• Addresses program generates = virtual addresses
• Addresses in memory = real or physical addresses
• Number of virtual addresses far exceed the number of real addresses
• Must mean that the real address space must be shared (reused) during the execution of a program
• Dynamic Address Translation : when virtual addresses translated to real addresses as process executes
• The concept of a virtual machine : machine larger than the real machine, sustained by the use of auxiliary storage (disks, tapes etc)
• At any given time, possible to have a diverse mix of many users programs in memory at the same time

  
Figure 2: Virtual to physical address mapping

  
Figure: Virtual to physical address mapping

• In base and bound systems, OS can relocate program in the program's back - transparency - a mechanism of indirection
• Only the Os gets to change the base and bounds
• Implementation cost : 2 registers, adder and comparator
• + : simple and fast
• - : hard to share between programs ; hard to grow address space, complex memory allocation (may need large amount of shuffling)

Solutions ?

 
• Segmentation
• Paging
• How do we replace pages or segments in cache ?
• The Working Set
• Page Fault Frequency
• Some Auxiliary Issues
• About this document ...

Back to the Contents Page