Cyrix Cx486

A Cyrix Cx486S processor. Compatible with the Intel 486SX.

Introduced in May 1993, The Cyrix Cx486S, codenamed M5, was designed to be compatible with the Intel 486SX and like the Intel part, did not have a floating point unit onboard which was of little concern to an average user at the time due to most games and applications using purely integer-based code. The processor did, however, sport a 2KB Write-back cache and a special "Write-Burst" signal which offered a slight performance boost in certain applications provided the motherboard was capable of utilizing the feature. The processors were labelled "FasCache" to emphasize this feature as most processors used slower Write-through caches.[4]

The Cyrix Cx487S FPU coprocessor, installed between CPU and motherboard socket.

Users could upgrade the processor to 486DX specifications through use of an accessory, the Cyrix Cx487S which was an x87 Floating Point Unit coprocessor which fit between the CPU and the socket on the motherboard.[5]

The table below lists the models of this CPU that were produced.

Die shot of Cx486S-33.

The Cyrix Cx486DLC is a derivative of this processor and was used to upgrade computers on the 80386 platform.

Codenamed M6, the Cyrix Cx486DX is essentially the same as the Cx486S except it featured an internal Floating Point Unit and quadruple the amount of cache memory. It was available in a form that allowed lower voltage operation. The processor was found to have comparable performance to competing solutions, but it varied depending on the application.

The table below lists models of this CPU that were produced.

Die shot of Cx486DX-40.

Despite the faster clock speeds, the 50 MHz version of this processor was less popular than the other models Cyrix had produced. This is likely due to VESA Local Bus being common in computers around the time of the processors introduction as the bus was notoriously unreliable when operating at speeds of over 33 MHz. Many peripherals and even the bus itself will exhibit undefined behaviors at higher speeds on a large number of motherboards.[6] This is reflected by Intel and AMD processors operating at 50 MHz where their sales show a similar trait of being less popular. Users could potentially negate this by increasing latency for the memory, cache and bus but this would also cause performance to drop.

Cyrix Cx486DX2 rated for 66MHz operation.

Introduced in September 1993 alongside the single clock version was a clock doubling version of the Cx486DX.[7] It was intended to compete with the Intel 80486DX2 and had similar capabilities. Its lower price point than equivalent Intel processors made it a popular choice.[8]

Die shot of Cx486DX2-V66.

Die shot of Texas Instruments TI486DX2-G66.

The 50 MHz version is again the least common, this time likely due to the slower bus speed rendering any upgrade questionable as the performance gains would be reduced by the lower bus speed.

The 80 MHz version was introduced in November 1994.

The final version of the Cx486 to bear the Cyrix name was a Clock Tripling CPU which was only available with a 100 MHz Internal Clock. Introduced in September 1995, it was otherwise not vastly different to the older designs and proved to be competitive both performance-wise and price-wise against offerings from AMD and Intel though it was later to market than its competitors.