2015-09-06
184
Translation Look-Aside Buffers:
The following show the use of a translation look-aside buffer.
The Translation Look-Aside Buffer (TLB) is a small amount of associative memory (16, 32 or 64 words). This memory very fast, very expensive and resides on the CPU chip so that no main memory references are needed to access its contents.
The left column of the associative memory is the key value. It holds the virtual page number.
The right column is the value of the associated memory cell and holds the physical page frame that contains the virtual page.
When the associative memory is presented with a key value it looks at all of the cells in the memory simultaneously for the key value.
If the key value is found the value in the associated memory cell is output.
When a memory request is made the page indices are sent simultaneously to the page translation system (could be a single level or a multi-level paging system) and to the Translation Look-Aside buffer (TLB).
The paging system begins its look up process.
The TLB also searches for the page index in its associative memory.
If the page index is found in the TLB then the physical page frame that is returned by the TLB is used.
When the page frame is found in the TLB the lookup by the paging system is canceled.
If the page index is not found in the TLB then the paging system continues its lookup process and finds the page frame in the same way that was discussed previously.
If the page index was not in the TLB it and the associated page frame are put into the TLB after the paging system finishes the lookup.
This may involve removing one of the page indices that is currently in the TLB to make room for the new one.
If a page index is removed from the TLB it might be the one that has not been used for the longest period of time. (There are lots of different algorithms for picking the page index to remove - these are very similar to the page replacement algorithms that we will see in the next class.)
