How to Really Save Computer Energy?

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Now, a modern desktop PC, such as DELL. Inspiron 530, has more powerful CPU, larger memory,. HDD, DVD-RW and LCD monitor instead of CRT. In.

How to Really Save Computer Energy? Vasily G. Moshnyaga Department of Electronics Eng. & Computer Science, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan Abstract—This paper discusses energy consumed by a typical desktop computer. We show that despite the high energy cost associated with computer manufacturing, the typical desktop computer is used only a fraction of its expected time. Increasing PC lifetime by reuse could amortize the energy required to build a computer. Keywords: computer, lifetime analysis, energy, design for reuse



Due to elevating problems of global warming, reduction of energy consumption becomes increasingly important. One of the most effective ways to improve energy management, increase energy efficiency and reduce waste starts with personal computer or PC. According to IDC, there were 900M desktop PCs in use worldwide in 2006[1]. Each of these machines consumes energy at every stage of life-cycle: manufacturing, usage and end-of-life. While electrical energy, taken from the wall during computer usage has been in focus of the design community for over a decade, manufacturing and end-of-life stages have not been considered. As study [2] shows producing a PC takes by 3 times more energy than using the PC for three years. And the study was conducted in 2002. Since that there have been significant advances in technology, computer architecture, design, etc. For example, transistor feature sizes have shrunk by 4 times (from 180nm to 45nm), chip sizes increased by 1.2 times, CPU clock frequency increased by 3-4 times, processor architecture advanced from Pentium3 and Pentium 4 to multi-cores, display technology has changed from CRT to LCD, and so on. Have these changes affected energy figures? What are the trends? Is manufacturing energy still the main source of energy consumption? Has the usage energy increased? What can be done to reduce total computer energy more efficiently? In this paper we tried to answer these questions. The goal of this paper is twofold. The first one is to examine trends in energy associated with a typical computer in perspective to advances in technology and computer architecture made over the last six years. The second is to present an approach capable of amortizing the energy increase. Based on our findings, we urge to explore energy reduction alternatives beyond the computer usage and advocate computer reuse as a possible approach. The work was sponsored by The Ministry of Education, Culture, Sports, Science and Technology of Japan, Grant-in-Aid for Scientific Research (C) No.19500049

The paper is organized as follows. Section 2 analyzes energy associated with modern PC in comparison to PC of the year 2002 to show the trend. Section 3 discusses our approach to energy reduction. Section 4 presents conclusions.


The Energy Associated with PC

The energy associated with computer lifetime can be expressed as the sum of the production energy, usage energy and the energy consumed at the end-of computer life. A comparative analysis of these three components reveals that energy required to manufacture a computer dominates the energy consumed over the computer lifetime.

2.1 Production Energy This component accounts for total energy and fossil fuels used in producing the computer and delivering it to its owner. According to [2], this energy can be expressed as, EPR =EP + ΕA + ER (1) where, EP defines the total “process-sum” energy required for carrying out fabrication processes, such as production of silicon wafers from row materials, production of bulk materials in computers and monitors, manufacture of microchips, manufacture of printed circuit boards, manufacture of display, and assembly of the computer from the parts; ΕA is the additive factor for calculating the energy of producing a unit from industries or sectors, which have specific economic (not process) data available (e.g. sectors producing passive components, chip-making equipment, electronic chemicals, etc); ER is the “remaining” factor which represents energy associated with the parts manufacturing, transport, wholesale trade and the other processes, not covered yet in the analysis. The additive factor (EA) is determined by multiplying the supply chain intensity Hj of the relevant sector (in terms of MJ/$) by the producer price (Cj) of the product, i.e. ΕΑ= Σj Cj*Hj (2) The remaining factor (ER) represents energy associated with the parts manufacturing, transport, wholesale trade and the other values, not covered yet in the analysis. It is computed as the sum over a set of input-output (IO) sectors that excludes those costs covered by the process-sum and additive factor, i.e. ΕR ={C- Σk (Ck*Vk) -Σj Cj}∗Σi Si*Hi. (3)





Item Codename Frequency C Technology Transistors P U Die Size Pin count Core voltage L1/L2-cache Size DisWeight play Power Graphic Card DRAM HDD CD/DVD



