Pentium II Overdrive

In the days of the 486, the Overdrive socket caused quite a stir, but the concept never delivered on its promise. The Pentium 83 Overdrive (reviewed issue 14, p145) was held back by the 486's 32-bit Socket 5 and consequently had a hard time outperforming the faster 486s. However, Intel hasn't lost faith in the idea, hence the release of the Pentium II Overdrive. This upgrades Pentium Pro 166MHz or 200MHz chips to 333MHz, and 150MHz or 180MHz chips to 300MHz, although you'd be best advised to jumper your motherboard for a 200MHz to get the full 333MHz benefit.

The Pentium II Overdrive fits directly into a Pentium Pro Socket 8. Although Slot 1 is essentially a different, 242-pin form factor from the 387-pins of the Pentium Pro, putting a Pentium II on a Pentium Pro motherboard is much less of a compromise than a Pentium on a 486 board. In fact, the first Pentium IIs arrived on motherboards sporting the same 440FX chipsets as Pentium Pros. The Pentium II has essentially the same core design as a Pentium Pro too, the major differences being that it has MMX instructions and half-speed Level 2 cache that's off-die on separate chips. The Overdrive, however, has its 512Kb of Level 2 cache back on-die and back at full speed, just like a Pentium Pro.

In other words, the Pentium II Overdrive, unlike its predecessors, isn't a poor man's upgrade. It's also very easy to install. The package comes with a utility you're supposed to run before and after upgrade, but this makes no changes to your system, merely testing that it's working correctly. In fact, the results can be misleading. The utility claimed my new system was running at 375MHz after upgrade, which wasn't corroborated by Windows NT diagnostics or benchmark results. As the Overdrive comes with heatsink and fan already attached, all you have to do is remove the Pentium Pro and its fan, hook up power for the Overdrive fan and slot it in. There have been reports of BIOS chips not recognising the Overdrive's 512Kb of Level 2 cache properly but I had no problems on the Micronics W6Li board I was using. The CPU was called a 75MHz Overdrive at bootup but worked perfectly.

Our test system had 128Mb of EDO RAM, dual-CPU sockets, a 4.3 IBM UltraStar hard drive, Hercules Terminator 3DGL graphics and was running Windows NT 4 Workstation. I ran the standard PC Pro benchmarks once with a Pentium Pro 200 installed and once with a single Overdrive 333MHz, after which I tested with the LightWave Asteroids render with a Pentium Pro 200, and both single and twin Overdrives. The 51 per cent increase in benchmark performance after upgrade is about what you'd hope for from a 67 per cent increase in CPU clock speed. And the overall result is about what you'd expect for a Pentium II/333, showing no loss from the non-Slot 1 interface. As for the LightWave Render, this showed a 41 per cent reduction in render time with the 333MHz - again, a commendable performance increase.

With such encouraging performance results, the question of whether the Overdrive is worth buying depends on price and hassle. Upgrading to a Pentium II motherboard and 333MHz CPU will cost you under £300 and has the bonus of supporting AGP graphics and UltraDMA. However, it will also mean major work inside your PC with a screwdriver, as well as the purchase of replacement SDRAM for your EDO. The chances are that your power supply will need to be upgraded too if you opt for a 440BX motherboard. An ATX 2.1 supply is required to support the 5V suspend mode of 440BX chipsets. Overall, there's not much in it in terms of price. On the other hand, as long as your BIOS supports the Pentium II Overdrive correctly, upgrading via this route takes about five minutes and should entail no extra purchases and no reconfiguration of other peripherals.