I started testing this ECS UV486 motherboard and DX4-100 overdrive processor I got recently.
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The board worked fine with a DX-33 and DX2-66 processors. However, when trying the DX4-100, during bootup into DOS it would halt with a "parity error".
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On a whim I decided to try a newer BIOS chip from another ECS board that uses the same chipset. That worked, and the system was able to boot just fine.
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However, it doesn't detect the L2 cache. Doing some research apparently this is an issue with this chipset that can be solved with a different BIOS. So I've order some EEPROM chips and will hopefully be able to try flashing a new BIOS once I receive those.