Pentium III Coppermine 733MHz, 180 nm 28M 126mm2 423 1.65V 256KB 17 inch CRT 17 Kg 128W / 20W ASUS Radeon 9600, 32MB 512KB 80GB CD-R

Core 2 Duo E6700 3GHz 67 nm 291M 143mm2 775 1.36V 4MB 20 inch LCD 6.5Kg 50W/ 2W GeForce 7200,128MB 4GB 320GB DVD WR



Graphic Card(single)

4X 1/3X 10X 1.13X 1.83X 0.82X 16X 1.23X 0.38X 1 4X 8X 4X

PRODUCTION ENERGY Total Energy (MJ) 2002


Increase (%)



Display HDD, CD, etc

1100 450


1100 1680 6420

2424 1232 450 1100 1680 6886

16 12 7.2



System component Processor Motherboard

Here C is the producer price; Ck is the monetary expenditure for implementing a process k; Vk is the valueadded value modified by adding energy and subtracting capital; Cj is the monetary expenditure for producing an IO activity j per unit product; Si is the relative value fraction of supply-chain purchases for each respective sector. An average desktop PC in 2002 had Pentium-III CPU, 80GB HDD, 17-inch CRT monitor etc. and required approximately 6400 Mega Joules (MJ) of energy to manufacture [2]. Now, a modern desktop PC, such as DELL Inspiron 530, has more powerful CPU, larger memory, HDD, DVD-RW and LCD monitor instead of CRT. In comparison to Pentium-III, the Intel Core 2 Duo CPU produced in 2007 has 4 times higher clock frequency, 1.8X more pins, 10X more transistors on chip, 13% larger die size and 4X larger cache (see Table 1). If we assume that (i) the energy of manufacturing is proportional to semiconductor die area (Edie =1/yield*area) [3]; (ii) yield is 65% [4], (iii) the energy of making processor die and DRAM die are identical, and (iv) the assembly energy is 5.9MJ regardless of die size, than the energy to produce a die of Core 2 Duo processor is to be 39.7MJ, and the total manufacturing energy to be 45.6MJ. Table 2 shows the total energy required to produce a desktop computer, like 2007 DELL Inspiron 530 with 20” TFT display, estimated based on same data sources as [2].

Memory HDD Power Supply Total

COMPONENT POWER (WATT) Best case 12-30 10-15 3-10 (integrated) 5-15 3-5 5-15 38-90

Worst case

60-120 30-50 25-180 (pci express) 30-50 10-15 40-60 195-475

The amount of energy required for to manufacture a PC is clearly considerable. Though we conservatively assumed that additive/remaining factors have not grown since 2002, the total energy to make a PC had increased over the last years by 7% due to the rise of energy to fabricate chips and change from CRT to LCD. Even though the CRT production uses more energy for glass manufacturing [5], LCD monitors have more printed wiring boards, more IC chips, lower yield (40-60%) [6], and so are more energyconsuming.

2.2 Usage Energy The electrical energy consumed by a PC system during operation or usage can be expressed by a simple equation: EUSE=Σ Pj * Tj+ PIDLE*TIDLE, (4) where Pj is the mean power consumed by an application j; Tj is the duration of the application; PIDLE and TIDLE are the power consumption and duration, respectively, of idling. Two key factors determine the operation energy: (1) power requirements of computer and monitor to deliver an application, and (2) usage pattern. The sections below look at both these factors and how their interaction influences annual energy consumption. 2.2.1 Power requirements of computer and monitor Due to large difference in power consumption of components, power requirements of PC system vary widely. Table 3 shows power ranges of main PC components determined based on the internet data [7]. In the worst case, the system power may reach almost a half of a kilowatt. However, it barely exceeds 250 watts unless a powerful processor or a gaming class video card is used. Fig.1 plots power consumption trend of mainstream Intel processors. Since adoption of Pentium-4 NetBurstTM microarchitecture at the end of 2000, the processor power consumption had grown by five times reaching 130W in 2006. With introduction of Core Duo microarchitecture and 45nm fabrication technology, the full-load power of highend processors (with the exception of Core-Duo Quad and Extreme) has been pushed-down to the level of Pentium-III and mobile processors. Furthermore, since Pentium-III, the CPU designers had implemented a variety of techniques capable of shrinking idle power. Examples include Enhanced Intel Speedstep and AMD’s Cool’n’Quiet, which apply dynamic clock throttling and voltage scaling to save energy. According to [8], the savings can be as much as



