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AMD 5x86 (80486DX5)
The 486DX4-100 was the fastest 486 processor made by Intel before they decided to leave the fourth generation and concentrate on the Pentium. AMD took clock multiplying one increment further with what it calls the "5x86" chip. Despite the implication that it is a fifth-generation chip, it is not--it is a high speed, clock-quadrupled 486 processor, that runs in 486 motherboards. It does use a rather advanced 0.35 micron CMOS process (advanced for a fourth-generation chip that is).
The AMD 5x86 was made available in one speed only, 133 MHz, for use in 33 MHz motherboards. The processor runs at four times the system clock and fits into a Socket 3. Since having a clock multiplier of four was not part of the original Socket 3 design, AMD made the 5x86 look for a two times setting from the motherboard and interpret that as four times instead. In other words, to use the 5x86 you want to set the motherboard to the 2x setting. This will actually cause the 5x86 to run at 4x. The chip will actually physically fit into an older 486 socket such as a socket 1 or 2 or the original 168-pin 486 socket, but doing this requires a voltage regulator since the AMD chip runs at 3.3 volts.
AMD calls this chip the "5x86-P75" because it offers performance comparable to low-end fifth-generation chips. In fact, it is comparable in integer performance to a 75 MHz Pentium. It is also called the 80486DX5-133, which is a more accurate reflection of what the chip really is. Architecturally, the chip is virtually identical to the 80486DX4, except that it matches Intel's 16 KB level 1 cache (the AMD 80486DX4 has only 8 KB). Note that this is not the case with Cyrix's 5x86 chip, which is quite different.
The 5x86-133 is the most powerful 486-class chip available (the Cyrix 5x86-120 is actually faster but was discontinued by Cyrix many months before AMD stopped mass-producing the 5x86-133). The 5x86-133 has traditionally been an excellent choice for an economy PC, especially for home-builders, because not only is the chip itself very inexpensive, so are the motherboards that use it. With 200+ MHz Pentium and Pentium-compatible chips coming well down in price, the 5x86-133 is following the rest of the fourth-generation chips to obsolescence, quickly. However, since it is the "king of the hill" for 486 motherboards, it remains a good choice for those who want to upgrade their 486 systems without replacing the motherboard. The chips are still out there, though they are getting much more difficult to find now.
Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.
Next: Cyrix 5x86 ("M1sc")