Hardware Reference
In-Depth Information
Figure 3.1. Alternating current signal showing clock cycle timing.
Note
The hertz was named for the German physicist Heinrich Rudolf Hertz. In 1885, Hertz
confirmed the
electromagnetic theory
, which states that light is a form of electromagnetic
radiation and is propagated as waves.
A single cycle is the smallest element of time for the processor. Every action requires at least one
cycle and usually multiple cycles. To transfer data to and from memory, for example, a processor
such as the Pentium 4 needs a minimum of three cycles to set up the first memory transfer and then
only a single cycle per transfer for the next three to six consecutive transfers. The extra cycles on the
first transfer typically are called
wait states
. A wait state is a clock tick in which nothing happens.
This ensures that the processor isn't getting ahead of the rest of the computer.
See the
Chapter 6
section, “
Memory Modules
,
”
p.
346
.
The time required to execute instructions also varies:
•
8086 and 8088
—The original 8086 and 8088 processors take an average of 12 cycles to
execute a single instruction.
•
286 and 386
—The 286 and 386 processors improve this rate to about 4.5 cycles per
instruction.
•
486
—The 486 and most other fourth-generation Intel-compatible processors, such as the AMD
5x86, drop the rate further, to about 2 cycles per instruction.
•
Pentium/K6
—The Pentium architecture and other fifth-generation Intel-compatible processors,
such as those from AMD and VIA/Cyrix, include twin instruction pipelines and other
improvements that provide for operation at one or two instructions per cycle.
•
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 multicore 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
