486 - 486 586 - Pentium 686 - Pentium Pro As for the difference between a Pentium and Pentium Pro? The Pentium Pro has a 256KB secondary cache (a portion of high speed static memory that stores frequently used commands) built-in rather than as an external option as it is on the Pentium. Its built-in status means it can run at the processor's internal clock speed (200MHz on a 200MHz Pentium Pro) rather than at 66Mhz, the speed of the external bus. (The external bus is a collection of wires through which data is transmitted from one part of a computer to another). The Pro also has lots of internal improvements including: a deeper instruction pipeline, multiple arithmetic units, better branch prediction logic and register renaming. Without going into this too deeply and driving you absolutely insane with techno-terms, these features allow the processor to execute more instructions per clock cycle, making it zippier. When it runs 32-bit applications that are calculation-intensive, the Pentium Pro typically runs about 1.5 times as fast as a Pentium of the same clock speed (which is measured by the Mhz rating). Some of the Pentium Pro's drawbacks? Intel decided to optimize the chip for 32-bit rather than 16-bit code. So 16-bit DOS and Windows applications suffer a bit. You might remember from an earlier column that 16-bit code moves across the computer in chunks half as big as 32-bit code. A 200 Mhz Pentium Pro may only achieve the speed of a 150 Mhz or 160 Mhz Pentium with 16-bit software. Meanwhile, many DOS and Windows programs are already 32-bit so they benefit from the Pro.