Double your pleasure. (clock doublers and triplers) (Hardware Clinic)
by Mark Minasi

The eternal question among computer users who bought machines in the late 1980s is, What can you do with an old 386 to jazz it up a bit?

Get a new motherboard. If you've been reading this column for the past few years, you know that I always recommend buying generic PCs so that motherboard upgrades are an option; if you buy from a maker of proprietary computers, then your computer will have an oddly shaped case and motherboard, rendering it incapable of accommodating a replacement motherboard.

But even if you can put a new motherboard in your computer, it'll cost about $300 to $800 for the board itself. And then you'll probably have to buy new RAM because the faster processor on the new motherboard will probably require it. Add the cost of eight 1MB SIMMs, and the upgrade price goes up about $250, making the total upgrade cost a minimum of $550 to $1,050. For turning your old 20-MHz 386 into a 486DX-266, $1,050 isn't a bad price. It may not be within your budget, however.

A less expensive answer is to replace your CPU with a clock-doubling chip.

The notion of a clock double first appeared in 1991, when Intel announced the OverDrive and the 486DX2 CPU chips. Computer buyers want greater and greater speed, and one of the most important determinants of computer speed is the CPU speed. Consequently, there is ongoing pressure on Intel and other chip companies to turn out faster and faster CPUs.

While CPU speeds have increased tremendously, peripheral speeds have not. Most PCs have a lopsided design that makes the wisdom of buying ever-faster processors a bit suspect. For example, I have a 486DX-50 on my desk. Let's compare it to a 1984-model XT. Speed benchmarks tell me that my CPU is 68 times faster than the XT's, and it can do floating-point calculations 1214 times faster than the XT. Let's contrast that with another area of computer technology--printing.

My laser printer prints about 270 characters per second (eight pages per minute, about 2000 bytes per page of text), in contrast to the 35-cps Spinwriter I used in 1984. As a result of the improvement in printer technology, my system is able to print about eight times faster than the XT.

Now consider mass storage. The XT's hard disk had a seek time of about 90 ms and a data transfer rate of about 150K per second. My more modern PC (which was, by the way, less expensive in 1993 dollars than the XT was in the more valuable 1984 dollars) has an access time of 8.9 ms and a data transfer rate of 1575K per second. Both printing and mass storage technology have gone through approximately a tenfold improvement over the past decade.

In short, the newer PC has an engine that can put out about 70 times the horsepower, but its tires--the printer and hard disk--can only let it drive about 10 times faster. If we were looking to make a faster PC, we'd be better off if we got a disk drive with a 1.4-ms access time and a data transfer rate of about 10MB per second, and a printer that turns out 100 pages per minute.

Before you point out that you have a disk drive that shows a 0.4-ms access time and a data transfer rate of 6MB per second, look again. If your disk drive reports those statistics, then it probably has been equipped with some kind of cache. Caches render the results of disk benchmark programs nonsensical.

If you want your computer to be faster, you should be spending your money on faster peripherals rather than faster CPUs. Having gotten that out of the way, however, let's see what your options are for getting faster CPUs into an existing system.

The best-known options are the Intel clock-doubling chips mentioned earlier. They are only usable in existing 486DX systems. As Intel built faster and faster CPUs, it became a victim of its own success. Why? A system's motherboard must run at the CPU's speed. The difficulty of building motherboards increases exponentially as speeds rise, so moving from 12-MHz motherboards to 25-MHz motherboards is much easier than moving from 25-MHz motherboards to 50-MHz motherboards. Consequently, Intel's announced intention back in 1989 to offer a 100-MHz 486DX "soon" caused engineers to worry. Could they build a 100-MHz system to wrap around the new chip?

The answer to this conundrum seemed to be a clock doubler--a chip that could run at high speeds but on slow motherboards. It accomplished this by communicating with the outside world (the motherboard) at one speed, then performing internal operations at twice that speed.

For example, the first of these chips released was the Intel 486DX2-50, which communicates with a motherboard at 25 MHz but operates internally at 50 MHz. Specifically, an operation that does not require outside data access, such as adding two numbers that are already contained in the processor's registers (registers are very small areas of memory in the CPU), executes at 50 MHz. An operation that requires outside access, such as writing or reading information to or from memory (including the program currently operating), happens at only 25 MHz.