Power (W)


Pentium 4 Pentium D Core-Duo Core Q (X)

120 100

Power (W)

Technology 130nm 90nm



30 20 10


0 60



50 Brightness (%)







Fig.4: Monitor power consumption vs. screen brightness

0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Year Fig.1: CPU Power Consumption Idle

Power (W)

Power (W)


Other Monitor CPU




150 100 50

100 0






Tu rio

n 6 In 4 M te l C T40 D or A AM thlo eD uo n D 6 AM Sem 4 3 0 0 pr D on 0 A 34 AM thlo 00 n D 64 A 40 AM thlo 0 n D 64 0 A M At 35 hl D on 00 At 6 hl on 4 3 In 0 0 6 te l P 4 x2 0 en 48 In tiu 00 te lP m 4 e In te ntiu 630 lP m D en In 8 te t l P ium 20 en D ti 93 In te um 0 lP P4 en tiu 950 m 4 67 0



Fig.2: The full-load and idle power consumption of PC vs. CPU type. 150

Standby Active

Power (W)

120 90 60 30 0 CRT 15"

CRT 17"

CRT 19"

CRT 21"

LCD 15"

LCD 17"

LCD 19"

LCD 21"

Monitor type and screen size

Fig.3: Power consumption vs. monitor type and screen size

28% for Intel Core Duo processors and 40% for Intel Core 2 Duo processors. Additionally, the new 45nm fabrication technology allowed shrinking the leakage current thanks to the innovative transistors with metal gate and high-k dielectric. As result, power consumption of new 2008 Intel WolfdaleTM processor under full workload is only 65W, which is equal to that of two 2-3 year-old processors in idle mode [9] The power consumption of PC core strongly depends on CPU. As Fig.2[10] shows, the full-load power vary with CPU by a factor of four while idle power by a factor of two. Monitors, like computers, also differ in the amount of energy they require. The size of monitor makes a significant contribution to its power requirement. Fig.3 shows variation of monitor power with the screen size, observed for 4 CRT monitors from SONY and 4 TFT LCD monitors from Samsung [11]. In average, CRT monitors burn between





Fig.5: Estimated PC power consumption

65W-145W when active, and 9-14W in standby, while LCD monitors take two or tree times less power than CRTs of equivalent screen size in active mode and less than 2W in standby. Besides, the LCD screens recover from standby faster and consume less power during the transition. The monitor power consumption also depends on screen brightness. Fig.4 shows the trend on example of 21” LCD monitor from Dell. Changing the brightness level from the highest to the lowest reduces the monitor power by a half. To summarize, computers and monitors vary significantly in their power requirements. Due to recent architectural and technological improvements, the power efficiency of computers and monitors has been considerably improved. New computers use much more power when they are active compared to when they are in a low-power mode. Fig.5 shows the total power consumed by 2007 DELL Inspiron 530 desktop system in comparison to power used by PC in 2002. In this figure, idle refers to a state in which all components are ON but do nothing; in the lp state all components reach their lowest level of power consumption. Even though the 2007 PC burns more electrical power at full-load than the PC 2002, it consumes less in the lowpower mode. When all low-power features of PC and monitor are enabled, graphic card, PSU and motherboard eat more power than the Core 2 Duo CPU. The total power of our 2007 Dell system depends on application load as shown in Table 4. Although the power difference between the maximal load (3D gaming) and idling is only 30Watt in our system, it may be quite large especially in systems employing powerful graphic cards. TABLE IV. Inet 117

Email 117


Game 140

Music 118

Video 127

Idle 110

2002 PC 2007 PC 2007 PC*

Reference [2] [10] [12] [13] [14] [14] [15]b [16]c [17]

System Home PC PC Office PC Office PC Home PC Office PC Office PC Office PC Office PC

USAGE PATTERNS Active 21h/w 10% 16h/w 15h/w 10.2 h/w 12.9 h/w 7.6 h/w 15 h/w 5h/w

Idle 0 0 0 9.5h/w 0 0 0 9.5h/w 35h/w

Low-power 0 90% 27.5h/w 15.5h/w 3.2h/w 8.95 h/w 32.4 h/w 15.5h/w 0

250 200 150 100 50 0 [2]


