Processor Types and Specifications - Upgrading and Repairing PCs - page 55

• P6/P7 and newer —Sixth-, seventh-, and newer-generation processors can execute as
many as three or more instructions per cycle, with multiples of that possible on mul-
ticore processors.
Different instruction execution times (in cycles) make comparing systems based purely
on clock speed or number of cycles per second difficult. How can two processors that run
at the same clock rate perform differently, with one running “faster” than the other? The
answer is simple: efficiency.
The main reason the 486 is considered fast relative to the 386 is that it executes twice as
many instructions in the same number of cycles. The same thing is true for a Pentium; it
executes about twice as many instructions in a given number of cycles as a 486. There-
fore, given the same clock speed, a Pentium is twice as fast as a 486, and consequently a
133MHz486classprocessor(suchastheAMD5×86-133)isnotevenasfastasa75MHz
Pentium! That is because Pentium megahertz are “worth” about double what 486 mega-
hertz are worth in terms of instructions completed per cycle. The Pentium II and III are
about 50% faster than an equivalent Pentium at a given clock speed because they can ex-
ecute about that many more instructions in the same number of cycles.
Unfortunately, after the Pentium III, it becomes much more difficult to compare pro-
cessors on clock speed alone. This is because the different internal architectures make
some processors more efficient than others, but these same efficiency differences result
in circuitry that is capable of running at different maximum speeds. The less efficient the
circuit, the higher the clock speed it can attain, and vice versa.
One of the biggest factors in efficiency is the number of stages in the processor's internal
pipeline (see Table 3.6 ).
Table 3.6 Number of Pipelines per CPU