Next: Segmentation
- 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
![figure27](img1.gif)
Figure 2: Virtual to physical address mapping
![figure32](img2.gif)
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 ?
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Omer F Rana
Thu Feb 13 12:31:55 GMT 1997