Energy (kWh)


Energy (kWh)


a. The computer is off at night and weekends; on weekdays it is 18% active and 81% in low power b. The computer is off at night and weekends, and will enter low-power mode after a period of inactivity. Abbreviations: hours (h), week (w)




Office 4% Game 11%

Music 4%

Inet 49%

Email 8%

Office 37%

I-net 25% Email 16%

Other 18%

Fig.6: The most frequent applications at home (left) and at work (right)

To summarize, computers and monitors vary significantly in their power requirements. Due to recent architectural and technological improvements, the power efficiency of computers and monitors has been considerably improved. New computer systems use more power than older PC at full load and less at a low power mode. 2.2.2 Usage patterns The usage pattern (i.e. number of hours used in what power modes) is a key determinant of PC energy consumption. Researchers usually differentiate patterns by the type of PC (desktop or laptop), place (home or office) time (night and day) by assigning different periods of idling and active time. Despite differences, all these usage patterns have one limitation in common; namely they assume that computer consumes maximum (or full) power when active. Since various applications burn different amount of power this may lead to an error. By interviewing 200 users we found that amid a large variety of applications, only a few are used almost all the time. The most frequent are Internet browsing, CD-recording/playback, video/image editing, gaming and Office tools (such as MS-Word, MS-Excel, Power-Point, etc). Fig.6 shows the breakdown of time spent in these applications at home and office per week. Clearly, the applications are used unevenly: the internet browsing takes almost a half of total time at home and 25% at work, while the office tools consume 40% of operation time at work and only 4% at home. Also there are many applications (e.g. CAD, CAM tools, etc), which are used at work only.



100 50 0 [10]


Fig.7: Annual energy use by office PC (top) and home PC (bottom) estimated on different usage patterns

PC Energy Use (MJ)

Music 16%


2002 PC 2007 PC 2007-PC*



Video 12%


Home(min) Office(min)

10000 8000

Home(max) Offie(max)

6000 4000 2000 0 1






Duration (years)





Fig.8: PC operation energy variation with the lifespan

Since the power consumption varies with application, we propose to differentiate applications in the usage pattern as follows: ΕUSE = Σj (Pj*mj)*TA+Pidle*TI (6) Here, mj is the ratio of time spent in application j to the total amount of active time TA; TI is the total time of idling. Fig.7 shows the annual energy use by office PC and home PCs estimated on different usage patterns. The bars marked by 2002_PC and 2007_PC show the energy consumption estimated based on patterns (Table 5), the bars marked 2007_PC* show the results obtained based on Equation (6) and distributions shown in Fig.6. Although the amount of energy varies with the patterns, the difference between modern PC and that of the year 2002 is small. Linking the usage pattern to application reduces overestimation by 312%. Finally, based on above data, we estimate the lower and upper bounds on energy consumed by PC in operation at office and at home during lifespan. Fig.8 plots the results assuming 1KWh=3.6MJ. If we assume a 3 year lifespan for a PC, as in [3], the operation energy consumption ranges from 780MJ to 1500MJ for a home PC and from 1404MJ to 2700MJ, for an office PC, respectively.

2.3 End-of-life Energy of PC This component accounts for the energy and fossil fuels required by PC reuse and recycle. “Reuse" refers to the



Cumulative energy (MJ)

Total PC Energy (MJ)

16000 14000 12000 10000 8000

Home-02 Office-02 Home-07 Office-07

6000 4000

16000 14000 12000 10000 8000 6000 4000 2000 0 1







Lifespan (years)


Fig.8 Potential benefits of PC reuse.












