Virtual memory #

How to share one set of memory between multiple processes?

Goals:

• Multitasking: memory shared between processes running at the same time
• Transparency: user processes should not have awareness of shared memory
  • Process should see only its own private pool of memory
• Isolation: one process can’t corrupt another
• Efficiency: don’t want to add extra runtime to OS code or complexity to use code

Load-time relocation #

When loading process,

• Find memory region (using first-fit or best-fit)
• Relocate program
  • Find all pointers in the program (linker identifies)
  • Add base
• Problems:
  • Fragmentation
  • Process’s memory fixed at load time
  • No isolation/protection
  • Relocation can be slow
  • Can’t move process once it starts executing