Processor Types and Specifications - Upgrading and Repairing PCs - page 54

Note
The hertz was named for the German physicist Heinrich Rudolf Hertz. In 1885, Hertz con-
firmed the electromagnetic theory , which states that light is a form of electromagnetic radi-
ation 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 ex-
ample, a modern 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 pro-
cessor isn't getting ahead of the rest of the computer.
See “ Memory Modules ,” p. 333 ( Chapter 6 , “ Memory ”).
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 5×86, drop the rate further, to about 2 cycles per instruction.
• Pentium/K6 —The Pentium architecture and other fifth-generation Intel-compatible
processors, suchasthose fromAMDandCyrix, include twin instruction pipelines and
other improvements that provide for operation at one or two instructions per cycle.