• Virtual Memory
• Paging
• Page request

• Comes from the basis that all of a process’ address space is not needed at once
• Thus, chop up the address space into smaller parts and only load the parts that are needed
• These parts need not be contiguous in memory!

• Relation beteen virtual addresses and physical memory addresses given by page table.

• Virtual memory is used in most modern operating systems:

  • Windows uses one or more “page files” to swap pages
  • Linux uses a hard disk partition (“swap partition”) to swap to.

• Pros/Cons

  • Since only the necessary parts of the process are loaded, processes load faster and it allows much better memory utilization
  • Needs lots of extra hardware to accomplish the job (efficiently)
  • In some cases too much paging (i.e. “thrashing”) can occur, which is very slow

• Virtual Memory
• Paging
• Page request

• Address relocation
• Page table entry

• The internal operation of the MMU with 16 4-KB pages.

Translate Logical Address (LA) to Physical Address (PA)

• Step 1: Page size 2^n Byte => LA offset n bits
  • Page size 4KB = 2^12 B => offset 12 bits
• Step 2: LA = page number + offset
  • LA 32 bits , offset 12 bits => page number 20 bits
• Step 3: look up the page table , page number is the index, if the Present bit is 1, get the page frame number, else page fault.
• Step 4: PA = page frame number + offset

• A typical page table entry.

• Address relocation
• Page table entry

• Speeding Up Paging
• Multilevel Page Tables
• Inverted Page Tables

• The mapping from virtual address to physical address must be fast.

• A TLB (Translation Lookaside Buffers) to speed up paging.

• Comparison of a traditional page table with an inverted page table.

• Speeding Up Paging
• Multilevel Page Tables
• Inverted Page Tables

• Chapter 3: Memory management, Modern Operating Systems . Forth Edition, Andrew S. Tanenbaum