The TAG bit is an extra bit used in cache memories to flag an entry -- typically a "line" containing several words -- as having been modified -- written to. TAG bits are not needed in Write Through caches because Write Through caches always flush modified data back to memory as quickly as is feasible. TAG bits are often used with Write Back caches that do not write data back into memory until data is removed from the cache. A Write Back cache with no TAG bit must write all flushed data back to memory as it has no way to distinguish between data that has been altered and that which has not. This may affect some applications/benchmarks much more than others depending on the percentage of unnecessary writes.
TAG ram must have as many bits as the cache has "lines" to store and in some cases the TAG RAM may need to be a few nanoseconds faster than the rest of cache memory.
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Copyright 1994-2002 by Donald Kenney.