Inside the Pentium D 805

The Pentium D 805 is based on the first Intel dual core processor, the Pentium D with the Smithfield core. Its predecessors in this family were rated at clock speeds of 2.8 GHz (D 820) to 3.2 GHz (D 840). Both cores in this CPU family come equipped with a 1 MB L2 cache, whereas the most current dual core processors in the 900 series make 2 MB available to each core. For the last year, Intel has brought no new models in the 800 series to market, because the company has switched its fabrication from a 90 nm process to a 65 nm one in the meantime, and has used this smaller building block size only for processors in the 900 series. But then out of nowhere, the old Smithfield core put in another appearance in the form of the Pentium D 805.

The Secret Of The Multiplier

The multiplier expresses the ratio between the processor clock speed and the FSB clock. For the Pentium D 805, the combination of FSB and processor clocks results in a multiplier value of 20x. By comparison with other CPUs with 200 MHz or 266 MHz FSB, this is a very high value - 12x and 14x are much more common. But this also makes the Pentium D 805 a good candidate for overclocking. Simply by raising the FSB clock rate to 200 MHz, the resulting CPU clock rate rises to 4.0 GHz once the 20x multiplier is applied (20x times 200 MHz equals 4.0 GHz).

How Is Breaking The 4 GHz Barrier Possible?

A closer look at the processing cores in the 805 CPU reveals that the device is fabricated with a B0 product code (stepping). The first core in the 800 series on which this CPU is based was fabricated using the older A0 version instead.
There aren't any significant differences between the two versions when it comes to energy saving functions or instruction sets, but it's easy to recognize from the B0 version that the Pentium D 805 really belongs to the second production series for this CPU. The motivation for a version update usually comes from further optimizations to the circuit layout or fabrication process, as well as corrections for design or behaviour errors. On all counts, the B0 version is an improvement over the older A0 version. Since this version could handle CPU clock rates of 3.2 GHz at the beginning of the series without any problems, a consequence of these improvements is that all B0 version CPUs should be able to handle CPU clock rates of at least 3.2 GHz. This was the theory we formulated before we started testing the Pentium D 805.
Processor Clock Rate Version sSpec Number
Pentium D 840 3.20 GHz A0 SL88R
Pentium D 830 3.00 GHz A0 SL88S
Pentium D 820 2.80 GHz A0 SL88T
Pentium D 840 3.20 GHz B0 SL8CM
Pentium D 830 3.00 GHz B0 SL8CN
Pentium D 820 2.80 GHz B0 SL8CP
Pentium D 805 2.66 GHz B0 SL8ZH


Which Memory Clock Speed Is Most Suitable?
The Pentium D 805 works with a front-side-bus clock rate of 133 MHz (533 QDR). Thus, the design of the chipset dictates a maximum memory clock speed of DDR2-533. But overclocking the front side bus also raised memory clock rates; the actual memory clock rate that results from increasing the FSB clock rate may be calculated using a memory multiplier value. Typical Intel platform memory multipliers vary from 2.0 up to 4.0, and may be set as such. Because our overclocking starts from a very low FSB clock speed, a special set of circumstances emerges for the memory multiplier. Namely, for an FSB clock set between 133 MHz and 148 MHz, the only valid memory multiplier values are 3.0 and 4.0 - lower values make no sense.
FSB (MHz) Multiplier Bandwidth
133 - 148 3.00 DDR2-400 up to 444
133 - 148 4.00 DDR2-533 up to 592
149 - 266 2.00 DDR2-298 up to 533
149 - 266 2.66 DDR2-396 up to 710
149 - 266 3.00 DDR2-447 up to 800
149 - 266 3.33 DDR2-496 up to 888
149 - 266 4.00 DDR2-596 up to 1066
We assume a multiplier of 4.0 is set, which raises memory clock rates for DDR2-533 for FSB clock rates of 133 MHz to the value consistent with DDR2-667, when the FSB clock is raised to 166 MHz. After that, as you increase the FSB clock stepwise up to 200 MHz, you'll also increase the memory speed up to DDR2-800. If you continue to increase FSB clock rates, you'll also set memory clock rates significantly higher as well.


Some motherboard vendors don't lay out their products to use very fast DDR2-1066 RAM; the reasons for this have to do with the components they choose to populate their boards. These must be of higher quality to deliver clean signals to memory at such high speeds, which makes motherboards more expensive. Even printed circuit board designs play a significant role: data lines designed to run at 500 MHz can be placed very close to one another with lengths of up to almost 4 inches (10 cm). A bad design can cause capacitance build-ups and resonance to develop among circuits, which in turn can cause instability or crashes when clock rates are cranked up.
At FSB clock rates below 149 MHz, memory multipliers lower than 3.0 are not available, because they make no sense in terms of the performance they deliver. With an FSB clock rate of 133 MHz, for example, a 2.0 multiplier produces memory speeds of only DDR2-266.
The various memory multiplier values available in the BIOS vary by motherboard vendor. The 4.0 value isn't always selectable, so buying the wrong motherboard can lead to performance degradation. In most cases, vendors refer to a memory multiplier of 4.0 as "Native DDR2-800."
Keeping Cool When Power Consumed Tops 150 Watts at 4.1 GHz
The dark side of overclocking a CPU is the heat produced when consuming enormous amounts of power. Since our Pentium D 805 CPU was built using 90 nm fabrication technology and implements a dual core design, power consumption at clock speeds over 4 GHz climbs to extreme levels.
According to the "Platform Compatibility Guide", this CPU consumes 95 W of power at its standard clock rate - the 05A abbreviation that appears on the packaging represents the PRB0 specification. Of course, this also means that the CPU cooler that's bundled with this processor is designed only for such relatively modest power consumption levels.

At a standard CPU clock speed of 2.66 GHz, Intel specifies maximum power consumption of 95 W. On a purely mathematical basis, this produces theoretical power consumption levels of 142 W at 4 GHz.

Conclusion: The 4.1 GHz Dual Core Delivers Peak Performance For Pocket Change
Those who first break this bit of news to their circles of friends can count on some surprised looks - in fact, they might even find their credibility getting questioned! But it is true: a cheap CPU that costs $130 outperforms the fastest processors from AMD (Athlon 64 FX-60) and Intel (Pentium Extreme Edition 965), each of which costs over $1,000.
We bought an Intel Pentium D 805 from a local retail outlet and overclocked it up to 4.1 GHz, even though this part runs by default at just 2.66 GHz. This represents a heretofore unattained clock rate increase of just over 54 %, for which only some additional cooling is required. The secret is in the FSB clock rate, which is raised from 133 MHz to over 200 MHz; the system remains completely stable, because modern motherboards with Intel 9xx chipsets are laid out to handle FSB clock rates of up to 266 MHz. In language that overclocking enthusiasts will love to hear, the Pentium D 805 ascends to the throne as the new King of overclocking, knocking out the AMD Opteron 144.

Here's how the system looks after being reworked: a pumped-up Pentium D 805 outperforms the flagship processors from AMD and Intel that cost more than $1,000. Talk about an investment with an immediate payback!
A short while ago we tested Intel's latest flagship processor, the Pentium EE 965 (Extreme Edition), which costs nearly $1,100 at retail outlets. Even this CPU, which still isn't available at too many locations, has to surrender first place to this stealth candidate. Things look the same for the top-of-the-line AMD processor, the Athlon 64 FX-60, which also fell behind in most of our benchmarking categories.

Labels: Technology

This entry was posted on Wednesday, May 24, 2006 at 2:35 PM. You can skip to the end and leave a response.