Both the Am486DX40 and Cx486DX40 were tricky on early motherboards because of their high bus speed. Think again and look at this 50MHz part that requires a 50MHz front side bus. 50MHz sounds like it will fly but unfortunatly it doesn't.
My UMC PCI motherboard automatically sets the PCI bus on 25MHz (1/2 of the FSB) when running a 50MHz FSB. This is because the PCI slots are rated to run at 33MHz and the next step would be 37,5MHz (1/3 of the FSB). This affects performance of the cards that are installed, especially the graphics card. With a 25MHz PCI bus Doom 1.9S will score 24.91 FPS. The same benchmark with 37,5MHz PCI bus gets 26.17 FPS. All the benchmarks run by this CPU shown below are done with a 37,5MHz PCI bus.
Notice that the DX50 isn't much faster than the DX2/50, even though it runs at twice the bus speed (50MHz over 25MHz)! Only Doom seems to run a bit better but that's because the graphics card can be accessed faster over the 37,5MHz PCI bus.
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