Am5x86

The Am5x86 processor is an x86-compatible CPU introduced in 1995 by AMD for use in 486-class computer systems. It was one of the fastest, and most universally-compatible upgrade paths for users of 486 systems.

Introduced in November 1995, the Am5x86 (also known as 5x86-133, Am5x86, X5-133, and sold under various 3rd-party labels such as "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 486 processors. Like all Enhanced Am486, the Am5x86 featured write-back L1 cache, and unlike all but a few, a generous 16 kibibytes rather than the more common 8 KiB. A rare 150 MHz-rated part may have also been released by AMD.

Since having a clock multiplier of four was not part of the original Socket 3 design, AMD made the 5x86 accept a 2x setting from the motherboard and instead operate at a rate of 4x. When using an Am5x86, the motherboard must be set to the 2x setting. The chip will actually physically fit into an older 486 socket such as a socket 1 or 2 or the original 168-pin 80486 socket, but doing this requires a replacement voltage regulator, since the AMD chip runs at 3.45 volts.

The combination of best-in-class clock speed and the relatively large 16 KiB write-back L1 cache allowed the 5x86 to equal or slightly exceed an Intel Pentium 75 MHz processor in integer arithmetic performance. Also, because it was based on a pure 486 design, it was compatible with older systems, something its slightly faster rival, the Cyrix Cx5x86, had trouble with. The CPU was commonly overclocked to 160 MHz, thereby giving performance similar to that of a Pentium 90 MHz system. Although it has been reported that individuals have successfully run the chip at 200 MHz, this would seldom have been achievable due to the rarity of VLB video cards and motherboards (especially the cache RAM) that could cope with a 50 MHz system bus. The image of the Socket 3 version above, of type ADZ, was in fact the preferred version of the chip because it was rated for higher temperatures and thus more forgiving of overclocking.

The Am5x86 is also notable for the first-ever use of the controversial PR rating. Because the 5x86 was 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.

AMD manufactured the Am5x86 processor for ordinary PC systems until 1999. It was popular for entry-level desktop systems, appeared in many different notebook models, and also sold separately as an upgrade processor for older 486 systems. Several companies made upgrade kits which packaged an AMD 5x86 with a voltage regulator and socket converter, which allowed its use on almost any 486 motherboard ever produced. The chips were even used on later Acorn RiscPC "PC card" second processors. The RiscPC's OpenBus only supported a 32-bit memory interface, which meant that the Pentium could not be easily interfaced to it. Intel's expensive Pentium Overdrive for 486 systems was a troublesome CPU, with many compatibility issues, and so was not used. The 5x86 therefore provided the acme of RiscPC Windows performance.

The chip remains in production today, and is a popular choice for use in embedded controllers. One derivative of the 5x86 family is the core used in the Élan SC520 family of microcontrollers marketed by AMD.

Models

Am5x86-P75 (X5-133)

* 350 nm CMOS process
* L1 Cache: 16 KiB unified. write-back or write-through. 4-way associative
* Socket 3, 168-pin Socket 1, or Socket 2 with voltage regulator
* Vcore: 3.45 V
* Clock rate: 133 MHz
* Front side bus: 33 MHz

External links

* [http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_1260_1268,00.html AMD: The Am5x86 Microprocessor]
* [http://www.interwb.com/486bench.txt Comparative performance benchmarks]
* [http://web.inter.nl.net/hcc/J.Steunebrink/amd5x86.htm CPU Upgrade: Give your 486 PC a kick with the AMD 5x86 - 133!]
* [http://www.cpushack.net/gallery/index.php?folder=/chippics/AMD/8048x/ Gallery including Am586 images] at [http://www.cpushack.net Cpushack.net]
* [http://www.cpu-collection.de/?l0=co&l1=AMD&l2=5x86 AMD 5x86 processor images and descriptions at cpu-collection.de]
* [http://www.amd.com/epd/processors/4.32bitcont/14.lan5xxfam/24.lansc520/index.html AMD's Élan µController family]