IBM 5x86 3V3100GB

The IBM 5x86 is technically a Cyrix 5x86 processor. The 5x86 is based on a scaled-down version of the M1 core used in the Cyrix 6x86. It provides 80% of the performance for a 50% decrease in transistors over the 6x86 design. It's some sort of 586 on a 486 socket as it uses the 32-bit memory bus of a 486 (Pentium had 64-bit) but internally it even had some (limited) support for Pentium instructions. Some features of the 5x86 that could improve performance are disabled because they could be potential bugs which were not fixed before the release of the CPU. These features can be re-enabled using some utilities.

The 5x86 doesn't run in every 486 motherboard. Besides support for the lower voltage the BIOS also needs to know how to handle the 5x86 CPU.