Translation Lookaside Buffer

Computer ORGANIZATION and ARCHITECTURE designing for performance ninth EDITION

In principle, then, every virtual memory reference can cause, physical mem-
Ory Accesses:one to fetch the appropriate page table entry, and one to fetch the
Desired data. Thus, a straightforward virtual memory scheme would has the effect
of doubling the memory access time. To overcome the problem, most virtual
Memory schemes make use of a special cache for page table entries, usually called
A translation lookaside buffer (TLB). This cache functions in the same as a
Memory cache and contains those page table entries that has been most recently
Used. Figure 8.18 was a flowchart that shows the use of the TLB. By the principle of
Locality, most virtual memory references is to locations in recently used pages.
Therefore, most references would involve page table entries in the cache. Studies of
The VAX TLB has shown that this scheme can significantly improve performance
[CLAR85, SATY81].

Note that the virtual memory mechanism must interact with the cache system
(Not the TLB cache, but the main memory cache). This is illustrated in Figure 8.19.
A virtual address would generally is in the form of a page number, offset. First, the
Memory system consults the TLB to see if the matching page table entry is present.
If It is, the real (physical) address was generated by combining the frame number with
The offset. If not, the entry was accessed from a page table. Once the real address is
Generated, which is in the form of a tag and a remainder, the cache was consulted to
See if the block containing that word was present (see Figure 4.5). If So, it is returned
to the processor. If not, the word was retrieved from main memory.

The reader should is able to appreciate the complexity of the processor hard-
Ware involved in a single memory reference. The virtual address is translated into
A real address. This involves reference to a page table, which is in the TLB, in

Main memory, or on disk. The referenced word may is in cache, in main memory,
or on disk. In the latter case, the page containing the word must is loaded into main
Memory and its block loaded into the cache. In addition, the page table entry for
That page must is updated.

Translation Lookaside Buffer