📗 Virtual Memory and Multi-Programming

🎤 Vocabulary

Virtual Memory: An abstraction layer for memory management, allowing each process to believe it has access to the entire memory space.
Page Table: A data structure used by the operating system to map virtual addresses to physical addresses.
Page Frame: A fixed-length contiguous block of physical memory.
Page Fault: An event that occurs when a requested page is not in memory, triggering a fetch from disk.
Translation Lookaside Buffer (TLB): A cache that stores recent translations from virtual memory to physical memory addresses.
Demand Paging: A memory management scheme that loads pages into memory only as needed.

The lecture focuses on how virtual memory and multi-programming work together to optimize CPU utilization and memory management.
Understanding virtual memory is crucial because it enables efficient multi-tasking, allowing multiple processes to run concurrently without interfering with each other’s memory.

CPU & Digital Design: Fast but limited by slower I/O operations.
Multi-Programming: Allows multiple processes to utilize the CPU effectively by switching between them when one is idle.
Page Tables:
- Virtual Address (VA) -> Physical Address (PA) mapping.
- Page Size: Determined by the frame size, e.g., 1K, 2K.
- Frame Number: Identifies where a page resides in physical memory.

Page Table Structure:
- Consists of entries mapping virtual pages to physical frames.
- Each process has its own page table.
Page Fault Handling:
- On a miss, the OS loads the required page from disk (swap space) into memory.
- Updates the page table to reflect the new frame location.

Page Table Size: Can exceed physical memory size, leading to innovative solutions like multi-level page tables.
TLB Usage: Reduces lookup time by caching recent translations, improving access times significantly.
Cache and Memory Hierarchy: Balancing speed and size in cache design affects system performance.

Potential Topics:
- How virtual addresses are translated to physical addresses.
- The role and management of page tables and frames.
- Implications of TLB misses and page faults.
Common Misunderstandings:
- Confusing virtual and physical memory addresses.
- Misinterpreting the purpose of page tables and TLB.

Operating Systems: Core to memory management in all modern OSs.
Performance Optimization: Methods like TLB and caching are central to system performance tuning.
Embedded Systems: Use simplified versions of virtual memory for efficient resource management.

Exercises:
- Calculate page table size given virtual address space and page size.
- Diagram the process of handling a page fault.
- Simulate TLB hits and misses using a trace of memory accesses.
Resources:
- Review chapters on memory management in OS textbooks.
- Watch tutorials on virtual memory implementations in Unix/Linux systems.