Hardware Reference
In-Depth Information
driving a car at 60 miles per hour (mph), it would take 1 minute per mile (mpm). At a
faster speed of 120 mph, it would take only 0.5 mpm, and at a slower 30 mph speed it
would take 2.0 mpm. In other words, you could give the speeds as either mph or mpm
values, and they would mean exactly the same thing.
Becauseitisconfusingtospeakinthesedifferenttermsforchipspeeds,Ithoughtitwould
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 The Relationship Between Megahertz (MHz) and Cycle Times in Nanoseconds (ns)
As you can see from
Table 6.2
, as clock speed increases, cycle time decreases proportion-
ately, and vice versa.
OvertheevolutionarylifeofthePC,mainmemory(whatwecallRAM)hashadadifficult
time keeping up with the processor, requiring several levels of high-speed cache memory
to intercept processor requests for the slower main memory. More recently, however, sys-
temsusingDDR,DDR2,andDDR3SDRAMhavememorybustransferrates(bandwidth)
