The management of cache memory is challenging, because the system must keep the contents of the cache
memory synchronized with the contents of main memory. Engineers call this cache ???coherency.??? As long as the
program is reading from memory, but not writing, there is no problem. When the program writes to memory,
however, both main memory and cache must be updated.
Also, when the program begins to access a new area of memory, one for which the contents are not already
reflected in the cache, the cache management algorithm will typically bring to the cache the needed word as
well as a number of following words from memory. At the same time, the cache management algorithm must
decide which contents of the current cache to discard. As complex as this management is, use of cache memory
usually makes a very noticeable difference in performance, with speedup of average memory access often in the
neighborhood of 50 percent.
INPUT AND OUTPUT (I/O)
Obviously, most data on which we compute resides outside of the computer itself; perhaps it??™s originally
on paper receipts, or in lists on paper. And when computation is complete, we want to see the results outside of
the computer??™s own memory; on a display, or on paper, for example.
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