Microprocessor Basics

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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


    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.