Am5x86

Lore & Background

The Am5x86, also known as the 5x86-133, X5-133, and sold under third-party labels such as Kingston Technology's 'Turbochip', is an Enhanced Am486 processor with an internally set multiplier of 4, allowing it to run at 133 MHz on systems without official support for clock-multiplied DX2 or DX4 processors. Like all Enhanced Am486 chips, it featured write-back L1 cache, and unlike most, a generous 16 KB rather than the more common 8 KB. A rare 150 MHz-rated OEM part was also released. Since a clock multiplier of four is not part of the original Socket 3 design, AMD made the 5x86 accept a 2x setting from the motherboard and instead operate at 4x; the motherboard must be set to the 2x setting. The chip physically fits into older 486 sockets such as Socket 1 or 2 or the original 168-pin 80486 socket, but requires a replacement voltage regulator because it runs at 3.45 volts.

The combination of clock speed and the relatively large 16 KB write-back L1 cache allows the 5x86 to equal or slightly exceed an Intel Pentium 75 MHz processor in integer arithmetic in benchmarks. Real-world performance varies, with later Windows operating systems and many FPU-sensitive games favoring the Pentium 75 MHz. Because it is based on a pure 486 design, it is compatible with older systems, something its slightly faster rival, the Cyrix Cx5x86, has trouble with. The CPU is commonly overclocked to 160 MHz, giving performance similar to that of a Pentium 90 MHz system. There are four main versions of the socketed CPU: the common ADW variety, and the later ADY, ADZ, and BGC. Later models were preferred because they were rated for higher temperatures and thus more forgiving of overclocking.

The Am5x86 made the first-ever use of the controversial PR rating. Because the 5x86 is the equal of a Pentium 75 MHz processor in benchmarks, AMD later marketed the chip as 'Am5x86-P75'. Sales of the Am5x86 were an important source of revenue for AMD at a time when lengthy delays in bringing the AMD K5 to production were threatening the company's profitability.

Reader's Guide

AMD manufactured the Am5x86 processor for ordinary PC systems until 1999. It was popular for entry-level desktop systems, appeared in many notebook models, and sold separately as an upgrade processor for older 486 systems. Several companies made upgrade kits with an AMD 5x86 including a voltage regulator and socket converter, allowing its use on virtually every socketed 486 motherboard ever produced. Several companies also provided upgrades for older 486 notebooks by replacing soldered 486 CPUs. The chips were even used on later Acorn RiscPC 'PC card' second processors. The RiscPC's OpenBus memory interface was only 32-bit, meaning the Pentium could not be easily interfaced to it. Intel's expensive Pentium Overdrive for 486 systems is a troublesome CPU with many compatibility issues, so it was not used; the 5x86 therefore provided the acme of RiscPC Windows performance. The chip remained in production for a long time as a popular choice for embedded controllers. One derivative of the 5x86 family is the core used in the AMD Élan SC520 family of SoC microcontrollers, which powers the original Cisco PIX 501 model.

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