Memory - Upgrading and Repairing PCs - page 319

translate from one to the other.
A nanosecond is defined as one billionth of a second—a short piece of time indeed. To put some
perspective on just how small a nanosecond really is, consider that the speed of light is 186,282
miles (299,792 kilometers) per second in a vacuum. In one billionth of a second (one nanosecond), a
beam of light travels a mere 11.80 inches or 29.98 centimeters—slightly less than the length of a
typical ruler!
Memory speeds have often been expressed in terms of their cycle times (or how long it takes for one
cycle), whereas processor speeds have almost always been expressed in terms of their cycle speeds
(number of cycles per second). Cycle time and cycle speed are actually just different ways of saying
the same thing; that is, you can quote chip speeds in cycles per second, or seconds per cycle, and
mean the same thing.
As an analogy, you could express the speed of a vehicle using the same relative terms. In the United
States vehicle speeds are normally expressed in miles per hour. If you were driving a car at 60 miles
per hour (mph), it would take 1 minute per mile (mpm). At a faster speed of 120mph, it would take
only 0.5mpm, and at a slower 30mph speed it would take 2.0mpm. In other words, you could give the
speeds as either mph or mpm values, and they would mean exactly the same thing.
Because it is confusing to speak in these different terms for chip speeds, I thought it would be
interesting to see exactly how they compare. Table 6.2 shows the relationship between commonly
used clock speeds (MHz) and the nanosecond (ns) cycle times they represent.
Table 6.2. Relationship Between Megahertz (MHz) and Cycle Times in Nanoseconds (ns)