Functionally all models except U5D are identical and only differed in their intended application, voltage rating or physical packaging.

As one of the largest chip foundry owners in Taiwan, UMC owns several fabrication plants which allowed them to fabricate their own designs, whereas some other manufacturers, notably Cyrix, had to contract this process out to third parties such as IBM and Texas Instruments.

[5] Due to an error in the microcode, Microsoft Windows 98 Second Edition identifies some processors as an Intel Pentium MMX which could cause the operating system and software running within it to crash or exhibit undefined behavior.

These processors were designed to compete with other clock doubling solutions such as the Intel 80486DX2, AMD Am486DX2 and Cyrix Cx486DX2, but due to ongoing legal troubles UMC withdrew the U486DX2 from production.

The processor was only ever produced as an engineering sample and never made it to market,[6] it was manufactured with a 0.35 μm CMOS process and is contained within a ceramic package.