Semiconductor & Computer Engineering
Am5x86 was a family of highest performance 486-based microprocessors introduced by AMD in 1995. These processors came with all the features of the Enhanced Am486s but had the highest clock speed available. These chips, which ended up performing better than the Pentium-75, became the de-facto upgrade for users with existing 486-based systems. This family solidified AMD position as the official Intel competition.
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History
In 1995 AMD was busy working on their first home-grown x86-compatible microarchitecture, K5. Has a sort of a stopgap between the Am486 and the yet-to-be-released K5-based processors, AMD released the Am5x86 family. The name came about due to competitors selling 5th generation-based x86 processors (e.g. Cyrix 5x86) - people were more likely to buy a system with "5" vs "4". AMD took this opportunity to utilize their latest technology and release a final, highest clocked 486 chip for much lower price than their competitor's latest chips. This is very similar to what they had done with their highly successful 40 MHz version of their Am386 family. Later, strong sales of Am5x86 played a crucial role in keeping AMD's profitability while their K5 microarchitecture was experiencing severe delays. AMD continued to sell Am5x86 chips for personal computers until 1999 and for embedded systems well into the 2000s.
Architecture
Contrary to what the name implied, the 5x86 was not a "5th-generation x86". Architecturally, the 5x86 is identical to the Enhanced Am486s which included various system and power management features (e.g. SMM & Stop-clock control). Additionally the 5x86 came with the larger 16 KB write-back L1$. The processors were manufactured on AMD's newly upgraded 350 nm process which is what allowed AMD to clock it as high and compete with Intel's low-end Pentiums. AMD had to clock the 5x86 at much higher frequency in order to compensate for its inefficiencies and deliver performance comparable to the Pentium-75. The clock multiplier was set to x4 (e.g. bus speed of 33 MHz would have a core frequency of 133 MHz). Essentially, one can think of the 5x86 as a "Am486DX5-133".
Being backwards compatible, the 5x86 could go in the older Socket 1, Socket 2, and Socket 3. However Socket 1/2 both required a replacement voltage regulator as the chips needed 3.45 V to operate. Depending on the motherboard, it either had a manual jumper one has to set or an autodetect version, voltage must be set to 3.45 (± 0.15 tolerance). Likewise the input clock must be set to 33 MHz with a jumper set to quadrupled clock (note that because there is no such provision in the socket, setting the "clock-doubled" setting should cause the CLKMUL pin to be driven high allowing the processor to operate at x4 multiplier). While set to 133 MHz, many in the hobbyist community have reported to have had it overclocked to 160 MHz which would put it at performance similar to the Pentium-90. The later versions had higher package temperature rating, making them easier to be overclocked.
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