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The Pentium II processor was Intel's first commercial P6 (also called 686 or 80686) core architecture offered to the general public. The P6 has internal RISC architecture and a CISC-RISC translator. Its design is based on the earlier, moderately successful, Pentium Pro processor. The most significant extension to the original design was the introduction of MMX (multimedia extensions) instruction support. Updates included the introduction of a slot interconnect interface, an integrated 512 KB Level 2 (L2) cache memory mounted directly on the slot circuit board, and various refinements for better cooling and heat dissipation.
The most controversial feature introduced in the Pentium II is full clock-multiplier locking, which severely limits overclocking potential. The earliest Pentium II chips lacked this locked design, but Intel integrated the needed circuitry within a few months. The locked multiplier forced enthusiasts to adopt front-side bus manipulation as a means of overclocking the Pentium II. The disadvantage of this approach is that any changes to the front-side bus speed lead to the subsequent overclocking of nearly all other system and subsystem components, ranging from memory to peripheral component interconnect (PCI) devices and accelerated graphics port (AGP) video cards.
The first Pentium II chips were manufactured with .28-micron circuit pathways. They require a core voltage of 2.8 volts and generate tremendous heat. These factors place intense demands on the power supply and motherboard voltage regulators; therefore, any increase above 3.0 core volts means that a heavy-duty cooling system must first be installed.
The extreme temperatures produced by .28-micron chips, combined with limited bus frequencies in the popular Intel LX motherboard chipset, lowered overclocking potential. The LX chipset was designed for 66 MHz front-side bus (FSB) operation; the maximum unofficial supported frequency is 83 MHz. This frequency can be useful for lower multiplier chips, such as the Pentium II 266 (83 □ 4.0 = ~333 MHz), but the danger of damaging system components becomes significant when other system bus rates change substantially.
The Intel BX chipset became the most popular motherboard choice after the release of 100-MHz front-side bus Pentium II models. Other 100-MHz chipsets, such as VIA's Apollo Pro133, debuted shortly thereafter. These new designs offered improved overclock flexibility, for both 66- and 100-MHz chip owners. The BX and Pro133 both ventured front-side bus support up to 133 MHz. Some companies extended the unofficial specifications to 166 MHz and beyond. The potential for overclocking became clearer as the new 100-MHz Pentium II models incorporated a much cooler and more efficient .25-micron circuit design. The ability to scale successfully above 400 MHz brought the Pentium II to the attention of the overclocking community.
When the .25-micron Pentium II was released, the primary concern became the bus limitations inherent in Intel's popular BX chipset. The BX design could effectively scale to 133 MHz while maintaining the proper FSB-to-PCI ratio, but the AGP bus suffered from a 2/3 divider. It would be overclocked to approximately 88.6 MHz, leading to a high failure rate for most early AGP video cards. (PCI video solutions were unaffected.) Competing chipsets from VIA, SIS, and ALI bypassed this limitation by adding a ½ AGP divider, though the early revision of these boards offered other minor compatibility and performance problems compared to Intel's architecture.
Front-side bus speeds near 124 MHz create a problem analogous to the 83-MHz issue. The standard divider of 1/3 generates a PCI bus frequency of approximately 41.3 MHz, thus threatening the stability of various PCI devices, especially hard drives. Some retail motherboards, designed specifically for overclocking, offered the ability to adjust the PCI divider manually at these "problem" frequency ranges. For example, with a ¼ divider at 124 MHz, the PCI bus reverts to ~31 MHz, a "safe" value, close to the default PCI v2.x specification of 33 MHz.
Restrictions on the processor heatsink's size and orientation also come into play, due to the Slot 1 processor-to-motherboard interface. The heatsink extends parallel to the surface of the motherboard, which imposes a size limit, as the heatsink surface must clear not only the socket, but all other motherboard components as well. Worse yet, many manufacturers have placed the processor slot close enough to the memory slots to block some of them. This factor clearly influences the size and type of heatsink you can use with the Pentium II.
Intel's standard cooling solution proves adequate for default operation, but this small heatsink often fails to deliver the dissipation needed to sustain successful overclocking at extended operating speeds. A more efficient aftermarket cooling solution is needed for any real return. The size of any replacement cooler should be considered carefully. As mentioned, some motherboards simply do not offer enough space for the larger coolers.
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