MMX-- the new standard

by Alan Zisman (c) 1997. First published in Canadian Computer Wholesaler, February 1997

Backward compatibility is either a blessing or a curse? it means that your new technology can still work with your old data, hardware, and software. That?s good. But it also means that you may be unnecessarily tying yourself to the old generation?s limitations. That can be bad.

CPU king, Intel, has been following the backwards compatibility route ever since the popularity of the original, 1981 IBM PC, using Intel?s processor. Even five generations of chips later, Intel?s x86 family of chips still shows its ancestry. Even the limited number of new processor instructions included in 1985?s 80386 chip are only now being included in popular 32-bit operating systems.

So when Intel decides to upgrade the instruction set, the collection of commands built into the processor, it?s news. In this case, the result is the MMX family of processors?replacements for today?s Pentiums and Pentium Pros, with new instructions optimized for multimedia. Computers including the first of the MMX family, the P-55C replacement for the standard Pentium should be just becoming available as you read this, following the official January 8th unveiling.

A P-55C machine will run even old applications a bit faster than a standard Pentium running at the same clock speed. But this modest (5- 10%) speed increase is primarily a result of the larger, 32k Level 1 (L1) cache built into the new design. As well, the MMX family offers new tricks, such as Single Instruction, Multiple Data (SIMD), which lets the CPU run operations in parallel. But to take advantage of these new features, software will have to be specially written. At the moment, virtually nothing is available that makes full use of the MMX instruction set, but Intel claims that properly optimized applications will be able to provide a performance boost of up to 60%.

As well, while the standard, P-54C Pentiums and the new, MMX P-55C Pentiums are pin-compatible, you can?t simply pop the new chip into an old motherboard?you need control chipset (from Intel, natch), and a BIOS that?s designed for the new CPU. The newest motherboard and BIOS designs anticipate this, however, and can be set for either Pentium model.

Users of older machines need not feel left out- Intel has announced plans for a series of MMX Overdrive chips to upgrade original Pentiums. Using clock-multiplying technologies, these will speed up the systems at the same time as adding the multimedia capabilities. These Overdrive chips will require an upgradeable motherboard, but will include built-in voltage regulators, so that they can convert older, 3.3 volt systems to the new 2.8 volt standard. The initial P-54TCB Overdrive processor will upgrade Pentium 75, 90, and 100 mhz models to speeds ranging from 125 to 166 mhz. Later models will be made available to upgrade more recent Pentiums to speeds up to 200 mhz. While Intel will be making these available through retail channels, there are no plans to make these chips, available later in the first half of 1997, for OEM sales.

Further down the road are other members of the MMX family. Intel is planning a version to replace the current, high-end Pentium Pro, code-named Klamath (a river in Intel?s home state, Oregon). Klamath will debut at 233 mhz, but will entail major changes in the Pentium Pro design. The current processor is really two chips in a single package?the actual CPU and a built-in Level 2 (L2) cache. Klamath will remove the L2 cache, resulting a chip that will sell for less, but will deliver less performance, even with the beefed-up 32 kb L1 cache. It?s expected that the release of Klamath will drive down prices for P-55C machines, with these Pentium Pro models replacing them as entry-level machines by the end of next year. Around the same time, look for P6T?Pentium Pro upgrade chips, expected to run at around 300 mhz.

And following Klamath, Intel will be bringing us Deschutes (also a river in Oregon), a low voltage, high speed (starting at 300 mhz) processor line that will be aimed to mobile computing. But notebook users won?t have to wait until then to take advantage of MMX technology?a variety of companies were showing demonstration machines at last Fall?s Comdex show, running 166-200 mhz P-55C CPUs; such notebook designs should be widely available by mid-1997.

Intel?s rivals won?t be left too far behind, however. Both Cyrix and AMD have announced their own, multimedia-enhanced CPU models that will be competitive with Intel?s designs. AMD?s K6 chip and Cyrix?s M2, for example, will both be offering a 64 kb L1 cache, double Intel?s offering (and quadrupling the cache on a standard Pentium). Each will also offer the MMX instruction set, and will be pinout compatible with standard Pentiums, allowing them to be usable in current motherboard designs (with appropriate BIOSs). The Cyrix chip will run at 2.5 volts (compared to 2.8 volts for Intel?s design), making it potentially a contender for mobile computers.

Intel and its competitors clearly hope that the MMX-enabled chips will provide next-generation power to the 80x86 family of processors?speeding up 3D and graphics rendering. This is vital for CAD and graphics professionals, and early reports suggest than an MMX processor may actually run Adobe PhotoShop faster than current high end PowerMacs. Even more important is the gaming market, which drives the bulk of home purchases. Intel is planning to encourage manufacturers to clearly label MMX computers, and is planning a major advertising campaign, reminiscent of its ?Intel Inside? push.

But there is an important flaw in the MMX strategy.

When using the MMX registers of the chip, the CPU has to turn off the floating-point registers?both can?t be used at once. This is needed to maintain compatibility with existing operating systems. And switching modes like this takes time?about 50 clock cycles. As a result, Intel suggests that application developers refrain from using floating-point operations as much as possible. This, however, may prove a problem, especially working with 3D. (Cyrix claims that its model can switch modes in a single clock tick).

As well, this causes potential problems for multitasking. Try running an MMX-optimized graphics application and (for example), a spreadsheet using floating-point calculations at the same time. Performance will suffer because of the frequent switches between floating-point and MMX modes, but as well, the spreadsheet may end up with the wrong data, as graphics data mixes with its calculations. Better hope those engineers aren?t playing games at the same time as calculation nuclear reactor designs!

Only time will tell whether this turns out to be a major limitation?certainly, floating-point calculations are rarely used by most traditional applications; even your spreadsheet will correctly calculate your tax return without ever once needing to use floating-point math.

Despite these potential problems, count on MMX-enabled designs to be the industry standards, from 1997 through to the end of the decade.