📄 unit 5-79%.htm
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<div class=Section1 style='layout-grid:15.6pt'>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan;mso-outline-level:2'><b><span
lang=EN-US style='font-size:18.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>Unit 5. Memory Operation and Performance<o:p></o:p></span></b></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
widow-orphan;mso-list:l7 level1 lfo1;tab-stops:list 36.0pt'><![if !supportLists]><span
lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-memory-systems');">5.1 Memory Systems</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-cache');">5.2 Caches</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-virtual-memory');">5.3 Virtual Memory (VM)</a>
<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan;mso-outline-level:3'><b><span
lang=EN-US style='font-size:13.5pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>Assessments<o:p></o:p></span></b></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:AssessmentByName('assm-qz-mc-memory-operation');">Multiple-Choice
Quiz 5</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:AssessmentByName('assm-exer-memory-operation');">Exercise 5</a>
<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:AssessmentByName('assm-qz-pr-memory-operation');">Practical
Quiz 3</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='margin-top:12.0pt;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:10.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>© Copyright 1999-2005 iCarnegie, Inc. All rights reserved.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan;mso-outline-level:2'><b><span
lang=EN-US style='font-size:18.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>5.1 Memory Systems<o:p></o:p></span></b></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-memory-technology');">5.1.1 Memory
Technology</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-reference-locality');">5.1.2 Locality of
Reference</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'><a
href="javascript:ContentByName('pg-memory-hierarchy');">5.1.3 Memory
Hierarchies</a> <o:p></o:p></span></p>
<p class=MsoNormal align=left style='margin-top:12.0pt;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:10.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>© Copyright 1999-2005 iCarnegie, Inc. All rights reserved.<o:p></o:p></span></p>
<span lang=EN-US style='font-size:18.0pt;font-family:SimSun;mso-bidi-font-family:
SimSun;mso-ansi-language:EN-US;mso-fareast-language:ZH-CN;mso-bidi-language:
AR-SA'><br clear=all style='page-break-before:always'>
</span>
<h2><span lang=EN-US style='font-family:"Times New Roman";color:black;
mso-ansi-language:EN-US'>5.1.1 Memory Technology<o:p></o:p></span></h2>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>There are many ways to store a bit of information. Current technologies
use semiconductors (memory proper), magnetic plates (hard disks), and
reflective puckered surfaces (CDs), but over time there have been many
technologies used for this purpose. It is very possible, perhaps even likely,
that within our lifetime, the dominant storage technologies will change. It has
happened in the past! Sometimes, when the technology changes, the efficiency of
systems changes drastically and forces changes in their design. This is one
reason to strive to understand concepts rather than conclusions: you can apply
concepts to new technology scenarios in which the conclusions of the old
situation become useless.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>One of the earliest computers, designed in <st1:country-region w:st="on"><st1:place
w:st="on">England</st1:place></st1:country-region> in the 1940s, used mercury
delay lines to store bits. These devices consisted of large (and, especially,
long) mercury-filled tubes with electronic circuitry attached to them, and the
devices "remembered" bits by having the electronic circuitry send
bits on a roundtrip through the tubes. The tubes would then
"remember" the bits only for as long as it took the pulse to travel
through them-at which time the circuitry would receive the pulse and resend it
on another roundtrip. The tubes were filled with mercury because it slowed down
the pulses and thus allowed them to be "remembered" for a longer
period of time. It would be as if we schemed to remember a phone number by
shouting it into a canyon trusting that, a second later, the number would be
echoed back to us. We would of course have to repeatedly "refresh"
the number by shouting it over and over again. This may seem silly, but it
wouldn't be <i>completely</i> silly, because in the time it took the echo to
return we could pay attention to other things and completely forget the phone
number.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>Mercury delay lines had a capacity of only about a hundred bits and
occupied an entire room. We have of course progressed a lot since that time,
but the concepts have not changed very much. Many of the technologies used in
computer storage nowadays also use transient physical phenomena (like the echo)
to store information for short amounts of time and repeatedly refresh the
information as necessary. All of a computer's main memory works like this,
which is the reason that a loss of power causes a loss of the data stored in
the computer's memory. When there is no power to refresh data, it decays
quickly. Hard disks and CDs, on the other hand, rely on the (mostly) stable and
even permanent rearrangement of material (magnetic domains or reflective
surfaces) and need not be refreshed every few milliseconds, which explains why
they preserve their data even when the power goes out.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>Without a doubt, the kind of storage device to be preferred is the
latter kind, the kind that preserves data even in the absence of power. So why
do we still use both kinds in today's computers? Think about it.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>The answer is: speed. For many reasons having to do with the physics of
the various technologies, permanent states that don't need to be refreshed
cannot be detected as fast as those that do. Because information is stored as a
state, the speed of detection affects the speed at which memory can be read.
Screaming a phone number is faster than painting it on the canyon walls, even
though the latter is certainly more permanent and is easier to maintain and
read at leisure. Sometimes we need speed more than persistence, sometimes it is
the other way around. So sometimes we use disks, and sometimes we cannot.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>This is an example of what is often called a <i>trade-off</i>. It is a
situation in which two or more desirable features are not attainable,
simultaneously, to the desired degree. We can choose to improve one at the
expense of the other, but we cannot get a "free lunch"-that is, we
cannot get something for nothing. Different memory technologies occupy
different points in this trade-off. Whereas one technology may be better than
the other for a specific need, it may not be possible to proclaim that one is
better than the other in general.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>There are currently several types of common memory technologies:<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'>Static Random Access
Memory (SRAM) <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'>Dynamic Random Access
Memory (DRAM) <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'>Magnetic disks <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'>Magnetic tapes <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;margin-left:36.1pt;text-align:left;text-indent:-18.0pt;mso-pagination:
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lang=EN-US style='font-size:10.0pt;mso-bidi-font-size:12.0pt;font-family:Symbol;
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style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=EN-US style='font-size:12.0pt;
color:black;mso-font-kerning:0pt;mso-ansi-language:EN-US'>Optical disks <o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>The graphs below compare the cost and speed of SRAM, DRAM, and magnetic
disk technology. There are, of course, other parameters of these memory
technologies that are worth comparing-for instance, their persistence or lack
of persistence in the absence of power. However, although this parameter is very
important, we will ignore it and others we have not mentioned in the discussion
below.<o:p></o:p></span></p>
<p class=MsoNormal align=left style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:left;mso-pagination:widow-orphan'><span lang=EN-US
style='font-size:12.0pt;color:black;mso-font-kerning:0pt;mso-ansi-language:
EN-US'>The numbers are approximations, because there are many factors that can
make each individual access either faster or slower than the value shown below.
Note also that the scale of the vertical axes is logarithmic, so that visible
differences are large indeed.<o:p></o:p></span></p>
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<v:f eqn="prod @6 1 2"/>
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