the data must be loaded from disk or secondary storage into RAM.
Examples & Analogies
Think of it like searching for a book in a library. If the book is on the shelf, you find it easily. But if it's checked out or misplaced, the library system tells you it's not available, and you’ll need to wait for it to be returned before accessing it.
Valid vs. Invalid References
Chapter 2 of 5
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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, that indicates the virtual address is not part of the virtual address space for the process itself. If the valid bit is 0 and the reference is valid, then the page is just not in memory and it has to be brought from disk.
Detailed Explanation
When a page fault occurs, the operating system must determine if the request is valid. If it isn't a valid request (the data isn't part of the process's address space), the system will abort the operation. If it is valid but still not in memory, it means the system will have to retrieve the data from storage.
Examples & Analogies
Imagine you're ordering a pizza. If you order a pizza that doesn't exist on the menu (invalid reference), the restaurant will let you know they can't fulfill that order. However, if you order something on the menu that they just don't have in stock currently (valid reference), they will go and fetch it from the storage.
Handling Page Faults
Chapter 3 of 5
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Chapter Content
If a valid reference is made and it is a page fault, the operating system must find a physical page frame in memory to load the page into. It may involve replacing an existing page that is currently in memory and swapping the needed page from the disk to RAM.
Detailed Explanation
When a valid reference results in a page fault, the operating system goes through a series of steps. It finds a spot in RAM to put the new data. If RAM is full, it may need to remove some existing data (page replacement) to make room for the new information being fetched from disk.
Examples & Analogies
This is similar to when your backpack is full, and you need to take something out to put in a new book you just bought. You can't carry everything at once, so you choose to remove something that you don’t need as much.
Updating Page Tables
Chapter 4 of 5
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Chapter Content
Once the desired page is loaded into memory, the OS must update the page table to indicate the new page's physical address and set the valid bit to 1, showing that the page is now in memory.
Detailed Explanation
After the data is successfully loaded into RAM, the system finishes by updating the page table. This update gives the address of the newly loaded page in RAM and changes its status from invalid to valid, showing it is now ready to be used in processing.
Examples & Analogies
Consider this like updating your calendar. Once an important meeting is confirmed and added to your schedule, you mark it down as confirmed. Before it was just a tentative note that might not happen, but now it's official.
Restarting Instructions
Chapter 5 of 5
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Chapter Content
After updating the page table, the instruction that caused the page fault can be restarted, and the data can be accessed from physical memory.
Detailed Explanation
Once everything is in place and updated, the system goes back to the instruction that originally triggered the page fault. With the needed data now in memory, the instruction can be processed without any further interruptions.
Examples & Analogies
This resembles a situation where you stop to refill your gas tank while on a road trip. Once you're done filling up, you can get back on the road and continue your journey without any other delays.
Key Concepts
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Page Fault: A critical event when a program tries to access data not loaded into memory.
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Valid vs. Invalid References: The difference between references that are accessed correctly versus those that are not due to being outside the process's scope.
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Role of the OS: The process that the operating system follows to handle page faults effectively.
Examples & Applications
A program encounters a page fault when it tries to access a memory location it hasn't loaded yet, triggering the OS.
An invalid reference occurs if a process attempts to access memory location assigned to another process.
Memory Aids
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Rhymes
When data is missed, and faults appear, the OS will check, then have no fear. Valid or not, it knows what's best, to fetch the data and give you rest.
Stories
Once, in a digital kingdom, a process tried to access a book (data) that was stored far away (in disk memory). The wise OS, serving as the librarian, understood the dilemma. It quickly checked if the book was valid. Upon finding it missing, the librarian sped off to fetch it, ensuring the process could continue reading again.
Memory Tools
V F R L: Validate the Fault, Retrieve it Later. Each step is crucial when we manage a page fault!
Acronyms
RESUME
Retrieve
Examine
Swap
Update
Memory access
Execute.
Flash Cards
Glossary
- Page Fault
An interruption that occurs when a program tries to access data that is not currently in physical memory.
- Valid Reference
A reference to a virtual address that exists within the address space of the process.
- Invalid Reference
A reference to a virtual address that does not exist within the address space of the process.
- Page Table
A data structure used by the OS to keep track of virtual memory mapping to physical memory.
- Disk Operation
The process of retrieving data from secondary storage to bring it into main memory.
Reference links
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