15 Discuss the concept of a ???memory hierarchy??? whereby memory closer to the CPU is faster, more
expensive, and smaller than memory at the next level. Arrange the different types of memory we have
discussed in such a hierarchy.
The smallest, fastest, most expensive memory locations are the registers on the CPU.
Adjacent to the CPU are cache memories that are smaller, faster and more expensive than main
memory, but larger, slower, and less expensive than the set of registers. The next level in the
memory hierarchy is main memory, which modern machines have in some abundance, often
measured in gigabytes. After main memory, the next level in the memory hierarchy is mass
storage, today almost always on magnetic disk drives. Such memory is more expansive, less
expensive per byte, and slower than main memory. Beyond magnetic disks, optical disk
memory today represents the next step in the memory hierarchy, with lower costs, higher
densities, and slower speeds.
ANSWERS TO REVIEW QUESTIONS 187
SOFTWARE
4.1. Why was it important to the history of programming languages that, even at its introduction, FORTRAN
generated efficient programs?
There was a widespread belief that a higher-level language could never approach the efficiency of
hand-coded assembly language.
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