Mechanism: Address Translation

Introduction to Virtualization

• Limited Direct Execution (LDE):
  • Programs primarily run directly on hardware.
  • OS intervenes at critical points (e.g., system calls, timer interrupts).
  • Aims to balance efficiency with control over hardware.
• Virtual Memory Strategy:
  • Similar approach to CPU.
  • Hardware support is crucial for efficiency (e.g., TLBs, page tables).
  • Ensures applications only access their own memory, providing protection.
  • Flexibility for programs to use address spaces optimally.

Address Translation

• Hardware-Based Address Translation:
  • Hardware transforms virtual addresses to physical ones.
  • OS manages memory, setting up correct translations.
  • Goal: Illusion of private memory despite shared physical memory.
• Interposition:
  • Technique where hardware intervenes in memory access.
  • Provides transparency without changing client interface.

Assumptions in Virtualizing Memory

• Address space must be contiguous and smaller than physical memory.
• Each address space is assumed to be the same size.

Example of Address Translation

• Address translation transforms virtual addresses to physical memory locations.
• Example code sequence demonstrates loading and storing values in memory.
• Physical memory layout example shows process relocation.

Dynamic (Hardware-based) Relocation

• Base and Bounds Registers:
  • CPU uses these for memory address translation.
  • Base register sets starting physical memory address.
  • Bounds register ensures memory references are within legal limits.
• Address Translation Process:
  • Adds base register value to virtual address to get physical address.
  • Uses bounds for protection against illegal memory access.

Hardware Support

• Requirements:
  • Privileged CPU modes.
  • Base and bounds registers as part of the Memory Management Unit (MMU).
  • Privileged instructions to set register values and handle exceptions.

Operating System Issues

• Responsibilities:
  • Memory management and allocation upon process creation.
  • Base/bounds management during context switches.
  • Exception handling for illegal memory accesses.
• Dynamic Relocation Challenges:
  • Efficient memory utilization, avoiding internal fragmentation.

Summary

• Address translation allows for efficient memory virtualization.
• Base-and-bounds system provides quick translations and protects processes.
• Provides the illusion of continuous memory while sharing physical memory resources.

References

• Various papers and resources detailing the history and techniques of dynamic and static relocation.

Homework (Simulation)

• Experiment with relocation.py to understand address translation using base and bounds registers.
• Various simulation questions to explore bounds, base settings, and memory allocations.