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APPENDIX 4A 157

A cache in the range of 1K to 128K words is generally adequate, whereas main
memory is now typically in the gigabyte range. When we consider virtual memory
and disk cache, we will cite other studies that confirm the same phenomenon,
namely that a relatively small M1 yields a high value of hit ratio because of locality.
This brings us to the last question listed earlier: Does the relative size of the
two memories satisfy the cost requirement? The answer is clearly yes. If we need
only a relatively small upper-level memory to achieve good performance, then the
average cost per bit of the two levels of memory will approach that of the cheaper
lower-level memory.
Please note that with L2 cache, or even L2 and L3 caches, involved, analysis is
much more complex. See [PEIR99] and [HAND98] for discussions.