PC lifespan (years) 20000 18000 16000

Total Energy (MJ)



14000 12000 10000 8000

H ome ( Up 3 , Rc 9 ) H ome ( Up 3 , Rc 6 ) O ffice ( Up 3 , Rc 9 ) O ffice ( Up 3 , Rc 6 )

6000 4000 2000 0 1









PC lifespan (years)

Fig.7 Impact of the end-of-life on the total PC energy: (top) no upgrade, no recycle; (bottom) upgrade every 3 years and recycle either after 6 or after 9 years of usage.

return of old computer to use by extending its lifespan. It may include upgrading of PC components (memory, HDD, etc.) or repairing of faulty parts, refurbishing and giving (or selling) computers to someone or some other organization to use. With upgrade and repair, additional energy may be required for manufacturing replaceable parts and system testing. “Recycling" refers to the processing of waste PCs for recovery of individual materials (plastics, aluminum, steel, etc.) to be used in the production of new computers. PC recycling combines four processes: collection, disassembly, pre-manipulation and refinery processing. A waste PC completely shredded at the pre-manipulation, enters refinery process as raw materials. Precious material extraction, identification, row material recovery, cleaning, sorting, hazardous waste disposal, etc. are often required before the recovered materials can be used again. Also a portion of solid waste is sent to waste-to energy incinerators for energy recovery. To estimate the end-of-life energy (EOL) of a PC, we used a simple model, which assumes that reuse extends the

life of old PC for a second lifespan after which the user recycles the old machine and buys a new one. EEOL = rRU*EUSE2 + rUP*EUP - rRC*ER. (7) where rRU, rUP, rRC are the rates (%) for PC reuse, upgrade and recycle; EUSE2 is the usage energy during the second lifespan; EUP is the energy embodied in the PC upgrade and/or repair; ER is the equivalent net energy saved by recycling materials from one PC. The energy of land-filling a computer was not considered because it is negligible compared to other factors [18]. Fig.8 shows the effect of the EOL on the total energy embodied in PC, assuming that the energy embodied in parts for an upgrade is 1750MJ and the energy gain from recycling is -280 MJ [19]. The initial energy of PC is attributed to production, while the energy increase is attributed to the usage energy which grows with the time. If neither PC is upgraded or recycled, the total energy consumed by PC increases linearly and after 7-8 years of usage overpasses the energy embodied at the production almost twice. If PC is upgraded every three years and recycled after 6 or 9 years, its cumulative energy consumption reaches the level of production energy just in 6 years. Nowadays, the PC lifespan is short. According to 1998 Dataquest study, an average lifespan of office PC was 3.44 years [20]. However, a recent survey of Japanese webusers indicates that they purchase a new PC every two years [21]. As the PC lifespan is getting shorter, the production energy dominates the total energy associated with a PC. Table 6 breaks down the total energy embodied in modern PC (no upgrade) over the lifecycle stages. As one can see, the amount of energy required for PC production exceeds the energy of 5-years-long PC usage! If we “really” want to save energy, we must reduce production energy as much as possible. We claim that drastic savings of PC energy are only possible if aspects of computer production are considered and therefore urge to explore the energy reduction approaches beyond the PC usage. One of such approaches is presented in the next section.

TABLE VI. Lifecycle stages Produce Usage EOL



3 years Home Office 82 70 18 30 -3.3 -2.8

PC’s lifespan and type 4 years 5years Home Office Home Office 77 63 73 58 23 37 27 42 -3.1 2.6 3.0 2.4

PC Reuse for Energy Reduction

Because computers are delivered to us as working or completed products, we traditionally associate computer energy with electrical energy or PC usage energy. This energy component can be reduced in two ways: by using components that require less power (e.g. replacing CRT monitors with LCD, desktops with laptops, using a single unit HDD unit instead of two HDD units of equal capacitance) or by using power management software to adjust the energy consumption of these components to workload requirements. Modern computers have a variety of power management options, for CPU, HDD, monitor, etc. Most users, however, do not enable these options; so many PCs consume nearly as much power after hours as it was during the day. According to [17], a half of the world’s PC users never adjust the power management settings of their PCs for fear that it will impede performance. HewlettPackard [22] inspected the settings of 183,000 monitors worldwide and found that almost a 1/3 was not set to take advantage of the energy saving features. Enabling the features after 20 minutes of inactivity could save up to 381 kWh for a monitor and 294 kWh for a desktop PC per year. Although these energy-saving options are effective, they affect only usage energy and therefore are limited due to a large energy cost of manufacturing. To reduce energy embodied in PC, we must exploit approaches beyond the PC usage. A possible approach is to extend the usable life of PC through reuse. PC reuse saves energy associated with production of a new PC by putting the old PC in use for extended period of time. For example, if a user disposes his 4-year-old machine and buys a new machine, the cumulative energy grows by the energy of producing this new machine, as it shown in Fig.8. Now assume that delaying purchase of the new machine postpones its production. If both machines require the same amount of energy for production then reusing the old PC for 4 more years lowers the total energy by 31%. The key issues regarding reused PCs are the extent to which they replace purchase of new machines and the length of the second lifespan. An important aspect here is the decision whether to repair/upgrade and reuse components of the old PC or dispose the old PC, buying a new one. As PC lifespan widens, probability of failures increases. While HDD drives, soundcards, PSU, etc. are usually compatible among PCs, processors, motherboards, graphic cards, etc, have limited compatibility along PC generations. To sustain

