Virtual Memory Overview
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What type of addressing do most modern machines use?

  • Indexed addressing
  • Word addressing
  • Byte addressing (correct)
  • Segment addressing

    • What is the function of the MMU in address translation?

  • It converts virtual addresses to physical addresses. (correct)
  • It generates random addresses for virtual space.
  • It allocates memory to programs.
  • It removes the need for a physical address space.

    • Which architecture uses paging in its virtual memory model?

  • ARM architecture
  • x86-32 architecture
  • x86-64 architecture (correct)
  • PowerPC architecture

    • In a word-addressed machine, how is address 0x1 interpreted?

    <p>As the second word in memory</p> Signup and view all the answers

    What does paging in virtual memory involve?

    <p>Breaking memory into fixed-sized pages.</p> Signup and view all the answers

    What is the initial step the kernel takes when handling a page fault?

    <p>Find an unused physical page</p> Signup and view all the answers

    How does demand paging improve the loading of executable files?

    <p>By marking pages as not present but backed by the file</p> Signup and view all the answers

    What happens when brk() or sbrk() is called in a process?

    <p>The metadata for page tables is updated only</p> Signup and view all the answers

    What is a key benefit of demand paging related to memory allocation?

    <p>It allows faster and more efficient memory updates</p> Signup and view all the answers

    Which of the following statements about unused pages in demand paging is true?

    <p>Unused pages are never loaded into memory</p> Signup and view all the answers

    What happens when a process attempts to access a page that is unmapped?

    <p>The MMU notifies the kernel to allocate a page.</p> Signup and view all the answers

    How does the kernel manage the program stack during process creation?

    <p>It marks required stack pages as not present and allocates them on demand.</p> Signup and view all the answers

    What occurs when a page is evicted due to low memory on the system?

    <p>The page is removed from the map without writing data.</p> Signup and view all the answers

    What role does the Memory Management Unit (MMU) play in managing the program stack?

    <p>It configures the kernel settings to handle the stack.</p> Signup and view all the answers

    What does it mean when a stack page is marked as 'not present'?

    <p>The page is not available but can be accessed if needed.</p> Signup and view all the answers

    During the growing of the program stack, what triggers the allocation of new pages?

    <p>Page faults as stack memory is accessed.</p> Signup and view all the answers

    Which method describes the process of removing a dirty page from memory?

    <p>Writing the page back to its backing storage before removal.</p> Signup and view all the answers

    What is the primary function of the process’s access to the page after a page fault has occurred?

    <p>It continues to access the page as if it had always been mapped.</p> Signup and view all the answers

    What action does the MMU take when a process tries to use a new page?

    <p>The MMU notifies the processor.</p> Signup and view all the answers

    What step follows after the MMU notifies the processor about a new page request?

    <p>The kernel finds an unused page.</p> Signup and view all the answers

    What must the kernel do before inserting a new page into the process's page tables?

    <p>Clear the unused page.</p> Signup and view all the answers

    What function is called by the program to adjust the program break?

    <p>sbrk(PAGE_SIZE)</p> Signup and view all the answers

    What happens to the new page after the break is moved?

    <p>The page is marked as not present.</p> Signup and view all the answers

    What does the kernel do with the unused page during the page allocation process?

    <p>It clears the unused page.</p> Signup and view all the answers

    In the context of page faults, what does 'unmapped' signify?

    <p>The page is not allocated to any process.</p> Signup and view all the answers

    Regarding page tables, what role do they play in memory management?

    <p>They track the mapping between virtual pages and physical memory.</p> Signup and view all the answers

    What is required for managing translation mappings effectively?

    <p>Tight integration between the kernel and hardware</p> Signup and view all the answers

    What defines an address space in computing?

    <p>A range of potentially valid memory locations</p> Signup and view all the answers

    How do linear address spaces differ from segmented address spaces?

    <p>Linear address spaces map to contiguous blocks of memory with unique addresses</p> Signup and view all the answers

    What is a characteristic of segmented address spaces?

    <p>Addresses consist of a segment identifier and an offset within that segment</p> Signup and view all the answers

    Which of the following statements about virtual address spaces is true?

    <p>They are restricted to 48 bits in current x86-64 processors</p> Signup and view all the answers

    Which of the following best describes a linear address?

    <p>It represents a single address in a contiguous block of memory</p> Signup and view all the answers

    What can be said about the relationship between virtual and physical addresses?

    <p>They can reside in completely different address spaces</p> Signup and view all the answers

    In which instance would a segmented address space typically be used?

    <p>In older systems or some modern systems for better memory organization</p> Signup and view all the answers

    What is the primary function of a memory management unit in a virtual memory system?

    <p>To translate virtual addresses to physical addresses</p> Signup and view all the answers

    What happens during a page fault in a paged system?

    <p>A mapped but unallocated page is brought into memory</p> Signup and view all the answers

    How does page eviction help simulate extra memory?

    <p>By writing evicted pages to disk storage</p> Signup and view all the answers

    In a paged system, what role do page tables play?

    <p>They allow the mapping of virtual to physical addresses</p> Signup and view all the answers

    Which of the following statements about virtual memory is false?

