This course is run at the The University of Nottingham within the School of Computer Science & IT. The course is run by Graham Kendall (EMAIL : gxk@cs.nott.ac.uk)
It is interesting to look at how the MMU works. In particular, to consider why we have chosen to use a page size that is a power of 2. Take a look at this diagram.
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The incoming address (20818) consists of 16 bits. The top four bits are masked off and make an entry into the virtual page table (in this case it provides an index to entry 5 (101 in binary) and finds that this page is mapped to physical page 011 (3 in decimal). These three bits make up the top three bits of the physical page address.
The other part of the incoming address is copied directly to the outgoing address.
Thus, the page table (courtesy of the MMU) has mapped virtual address 20818
to physical address 12626.
If you look at the diagram that shows how virtual memory operates you will be able to follow this conversion.
The only other point we should, perhaps, consider, is why the first twelve bits of the incoming address can be copied directly to the output address. See if you can work it out before looking at the answer at the end of this section (question 2).
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Last Updated : 23/01/2002