Virtual Memory

• Demand paging

—   Do not require all pages of a process in memory

—   Bring in pages as required

• Page fault

—   Required page is not in memory

—   Operating System must swap in required page

—   May need to swap out a page to make space

—   Select page to throw out based on recent history

Thrashing

• Too many processes in too little memory


• Operating System spends all its time swapping


• Little or no real work is done


• Disk light is on all the time

•  Solutions

—   Good page replacement algorithms

—   Reduce number of processes running

—   Fit more memory

Page Table Structure

Translation Lookaside Buffer

• Every virtual memory reference causes two physical memory access

—   Fetch page table entry

—   Fetch data

• Use special cache for page table

—   TLB

TLB Operation

Segmentation

• Paging is not (usually) visible to the programmer


• Segmentation is visible to the programmer


• Usually different segments allocated to program and data


• May be a number of program and data segments

Advantages of Segmentation

• Simplifies handling of growing data structures


• Allows programs to be altered and recompiled independently, without re-linking and re-loading


• Lends itself to sharing among processes


• Lends itself to protection


• Some systems combine segmentation with paging