ENVS 1800 Lecture Notes - Lecture 6: Addressing Mode
14 May 2018
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ENVS 1800 Tutorial 6 Notes – Page Table Implementation
Introduction
• You should realize that data in the page table must be accessed during the fetch-
execute cycle, possibly several times
• If the fetch-execute cycle is executing an instruction with a complex addressing mode.
• Thus, it is important that the page table be accessed as quickly as possible
• Since the use of paging can negatively affect the performance of the system in a major
way otherwise.
• To improve access, many systems provide a small amount of a special type of memory
called associative memory.
• Associative memory differs from regular memory in that the addresses in associative
memory are not consecutive.
• Instead, the addresses in associative memory are assigned to each location as labels.
• When associative memory is accessed, every address is checked at the same time
• Only the location whose address label matches the address to be accessed is activated.
• Then the data at that location can be read or written.
• Cache memory lines are accessed similarly.
• A mailbox analogy might be useful in helping you to understand associative memory.
• Instead of having mailboxes that are numbered consecutively, picture mailboxes that
have those little brass inserts that you slide a paper label into.
• On each label is written the address of that particular box.
• By looking at all the boxes, you can find the one that contains your mail.
• For a human, this technique would be slower than going directly to a mailbox in a
known location.
• The computer, however, is able to look at every address label simultaneously.
• Suppose, then, that the most frequently used pages are stored in this associative
memory.
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