Virtual Memory Lecture Notes

Introduction to Virtual Memory

• Virtual memory is a fundamental concept in computer science.
• Provides an abstraction of memory to programmers, allowing them to assume a large, infinite memory.
• Solves multiple problems: managing memory capacity issues and providing access protection and isolation.
• It hides physical memory constraints from programmers, making programming easier.

Programmer's View on Memory

• Programmers see memory as a large space they can use without worrying about physical limitations.
• Physical memory is much smaller, and virtual memory management hides this disparity.

Virtual Memory System

• Consists of virtual addresses mapped to physical addresses.
• The system maps using software and hardware, providing a seamless experience for programmers.
• Allows programs to use large addresses even when physical memory is small.

Benefits of Virtual Memory

• Automatic Memory Management: Programmers don't need to deal with physical addresses directly.
• Isolation: Processes have independent virtual address spaces.
• Flexibility: Data can be located anywhere in physical memory.
• Security: Protects memory access between different processes.

Challenges in Virtual Memory

• Page Table Size: Page tables can be large, especially with 64-bit addressing.
• Performance: Address translation can introduce delays due to additional memory access requirements.

Address Translation

• Virtual addresses are translated to physical addresses using a page table.
• A page table entry includes a valid bit, a physical page number, and other metadata.
• Multi-level page tables are used to manage size and efficiency.

Translation Lookaside Buffer (TLB)

• A cache that stores recent translations to speed up the process.
• Reduces the need for repeated memory accesses for translation.

Page Faults

• Occur when a page is not found in physical memory, requiring it to be fetched from disk.
• Page faults trigger an exception and are handled by the operating system.

Page Replacement Algorithms

• Used when physical memory is full.
• Clock algorithm is a common method for approximating least recently used (LRU) page replacement.

Memory Protection

• Ensures isolation and security between different processes.
• Access control bits in page tables prevent unauthorized memory access.

Security Concerns: Row Hammer

• A vulnerability that can be exploited to flip bits in memory, potentially bypassing memory protection.
• Demonstrates the importance of hardware reliability for security.

Modern Virtual Memory Systems

• Examples from processors such as Intel's Skylake show complex virtual memory management mechanisms.
• Use of multiple levels of TLBs, page table walkers, and page walk caches.

Conclusion

• Virtual memory provides significant benefits in abstraction and protection but comes with complexity and performance challenges.
• Future systems need to address these challenges to scale effectively.