You might think that since a normal CPU constantly accesses memory to read programs and data, most of the operations would end up taking place at 25 MHz, and very few would run at 50 MHz. That would be true except for the existence of a small area of memory called a processor cache (which is generally 8K). Often-used code or data can be kept in the internal cache, allowing small programs to run entirely inside the CPU at 50 MHz. (Communication to the external processor cache--the 46K or 256K of cache referred to in the computer ads--still operates, unfortunately, at 25 MHz.)

The beauty of the DX2 chip is that you can take an existing 25-MHz 486DX, replace the CPU with a 486DX2, and instantly get a speed boost. It won't be a 100-percent speed boost, but the 60- to 80-percent improvement you'll see isn't bad. The processors aren't inexpensive, however. A 50-MHz DX2 costs around $350 (street price) and a 66-MHz DX2 costs around $500.

Don't confuse a 486DX-50 and a 486DX2-50; the DX-50 is a 50-MHz chip that requires a 50-MHz motherboard, while the DX2-50 is a hybrid that runs at 50 MHz internally but only at 25 MHz on the motherboard. The DX2-66 will instantly upgrade any 33-MHz 486DX machine. Unfortunately, Intel doesn't offer an upgrade for the DX-50; there is no DX2-100.

For those with 386DX systems, there are clock doubler chips as well, but not from Intel. These chips come from Cyrix, makers of clones of Intel processors. The 386 clock doubler comes in two varieties, a 386DX doubler and a 386SX doubler, respectively called the 486DRx2 and the 486SRx2. These are not 486s, and they don't convert your 386 to a 486, no matter what the writing on the outside of the box says. There's no math coprocessor and no 8K cache. There is a 1K cache built into these chips, which helps speed them up. That cache requires a driver to wake it up, so you have to install cache support software along with the chip.

Installing the DRx2 is pretty easy. The 386-to-486 kit comes with a chip puller that makes extracting the old 386 remarkably easy. (I say remarkably easy because previously I've removed CPUs with a small screwdriver and a lot of patience. You see, you pry up one corner a trifle, then you move on to the next corner and pry it up a trifle, and so on. It works, but the chip puller is better.)

What about installing an upgrade for a 386SX, though? Most 386SXs are surfac-mount soldered right onto the motherboard, so no chip puller will be of help here. Cyrix has packaged its 386SX upgrade with a nifty piggyback package: You needn't remove the old CPU; you need only clip the CPU upgrade right on top of the existing chip. It's pretty easy. The result for a 386SX or 386DX is an improvement of about 50 to 80 percent, depending on which benchmarks you run. You can find the Cyrix upgrade kits from mail-order houses for about $250-$300, depending on what speed chip you need. At this writing, there are only clock doublers for 16-MHz and 25-MHz 386 systems (you can use the 25-MHz upgrade for 20-MHz systems); if you've got a 33-MHz system, then you'll have to wait until the 33-MHz clock doubler ships, and if you've got a 40-MHz 386, then you're out of luck, at least for the moment (but keep your eyes open). By the way, in most cases your current math coprocessor will continue to work fine, if you've got one, but some really old 387DX chips may not work with the doublers. If you've got an old 16-MHz 386 system, call Cyrix to find out if you'll have compatibility problems.

Intel is not letting grass grow under its feet. It will soon ship a clock tripler, accelerating 25-MHz 486 systems to 75 MHz and 33-MHz systems to 99 MHz (the ads will say 100 MHz). Despite the fact that it's a clock tripler, the name of the chip is the 486DX4. I can't wait to pop one in my 33-MHz 486DX server and watch it fly.

There's more to the DX4 than just clock tripling. For one thing, Intel went back to the drawing board and redesigned much of the chip from scratch, yielding a chip that runs cooler than the DX2. That's particularly important for laptop users: I know of some DX2 laptops that have actually had keys on the keyboard melt because of the heat of the processor. Further, the DX4 doesn't have 8K but rather 16K of internal cache.

Should you buy a doubler or a tripler? It depends on what you're doing, and what your system looks like currently. If you're doing graphical things, then spending $300 to upgrade to a very fast video card may be a better buy. If you're running Windows on a 4MB machine, then upgrading to 8MB will probably produce better speed improvements. But for a kick in raw processing power, it's probably worth getting the doublers. They're reasonably priced and very compatible.