Dynamic Address Translation #

Goal: Make a process think it has its own memory space independent from all other processes

• Process’s memory would start at 0

Hardware has

• CPU core
• Memory management unit
  • Converts virtual addresses (process memory space) to physical addresses (actual hardware memory address)
• Memory

Base and Bound #

• Virtual address space goes from 0 to Bound
• Physical address for same process goes from Base to Base+Bound

Properties:

• Private memory per process
• Memory isolation
• Base and bound saved in PCB, set on context switch
• No load-time relocation

Notes:

• OS-level tasks have the MMU disabled
• User processes can’t see base or bound
  • Processor Status (PS) bit: kernel vs user

How does OS regain control? Special instructions (traps)

• Branch into OS code (address in special memory location)
  • Save instruction pointer, PS bit
• Reset mode to kernel
• Return from trap:
  • Jump back to user
  • Reset PS bit

Advantages:

• Cheap
• Efficient
• Swapping to disk possible

Disadvantages:

• Fragmentation
  • Growing memory for process difficult/expensive
• One contiguous region per process: sharing impossible
• Limited stack growth
• Can’t have read-only code