Hardware Reference
In-Depth Information
No matter how big the main memory is, it is always way too small. People al-
ways want to store more information than it can hold, primarily because as tech-
nology improves, people begin thinking about storing things that were previously
entirely in the realm of science fiction. For example, as the U.S. government's
budget discipline forces government agencies to generate their own revenue, one
can imagine the Library of Congress deciding to digitize and sell its full contents
as a consumer article (''All of human knowledge for only $299.95''). Roughly 50
million topics, each with 1 MB of text and 1 MB of compressed pictures, requires
storing 10
14
bytes or 100 terabytes. Storing all 50,000 movies ever made is also in
this general ballpark. This amount of information is not going to fit in main mem-
ory, at least not for a few decades.
The traditional solution to storing a great deal of data is a memory hierarchy,
as illustrated in Fig. 2-18. At the top are the CPU registers, which can be accessed
at full CPU speed. Next comes the cache memory, which is currently on the order
of 32 KB to a few megabytes. Main memory is next, with sizes currently ranging
from 1 GB for entry-level systems to hundreds of gigabytes at the high end. After
that come solid-state and magnetic disks, the current workhorses for permanent
storage. Finally, we have magnetic tape and optical disks for archival storage.
Registers
Cache
Main memory
Magnetic or solid state disk
Ta p e
Optical disk
Figure 2-18.
A five-level memory hierarchy.
As we move down the hierarchy, three key parameters increase. First, the ac-
cess time gets bigger. CPU registers can be accessed in a nanosecond or less.
