!

NWEN$242$Revision$$

1. Introduction$$$

Embedded!computers!are!computer!inside!another!device.!It!is!less!tolerance!to!failure!

compare!to!other!type!of!computers.!!

!+!The!performance!of!the!program!is!determined!by:!

< The!algorithm!run!by!the!program!!< Compiler!!

< Effectiveness!of!the!computer!in!executing!the!instructions!

(Memory/System/Processor)!< I/O!performance!!

< The!OS!(Operating!System)!!

!From!the!above!factors!we!cannot!say!one!factor!influence!the!program!performance!more!

than!the!other,!it!depends,!but$usually$the$I/O$and$the$OS!(depend!on!the!app!running).!!!

! What!is!a!Compiler?!!< A!compiler!is!a!program!that!translates!high<level!language!into!assembly!language.!!

!

The!OS!handles!the!basic!I/O!operation.!Allocates!storage!and!memory,!and!provide!protection!of!sharing!the!computer!with!multiple!applications!using!it!simultaneously.!!

!

+!Note:!App!cannot!access!the!hardware!directly.!!!

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!

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Assembler!–!converts!the!assembly!statement!into!binary!machine!codes.!Assembly!

statement!corresponds!to!the!machine!instructions.!Example!of!the!assembly!statement!could!be!like!this:!add!$t0,!$t1,!2!!

!+!The!5!major!components!of!the!computer!are:!!

< Control!(command!the!datapath,!memory,!I/O!devices!according!to!the!instructions!

of!the!program)!!< Input!(computer!fed!in!the!information!such!as!mouse,!keyboard)!!

< Output!(conveys!the!result!of!computation!to!the!user)!!< Datapath!(performs!the!arithmetic!and!logical!operation)!

< Memory!(DRAM!&!SRAM)!!

!The!CPU!contains!the!datapath!and!the!control.!!

!

Note:$The!main!memory!is!volatile,!this!means!that!the!component!can!only!keep!information!when!there!is!a!power!supple!to!it,!the!information!will!disappear!if!there!is!no!power!supply.!

Cache!is!Volatile,!and!it!uses!the!SRAM!which!build!fast!memory!but!costly.!On!the!other!hand,!secondary!storage!such!as!Hard!Disk,!Flash!Drive!is!non<volatile!and!uses!the!DRAM.!!

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Application!Software!

System!Software!!

Hardware!!

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2. Understanding$Performance$$$

The!performance!of!the!computer!can!be!seen!as!the!“time!it!takes!to!execute!given!a!single!

task.!!

!! Concepts$

$

< Throughput:!total!amount!if!work!done!in!a!given!time.!!

< Response!time:!The!time!between!the!start!and!the!completion!of!the!task.!!

< Execution!time:!The!time!it!takes!to!execute!the!task.!!< Time:!is!the!measure!of!the!computer!performance.!!

< CPU!time:!the!time!the!CPU!spends!computing!for!a!task.!!

< Clock!cycles:!how!fast!the!hardware!can!perform!basic!functions.!!< Clock!period:!a!time!for!a!complete!clock!cycle.!!

< Clock!rate:!the!inverse!of!clock!period.!!< Clock!signal:!can!consider!as!a!sequence!of!pulses.!!

!

For!example,!if!we!replace!the!processor!of!the!computer!say!from!core!i3!to!core!i7,!then!it!will!help!reduce!the!response!time.!What!if!we!add!more!processors,!say!from!1!to!8?!What!is!

the!benefit?!This!really!depends,!but!at!least!it!should!give!a!better!throughput.!It!won’t!really!improve!the!response!time,!if!we!were!to!just!execute!one!task!then!one!processor!is!enough!

but!if!we!were!to!execute!multiple!tasks!then!more!processor!is!better,!so!less!response!time.!

What!we!are!trying!to!say!is!that,!if!we!only!execute!one!task,!only!one!processor!is!use!anyway!so!there!is!no!point!in!having!lots!of!processors,!hence!no!improvement.!!

!

