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Intel Pentium with MMX Technology ("P55C")

In January of 1997 Intel released its newest, and presumably last, fifth generation processor, the Intel Pentium with MMX Technology. It is an evolutionary design, adding some enhancements and new capabilities to the "classic" Pentium. In most ways however, it is the same chip.

The chief (well-publicized) difference between the Pentium with MMX and the Pentium Classic is the inclusion of the MMX instruction set extensions. Running software that is designed for MMX on a Pentium MMX will result in a significant increase in performance over the regular Pentium; some claim up to 70%. Even running non-MMX software on the MMX chip results in an improvement of about 20% over a regular Pentium of the same clock speed, due to the following other improvements over the older chip:

• Doubled Primary Cache: The Pentium with MMX has 16 KB for each of the level 1 data and instruction caches, as opposed to 8 KB each for the regular Pentium.
• Improved Cache Mapping: The primary cache is now 4-way set associative instead of 2-way.
• Deepening of Internal Pipelines: Both of the internal integer executions units are increased from 5 to 6 stages.
• Better Use of Internal Pipelines: More types of instructions can be run in parallel down the two execution pipes than on the older Pentium, so more use is made of the second pipe.
• Improved Branch Prediction Unit: The branch prediction unit's accuracy is enhanced over the classic Pentium.
• Improved Instruction Decoder: The instruction decoder is more efficient than the Pentium's.

The Pentium with MMX is pin-compatible with the regular Pentium and goes in a Socket 7 just like the old chip. However, there is one important difference between the two: the Pentium with MMX uses split-rail voltage: 3.3 volts for the external voltage and 2.8 volts for the core. Not all motherboards with a socket 7 support this dual voltage setting, because before the Pentium with MMX came out no processors needed it, so some manufacturers skimped by not planning for the future. Intel is making OverDrives for motherboards that don't support the lower core voltage.

Warning: Some unscrupulous vendors are misleading buyers by telling them that the Pentium with MMX will run with 3.3 volts applied to both external and internal voltage. In many cases this will not instantly fry the chip; it may even work for a little while. Eventually however it will fail, and you could void the warranty on the chip.

The MMX capability is implemented on the Pentium with MMX by sharing the registers used by the floating point unit. This has led to silly rumors that using MMX would cause a "major" performance penalty when switching between applications using MMX and those using floating point applications. This is a myth; aside from the fact that very few applications make extensive use of the floating point unit, a switch between MMX and non-MMX applications takes no more time than a switch between floating point and non-floating-point applications. And the amount of time for a switch is less than 100 cycles on a processor that is usually running at 200,000,000 cycles per second or more.

Originally, the Pentium with MMX was supposed to be available only in 166 and 200 MHz versions, after which Intel was going to stop production on Socket 7 chips entirely. Intel surprised the PC world a bit a few months after the introduction of the original 166 and 200 chips by announcing a Pentium with MMX 233, running at 66 MHz and 3.5x multiplier. They now say that this will be the last Pentium, but who knows. :^)

Since socket 7 boards don't support a 3.5x multiplier, Intel made the chip respond to a 1.5x signal as if it were 3.5x. This is becoming a common trick, which I believe AMD used first in creating its 5x86-133 (which runs in 486 motherboards as 4x when configured as 2x). So to set up a Pentium with MMX 233, you jumper its system bus speed and multiplier (but not core voltage!) as if it were a Pentium 100.

Overall, the Pentium with MMX is a very good chip, and with Intel aggressively cutting prices it has become pretty much the mainstream processor of choice over the last year or so. In recent months, however, with new sixth-generation chips by AMD (the K6) and Cyrix (the 6x86MX) that will run in fifth-generation motherboards becoming commonplace, the high-end performance picture on fifth-generation motherboards has become much more cloudy. For its part, Intel has moved on to the Pentium II, and as a result the Pentium with MMX is being used on fewer and fewer new systems, in favor of the newer chip.

Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.

Next: Intel Pentium with MMX Technology OverDrive

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