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Characteristics of a Microprocessor

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The microprocessor is the defining trait

of a computer, so it is important tounderstand the characteristics used to

describe microprocessors.

This module provides an introduction to

these characteristics.

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Clock speedAlso called clock rate, the clock speed

is the speed at which a microprocessor

executes instructions. Every computercontains an internal clock that regulates

the rate at which instructions are

executed and synchronizes all the

various computer components.

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The faster the clock, the moreinstructions the microprocessor canexecute per second. The

microprocessor requires a fixed numberof clock ticks (or clock cycles) toexecute each instruction.

Clock speed is stated in either MHz orGHz. 1 MHz is equal to 1 million cyclesper second, while 1 GHz is equal to 1billion cycles per second.

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At the present time the most common

microprocessors run from 1.8 GHz (1.8

billion cycles per second)to 3.2 GHz(3.2 billion cycles per second.

Clock speed is a major factor in determiningthe power of a computer.

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Instruction SetThe possible operations amicroprocessor can performs is basedon its instruction set. Programs are

written for a microprocessor based onits instruction set. For example, theSIMP computer understands 10

instructions, and any program written forit uses those ten instructions in variousways to accomplish some surprisinglycomplicated tasks.

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Meanwhile, advanced processors canhave from 150 to over 200 instructions,allowing for extremely complicated

tasks.

Since software is written with theinstruction set in mind, sometimes a

larger instruction set will equal betterperformance. For example, onedifference between Pentium 4 andPentium 5 is that Pentium 5 has a larger

instruction set.

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When comparing a 2GHz Pentium 4and 2GHz Pentium 5, if they both runsoftware designed with the new

instruction set in mind, the Pentium 5will outperform the Pentium 4, despitehaving the same clock speed. However,

if the two are compared while runningolder software, which does not use thenew instructions, their performance willbe similar.

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CacheMost programs access the sameinformation repeatedly while running.Cache memory is intended to take

advantage of this fact. Memory cache isa high speed storage mechanism thatholds recently read data and

instructions from main memory, whicheliminates the processor from having toconstantly access main memory.

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The program first checks the cache tosee if the desired information is alreadypresent there. If it is, the cache sends

the information back to themicroprocessor, bypassing the mainmemory.

95% of the time the processor isworking, it is accessing information fromcache.

There are two kinds of cache L1 and

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L1 cache (also called primary cache) is

built directly into the microprocessor, a

location referred to as "on-die". Since itis "on-die", it is part of the

microprocessor and, is usually smaller

in size than L2 cache, but since it is built

in it runs at the same speed as theprocessor.

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L2 cache (also called secondary cache)is not usually built into microprocessor,but it is found within the processor's

external packaging. This location isreferred to as, "off-die." Since "off-die"L2 cache is not included in the

processor's architecture, it can often beof greater sizes. However, since it is notincluded on-die, L2 cache is usuallyslower than L1 cache.

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Bus SpeedThe processor communicates with other

devices via the data bus, sometimes

called the front side bus. Bus speed ismeasured in MHz, the same unit used

to measure clock speed. While a

processor might be working at up to 3

GHz, quite often the performance of thecomputer is hampered by a slower data

bus speed.

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Recently much effort has been put into

making the data bus have speeds more

comparable to the microprocessor.

At the present time data bus speeds

range from 200 MHz up to 1GHz.