Intel Pentium II OverDrive

The original Pentium Pro suffered from extremely poor performance under 16-bit applications and even under situations where mixed 16/32-bit application code was present. This kept the Pentium Pro from excelling above the level of the Pentium, clock for clock, under Windows 95 and even under Windows 98. The problem was much less pronounced under Windows NT, which is entirely 32-bit native code, however one thing was for sure, the Pentium II OverDrive for Pentium Pro users had to address this issue.

By using the 0.25 micron Deschutes core of the Pentium II, Intel managed to put a quick end to the 16-bit weakness of the Pentium Pro, and in doing so, they also managed to double the amount of L1 cache on the processor. If you recall, the Pentium Pro, like the original Pentium processors, was only outfitted with 16KB of L1 cache split evenly between Data and Instruction Set caches. The Pentium II OverDrive, like the Pentium MMX, the Pentium II, and the Pentium III, features a full 32KB of L1 cache, once again, split evenly among its Data and Instruction Set caches (16KB + 16KB).

A problem with using the Pentium II core on the OverDrive processor was that there would be no way a Pentium Pro user would agree to making an upgrade to a processor whose cache ran at 1/2 clock speed while their aging Pentium Pro ran its L2 cache at full clock speed. Intel worked around this little problem by making the Pentium II OverDrive nothing more than a Pentium II Xeon on a much smaller scale, using the Pentium II's design to place the L2 cache externally off of the CPU, but on a card that would allow it to operate at the same clock speed as the CPU itself. Owners of the Pentium Pro 200 with 1MB of L2 cache may want to think twice before considering this upgrade, as the Pentium II OverDrive is outfitted with 512KB of L2 cache which should be an improvement for most Pentium Pro users as the 256KB processors were quite popular.

The Pentium II OverDrive, although officially offered in two flavors, a 300MHz and a 333MHz version, is essentially a single 333MHz processor with L2 cache that is intended to run at 333MHz. The chip has a built in clock multiplier of 5.0x, and it derives its clock speed based on your motherboard's set FSB frequency. It is because of this that owners of 150MHz and 180MHz Pentium Pro processors can stick the OverDrive in their systems and have it run at 300MHz, while 166MHz and 200MHz Pentium Pro owners can do the same and have a 333MHz upgrade. Since the 150/180MHz chips use a 60MHz FSB, and 60MHz x 5.0 = 300MHz, and since the 166/200MHz chips use a 66MHz FSB, and 66MHz x 5.0 = 333MHz, the processor works both ways. If you happen to be the owner of a 60MHz FSB Pro and if you happen to have a little knowledge about your motherboard, you're better off jumpering your board as if you had a 66MHz FSB processor (i.e. setting up your 150 as if it were a 166) and then sticking in the OverDrive processor for a small performance improvement.