インテル 486DX2 CPU

The Intel 486DX2, introduced March 3, 1992, was the first clock-multiplied x86 CPU. It was a clock-doubled version of the earlier 486 CPU. A DX2 ran at speeds of 50 or 66 MHz, using a 25 or 33 MHz front side bus. It was pin-compatible with the earlier 486 CPUs, using the same 168-pin socket, but the use of a clock multiplier let it run at double the clock rate, yielding a 50-70 percent speed improvement over running the CPU at the bus speed. Much of the speed gain came from taking advantage of the 486’s on-die 8KB L1 cache.

The 50-MHz Intel486 DX2 cost $550 each in 1,000-pieces quantities at the time of introduction.

Borne from a compromise

The 486 had difficulty running at a bus speed of 50 MHz. Introducing a clock multiplier was a compromise to ramp up performance while maintaining stability. A no-compromises 486DX-50 wasn’t possible to build, partially because the VESA local bus couldn’t run reliably with more than one add on card at 50 MHz. You had to choose between IDE or video on the local bus. At 33 MHz, it was possible to have both.

The Intel 486DX2 was also available as an upgrade chip for existing owners. Intel marketed them as an Overdrive processor and sold them in retail packaging.

AMD and Cyrix both made their own versions of the 486DX2 to compete with Intel. The AMD version was close to Intel in performance, within one percent. The Cyrix version was about 10 percent slower.

Intel did not initially produce a clock-doubled 486SX. They introduced a 50 MHz 486SX2 without a coprocessor after AMD introduced one. AMD’s SX2 topped out at 66 MHz.

486DX2 P24 vs P24D

There are two major versions of the Intel 486 DX2: P24 and P24D. The original P24 version offered only the slower write-through cache mode. The P24D version has a faster L1 cache mode, called write-back, that improves performance. To identify them, look for the marking SX954 or SX955 on the line above the copyright date on the top side of the CPU.

486DX2 P24D CPUs will return CPUID 0436h when the L1 cache is in write-through mode, and CPUID 0470h when in write-back mode.

Late model 486 motherboards included support in the BIOS to enable writeback mode. Earlier systems without that support need a utility to enable write-back mode for better performance.

The difference is slight. In DOSBench, for example, the improvement is about 2.44 percent. In Doom, the difference is only one percent. If you have a motherboard and CPU combination that supports it, it’s worth enabling, but the difference isn’t enough to be worth expending too much effort to find a P24D over a regular P24.

The 486DX2 as a mainstream CPU

The Intel 486 DX2-66 is a classic CPU for running MS-DOS. Although it was only the fastest x86 CPU on the market for about a year, having been overshadowed by the Pentium when it was released in late March 1993, the DX2 was much more cost effective than the Pentium. It took about three years for games that ran significantly better on a Pentium than on a DX2-66 to appear.

I have seen assertions that the 66 MHz version was much more popular and sold much better than the 50 MHz version. It is completely true that the 66 MHz version gives a better overall performing system because of the 25% faster bus speed. But this wasn’t something that everybody understood in the mid 1990s. Some of the salespeople I worked with didn’t understand the concepts of clock doubling or bus speed, let alone the consumers we sold to.

I will agree that late in the 486’s life, the 66 MHz version was much more popular than the 50 MHz version. But when the price difference was $100 or more, the 50 MHz version was a popular choice for cost conscious customers. When someone asserts that nobody bought the 50 MHz version of the 486DX2, that tells me their perspective starts in the late 90s and they aren’t considering the first half of the decade.

There was a time when the 486DX2-50 made a lot of sense.

In defense of the 486DX2-50

When I sold computers at Best Buy, we had no difficulty selling 50 MHz DX2s at all. The 50 MHz DX2 was still 25% faster than a 33 MHz system.

It’s also important to remember that there was a price difference between the two systems. In ads I found in the January 10, 1995 issue of *PC Magazine*, the price difference was over $200. It was very tempting to buy the slower system and just save the difference, or buy the slower system and buy more RAM or a larger hard drive.

The 50 MHz DX2 had an important place in computer manufacturers’ lineups. When the DX2 hit the market, Intel phased out the 25 and 33 MHz 486DX models, keeping the SX models in place. That meant after supplies of 33 MHz 486DXs ran out, the cheapest 486 with a math coprocessor was the DX2-50. Some people bought 50 MHz systems for that reason alone. The DX2-50 wasn’t a low-end CPU, it was a midrange CPU for most of its life.

David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.