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Several physical properties directly influence a processor's speed potential, but the die fabrication size of the processor's core circuitry is the most important. The core die size represents the actual physical distance between each trace signal route used to construct transistor pathways within the processor. A smaller die size means that the processor can generally operate at higher clock frequencies while using less voltage and producing less heat.
The current industry standard die size is .18 micron (µ), which represents a balance between electrical and thermal constraints, yet retains scalability beyond 1 GHz. Popular designs using the .18 -micron die size include Intel's Pentium III Coppermine and Pentium 4 Willamette, and AMD's Athlon Thunderbird.
A significant number of PCs still in use contain processors fabricated with the much older .25-micron core die size. These include processors like the Pentium II and K6-2. Massive cooling systems are needed when overclocking .25-micron processors, because these chips demand much higher voltage levels compared to those required by their .18-micron counterparts. It is difficult to scale these older designs beyond 600 MHz.
The core die size for the latest generation of processors, like Intel's Pentium III Tualatin and Pentium 4 Northwood, is the radically small .13 micron. These chips offer relatively low thermal dissipation rates (up to 50% lower than .18-micron models), as well as significantly lower core voltage requirements. Improved MHz scalability is the direct result of these advancements, and many .13-micron designs are expected to scale to 3 GHz and beyond before the next-generation fabrication process, which should be in the range of .7 to .9 micron, is introduced.
These overclocked chips need more power to keep them stable at extended MHz operating speeds. Current designs are built atop a split-voltage architecture. Core voltage represents the internal electrical properties of the processor and corresponds with the die size employed during fabrication. Input/output voltage represents the operational voltage of the processor-to-chipset bus. It usually includes the power levels of other front-side bus components within a traditional system configuration.
The thermal dissipation rate is a measurement of heat generated within an electrical circuit. The actual thermal unit employed in this measurement is watts. Assuming that the core die size remains consistent, the thermal rate increases proportionally to rises in operational speeds and core voltage levels. The processor's heatsink cooling mechanism is worth examining in any PC.
Most designs use a large metal heatsink coupled with a fan to provide a forced-air cooling system for maximum heat dissipation at a relatively low cost. Other cooling systems are available, including vapor-phase and thermoelectric technologies, but their associated costs are usually prohibitive for the average desktop PC user. Whatever the cooling system, efficient thermal regulation is an important factor in successful overclocking. If core temperatures exceed normal operating specifications, the system can become unstable. Circuits can also be damaged during prolonged periods of intense heat.
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