    <p>Virtual memory uses physical memory exclusively without paging</p> Signup and view all the answers

    What is a key characteristic of how pages are handled in a paged system?

    <p>Pages can be mapped in any order between virtual and physical addresses</p> Signup and view all the answers

    What occurs after a page is evicted in the paging process?

    <p>The evicted page is remapped to a new location</p> Signup and view all the answers

    Which of the following best describes paging in the context of virtual memory?

    <p>Paging breaks memory into fixed-size units called pages</p> Signup and view all the answers

    Study Notes

    Virtual Memory Overview

    • Virtual memory is a system mechanism that separates program address space from physical memory layout.
    • Virtual addresses represent locations within a program's address space.
    • Physical addresses represent locations in actual hardware RAM.
    • Virtual memory allows these two address spaces to be different.

    Process Layout

    • Each process has unmapped memory near NULL.
    • Processes can potentially access the entire address space.
    • Each process is denied access to memory used by other processes.
    • Virtual memory resolves the apparent contradiction.
    • Every address in a process's address space is a virtual address.

    Physical Layout

    • Hardware RAM layout varies significantly between machines and platforms.
    • Certain memory locations may be restricted.
    • Devices may appear within the memory address space.
    • Different RAM amounts may be present.
    • Historically, programs needed to be aware of these restrictions.
    • Today, virtual memory hides these details from programs.
    • The kernel must still be aware of the physical layout.

    Memory Management Unit (MMU)

    • The MMU translates addresses.
    • It uses a per-process mapping structure to change virtual addresses to physical addresses.
    • The MMU is hardware between the CPU and memory bus.
    • Address translation needs to be very fast, but occasional performance penalties are possible.
    • Kernel and hardware need to tightly integrate for translation management.

    Paging

    • Multiple virtual memory models are possible.
    • x86-64 architectures support several models.
    • Linux on x86-64 uses paging.
    • Paging divides memory into fixed-sized pages.
    • Multiple page sizes might exist in a system.
    • Page sizes are typically powers of two.
    • x86-64 small pages are 4 KB.

    Page Metadata

    • Page table metadata describes page features like readability/writability, code execution permission, and presence in memory.
    • A page fault occurs when a memory access violates this metadata (e.g., write to a non-writable page).
    • The MMU notifies the processor.
    • The processor jumps to a specific kernel routine for processing.
    • The kernel tries fixing the problem or informing the offending process.

    Page Backing

    • Virtual pages can use files, physical pages, or both for backing.
    • A backed page's contents depend on its backing.
    • Backed pages don't always need to be in memory.
    • Clean pages are identical to their backing; dirty pages are different.
    • Clean pages can be recreated from their backing at any time.

    Demand Paging

    • A virtual page might have backing but not be present in memory.
    • Such pages are marked "not present" in page tables.
    • Attempts to access these pages notify the kernel. (This is a page fault.)
    • The kernel pages in the page by: finding an unused physical page, locating the virtual page's backing, reading the backing data into the physical page.

    Demand Paging Benefits

    • Physical memory allocation is quicker, simply by updating page tables.
    • Executable file loading is faster; pages are marked present but backed by files.
    • Accessing these pages causes the file to be read into memory.
    • Unused pages are never loaded.

    Program Break

    • Calling brk() or sbrk() changes a process's memory map.
    • Pages adjacent to the old program break are marked as "Not present, Readable and writable".
    • This only affects page table metadata; the pages themselves aren't allocated.
    • On a new page request, the MMU notifies the processor.
    • The kernel locates and clears an unused page.
    • The kernel adds the new page to the process's page table.

    Moving the Program Break (Page Fault)

    • The process's access to new pages is handled normally after the program break is moved.
    • The MMU notifies the kernel when the process tries to access the new page.
    • The kernel allocates the requested page.

    C Stack

    • The kernel manages the program stack.
    • The stack grows as needed (not explicitly sized by the program).
    • The kernel configures the MMU to manage the program stack.
    • Similar to newly allocated memory, the kernel determines the stack size.
    • Stack pages are marked "not present" but "readable and writable"

    Page Eviction

    • When memory is low, the system can evict a page.
    • Clean pages are removed from the map.
    • Dirty pages are written to their backing before removal.
    • Swap space can back un-backed dirty pages.
    • This allows the system to simulate more memory than physically available.

    Address Spaces Summary

    • Virtual memory uses a memory management unit.
    • The CPU operates in a virtual address space separate from physical memory.
    • The MMU translates virtual addresses to physical addresses.
    • Paging divides both virtual and physical address spaces into pages.
    • Pages can be individually mapped between virtual and physical addresses.
    • Page tables allow the MMU to translate addresses.
    • Page faults handle situations where mapped pages may not be currently allocated in physical memory.

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    Virtual Memory PDF - CSE 220 Systems Programming

    Description

    This quiz explores the concepts of virtual memory, focusing on the distinction between virtual and physical addresses. It covers the layout of processes and hardware RAM, including access restrictions and the importance of virtual memory in modern programming. Test your understanding of how virtual memory manages address spaces.

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