Lecture 10 - Virtual Memory

How many bits are used for offset in a 4096-byte page?

12 bits (log2(4096)) are used for the offset within a page.

How does a 64-bit system handle virtual addresses?

A 64-bit system typically uses 48 bits for addresses, with 36 bits used for the Virtual Page Number (VPN) and 12 bits for the page offset.

What is a Page Table Entry (PTE)?

A Page Table Entry (PTE) contains information about the physical location of a virtual page, whether it's in memory or on disk, and status bits like the valid bit.

What is a TLB miss, and how is it resolved?

A TLB miss occurs when the requested virtual-to-physical address translation is not found in the TLB. The system then looks up the page table to find the correct mapping, which takes more time.

What is the difference between a Cache Miss and a Page Fault?

A cache miss occurs when data is not found in the CPU cache but can be retrieved from memory. A page fault occurs when data is not found in memory and must be retrieved from disk.

What is a context switch, and how does it affect memory access?

A context switch is when the operating system switches between different processes. It involves switching page tables and may require flushing the cache and TLB, slowing down memory access temporarily.

What is the difference between a hard page fault and a soft page fault?

A hard page fault occurs when the requested page is not in memory and must be loaded from disk, while a soft page fault happens when the page is found elsewhere in memory (such as in a cache).

What is a page fault, and how does the operating system handle it?

A page fault occurs when a process accesses a page that is not in physical memory. The OS handles it by loading the page from disk into memory and updating the page table.

What is a Page Fault?

A page fault occurs when a program accesses a page that is not currently in RAM, causing the operating system to load the page from disk. Page faults are expensive in terms of time.

What is a Page Table?

A page table is a data structure used by the operating system to store the mapping between virtual addresses and physical memory locations (frames). Each process has its own page table.

What is the purpose of a swap file or swap partition?

A swap file or partition is used to extend the amount of available memory by storing pages that cannot fit in physical RAM, allowing them to be loaded back into memory when needed.

What is the purpose of a swap file or partition?

A swap file or partition is used to store pages that are not currently in RAM, allowing the system to free up physical memory and load the pages back when needed.

What is Controlled Sharing in Virtual Memory?

Controlled sharing allows multiple processes to share certain pages, such as library code, while keeping other parts of memory isolated.

What is Copy-on-Write (CoW) in the context of fork()?

Copy-on-Write allows the parent and child processes to share memory pages after a fork() until one of them modifies a page, at which point a copy of the page is made.

What is Copy-on-Write (CoW)?

Copy-on-Write is an optimization where processes initially share memory pages and only get their own copies when they modify a page, reducing memory usage until writes occur.

What is Demand Paging?

Demand Paging is a virtual memory system where pages are stored in disk (swap) and only loaded into RAM when needed, allowing processes to use more memory than is physically available.

What happens during a TLB miss?

During a TLB miss, the system must look up the virtual-to-physical mapping in the page table, which takes more time than a TLB hit.

How does Context Switching impact memory?

During context switching, the current process's page table is replaced with the next process's page table, and the TLB and cache may be flushed, leading to performance overhead.

What is External Fragmentation?

External fragmentation occurs when free memory is scattered in small, non-contiguous blocks, making it difficult to allocate large continuous blocks of memory.

Why do we use multi-level page tables?

Multi-level page tables are used to reduce memory overhead by dividing the large page table into smaller tables, improving memory efficiency.

What is the role of the Page Table Base Register (PTBR)?

PTBR stores the starting address of a program's page table in memory and is managed by the operating system, not visible to the user.

What are Signals in an Operating System?

Signals are software notifications of events (like interrupts). They can terminate a process, pause/resume execution, or be ignored. Signals are asynchronous, and processes can block or handle them.

What is Swapping in modern systems?

Swapping moves memory pages that are not currently needed by processes from RAM to disk, freeing up space for other pages, though modern systems do this less frequently due to large RAM capacities.

What is a Memory Management Unit (MMU)?

The MMU is hardware that maps virtual addresses to physical addresses. It performs translations on-the-fly as addresses are accessed by the CPU.

How does the Memory Management Unit (MMU) work?

The MMU translates virtual addresses into physical addresses by looking up the page table, and sends the physical address to memory hardware for data access.

How does the OS decide which pages to swap out during paging?

The OS uses a page replacement policy, such as Least Recently Used (LRU), to decide which pages to swap out when memory is full and a new page needs to be loaded.

What is a Translation Lookaside Buffer (TLB)?

The TLB is a cache that stores recent mappings of virtual addresses to physical addresses to speed up memory access and avoid repeated page table lookups.

What is the purpose of the fork() system call?

The fork() system call creates a new process by duplicating the calling process. The child process is an exact copy of the parent process, sharing memory until one process modifies it.

What is mmap() used for?

The mmap() system call allows a program to map a file or device into memory, making file I/O more efficient by allowing direct access to file contents via memory addresses.

What is the purpose of the page replacement policy?

The page replacement policy determines which page to evict from memory when new pages need to be loaded, typically based on usage patterns like Least Recently Used (LRU).

What is the role of the sigprocmask() function?

The sigprocmask() function is used to block or unblock certain signals, preventing them from being delivered to a process until it's ready to handle them.

What is the purpose of the valid bit in demand paging?

The valid bit in demand paging indicates whether the page is currently loaded into memory (valid) or needs to be fetched from disk (invalid).

What is the significance of the "valid bit" in a page table entry?

The valid bit indicates whether a page is currently loaded in memory. If it's not, a page fault will occur.

What is the role of the valid bit in a Page Table Entry?

The valid bit indicates whether a page is in RAM (1) or on disk (0). If a page is on disk, the entry provides the disk location.

What is Virtual Memory, and why is it used?

Virtual Memory allows programs to use more memory than is physically available by abstracting physical memory and enabling features like isolation and memory protection. It addresses issues like fragmentation and provides flexibility.

What is the advantage of Virtual Memory?

Virtual Memory provides isolation between processes, prevents fragmentation, allows for efficient use of memory, and enables the illusion of more memory than physically available.

What happens when a signal is delivered to a process?

When a signal is delivered, the process may terminate, pause, ignore the signal, or handle it with a custom signal handler, depending on the signal type and process setup.