Virtual Address Space and Physical Address Space

Virtual Address Space

• virtual address spaces are shared by all threads in the process, but protected from other processes
• process = program = thread + virtual address space
• thread = register values + stack
• because they are logical, they can be up to $2^{64}$ bits for 64 bit system, approximately 16 million terabytes.

Physical Address Space

• if the memory is big enough, the OS uses the physical memory (RAM)
• else, least-recently-used data is dumped to the disk
• only OS can access PAS directly!

VAS and PAS

• broken into "pages", usually 4 KB. Most pages will not be used.
• broken into page-sized block called frames.
• virtual page can be unallocated, uncached (allocated in disk), cached (allocated in memory).
• If we access a page not in physical memory, HW generates a page fault exception. Then the OS will bring in the page to physical memory (possibly evicting another page)
  • page size should be fairly large
  • pages in main memory should be fully-associative to reduce conflicts and maximize page hits
  • use OS instead of HW
  • write-back policy