Page 69 - A Practical Guide from Design Planning to Manufacturing

Computer Components 45

clock, neither FSB400 bus uses a 400-MHz clock, even though both are
commonly referred to as “400-MHz” buses. From a performance per-
spective this makes perfect sense. The data buses for both processors
have the same width (64 bits), so the data bandwidth at 400 MT/s is the
same regardless of the frequency of the bus clock. Both FSB400 stan-
dards provide a maximum of 3.2 GB/s data bandwidth. Where the true
bus clock frequency makes a difference is in determining the processor
frequency.
Multiplying the frequency of the bus clock by a value set by the man-
ufacturer generates the processor clock. This value is known as the bus
multiplier or bus ratio. The allowable bus ratios and the processor bus
clock frequency determine what processor frequencies are possible.
Table 2-2 shows some of these possible clock frequencies for the Athlon
XP and Pentium 4 for various bus speeds.
The Athlon XP allows for half bus ratios, so for a 200-MHz bus clock,
the smallest possible increment in processor frequency is 100 MHz. The
Pentium 4 allows only integer bus ratios, so for a 200-MHz bus clock the
smallest possible increment is 200 MHz. As processor bus ratios get
very high, performance can become more and more limited by commu-
nication through the processor bus. This is why improvements in bus
frequency are also required to steadily improve computer performance.
Of course, to run at a particular frequency the processor must not only
have the appropriate bus ratio, but also the slowest circuit path on the
processor must be faster than the chosen frequency. Before processors
are sold, their manufacturers test them to find the highest bus ratio they

TABLE 2-2 Processor Frequency versus Bus Ratio
Athlon XP freq (GHz)
FSB267 FSB333 FSB400
Bus ratio (133 MHz) (167 MHz) (200 MHz)
10.0 1.33 1.67 2.00
10.5 1.40 1.75 2.10
11.0 1.47 1.83 2.20
11.5 1.53 1.92 2.30

Pentium 4 freq (GHz)
FSB400 FSB533 FSB800
(100 MHz) (133 MHz) (200 MHz)
14 1.40 1.87 2.80
15 1.50 2.00 3.00
16 1.60 2.13 3.20
17 1.70 2.27 3.40