+!In!order!to!maximize!the!performance,!we!need!to!minimize!the!execution!time.!!!

!"#$%#&'()" = ! 1!"!#$%&'(!!"#$!!

!+!If!say!we!have!two!computers,!x,!and!y.!Say!x!has!a!better!performance!than!y,!then!we!have:!

!!"#$%#&'()"! ! > !!"#$%#&'()"!(!)!

!1

!"!#$%&'(!!"#$!(!) > !1

!"!#$%&'(!!"#$!(!)!!

!"!#$%&'(!!"#$! ! > !!"!#$%&'(!!"#$!(!)!!

+!If!x!is!n<times!faster!than!y,!then,!!!

! = !!"#$%#&'()"!(!)!"#$%#&'()"!(!) = !!"!#$%&'(!!"#$!(!)!"!#$%&'(!!"#$!(!)!

!

Different$time$concepts:!Clock!time!/!Elapsed!time!/!Response!time,!they!all!refer!to!the!same!idea!in!which!means!the!total!time!spent!when!executing!a!task.!The!time!included,!the!

disk!access,!memory!access,!I/O!operations,!etc.!On!the!other!hand!the!CPU!time!does!not!include!the!time!spent!waiting!for!the!I/O!or!running!other!programs.!There!are!‘User!CPU!

time’!(Time!spent!by!the!CPU!in!user!mode!–!has!limited!access)!and!the!‘System!CPU!time’!

(Time!spent!by!the!CPU!in!kernel!mode!–!can!access!all!resources!in!the!computer).!!!

Note:!Almost!all!the!computers!construct!using!the!clock!to!determine!when!events!take!place!

in!hardware.!!

!

CPU$Performance$

!To!get!the!CPU!execution!time,!let!x!=!CPU!execution!time!for!a!program!

! ! ! ! !!!!!!!!!!!y!=!CPU!clock!cycles!for!a!program!!

! ! ! ! !!!!!!!!!!!z!=!Clock!cycle!time!!!! ! ! ! !!!!!!!!!!!a!=!Clock!rate!!

!! = ! ∗ !!

!

Note:!CPU!clock!cycles!means!how!may!clock!cycle!is!required!to!execute!a!task.!Clock!cycle!time!is!the!inverse!of!the!clock!rate.!!

! = !!!!!

Instruction$Performance$$

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+!Clock$cycle$per$instruction$(CPI):!average!number!of!clock!cycles!per!instruction!for!a!program.!!

!!"#!!"#$%!!"!#$% = !"#$%&'$()"# ∗ !"#!!

!

It!is!noted!that!different!instructions!may!take!different!amount!of!time.!CPI!can!be!seen!as!a!way!of!comparing!two!processors.!We!can!use!the!CPI!to!tell!us!the!performance.!!

!Note:!Instruction!mix!is!a!measure!of!the!dynamic!frequency!of!instructions!across!one!or!

more!programs.!

!The$power$wall$$$$

$

+!Dynamic!power!is!the!power!that!is!consumed!during!the!switch.!!!

!"#$% = !"#"$%&%'(!!"#$ ∗ !"#$%&'! ∗ !"#$%#&'(!!"#$%ℎ!"!!

+!Lower!the!voltage!can!help!reduce!the!dynamic!power!but!it!may!make!the!transistors!too!leaky.!About!40%!of!the!power!consumption!is!due!to!leakage.!!

!Amdahl’s$Law$$

$

Let!a!=!“Execution!time!after!improvement”!!!!!!!!!b!=!“Execution!time!affected!by!improvement”!

!!!!!!!c!=!“Amount!of!improvement”!!!!!!!!d!=!“Execution!time!unaffected”!

!

! = !!! + !!!!!

Millions$instructions$per$second$(MIPS)$=>$!"#$ = ! !"#$%&'$()"!!"#$% !"#$%&'()!!"#$ ∗ !10!$< There!are!problems!if!we!use!MIPS!to!compare!computers,!one!is!different!

instruction!sets,!two!is!MIPS!varies!between!programs!on!the!same!computer.!!!

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