the energy balance of the reuse, old components have to be used as spare parts for repair. Establishing an infrastructure capable of supplying used components to the users is quite a challenge. Another challenge is upgrading of PC. Unfortunately, up to now computers have been designed for performance, not reuse. Upgrading a PC frequently deals with problems related to software or hardware compatibility. Some solutions could be found in replacing Windows with an open source system. If memory and hard-drive upgradeability possible, obsolete computers can be turned into servers [23] to store movies, images, or songs or used as wireless routers [24].However, these are only particular solutions. We claim that to avoid the gross wastefulness of obsolete PCs, computers must be designed for Longevity, Upgradability, and Reuse. In contrast to traditional design that targets high-performance or low-power, design for reuse and long-life means solutions, capable of being upgraded and compatible through time. In that way, only small component parts of a computer would need to be replaced rather than the whole machine. For example, when technological advances create faster processors, it should be made easier to insert the new ones in place of the older ones rather than throwing out the whole computer or motherboard. The main features of the design for reuse are: 1. Modular structure to allow components to be removed and replaced. 2. Upgradeability of modules in time. Compatibility with hardware and software. 3. Eliminating glues and adhesives by using, for example, snap-in features. 4. Accessibility of modules with commonly available tools. The user can change the modules without any special tools. Many companies have the technological and financial resources to implement such changes; they often lack the political will. Hewlett-Packard, Dell, Fujitsu, Sony, Apple, et al, have been reported recently on efforts to change PC design for recycling. However, in contrast to reuse, the goal of design for recycling is different; i.e. to ensure clear, safe, and efficient mechanisms for recovering its raw materials. An example is Bamboo PC [25], whose box is made from wood to minimize recycling energy. Such a design maximizes PC ability to be recycled but not reused. Reuse is always the best option for disposal of PC, rather than recycling. It is cheaper, simpler and requires less energy. To “really” save PC energy, we must reuse the PC! We suggest that an old PC has to be reused until the production energy for a new PC is offset by the low cumulative energy of usage, upgrade of the old PC. This relation is shown in Fig.9. An old PC refurbished and/or upgraded with new components has to be used a certain time until a break point (T) is reached. After that recycling the old PC and purchasing a new one becomes a net benefit.

[3] Energy



Production energy of new PC




[5] Production energy of old PC


Reuse is a net benefit

Reuse is a net loss T


Fig.9: PC lifespan calculation

[6] [7] [8] [9]

4 Conclusions The purpose of this work was to determine the trend in energy consumption associated with PC and a possible approach to PC energy reduction. By analyzing production, usage and the end-of-life stages of modern PC in perspective to its 2002 prototype, we found that • The energy used for PC production has increased by 7% in comparison to the year 2002. • The modern computers consume more energy at fullload than the old ones while in a low-power mode they take less energy than the old computers. The total energy used by 2007 PC per year is almost same as it was in 2002. • The energy consumed at the end of computer’s life depends on recycling, upgrading and reuse options. • The total energy associated with modern computer is dominated by energy embodied at the production stage. Reducing computer lifespan leads to heavy loss of unused energy. • PC reuse is cheapest and simplest way to save energy and therefore must be applied as much as possible. • To ensure computer reusability, new design methodology (design-for-reuse) is needed.

[10] [11] [12]




[16] [17]



[20] [21]

5 References [1]


L.Webber, “Computer Use Expected to Top 2 Billion”, July 2007, available from E.Williams, “Energy Intensity of Computer Manufacturing: Hybrid Assessment Combining Process and Economic Input-Output Methods”, Environ. Sci. Technol, vol.38, pp. 6166-6174, 2004


[23] [24] [25]

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