The Windows NT 3.1 kernel is incompatible with enhanced 486 processors that provide the CPUID instructions. This bug affects the KiSaveProcessorControlState function, resulting in incorrect detection of processor type. Specifically, the byte at offset 19h in the KPRCB is considered part of the model number, causing Windows NT 3.1 to treat a CPUID capable 80-4-86 processor as 80-260-86 processor. To fix this, the CMP instruction in the KiSaveProcessorControlState and KiRestoreProcessorControlState functions needs to be patched to be a byte compare instead of a word compare. This fix applies to both the original NT 3.1 Advanced Server distribution and NT 3.1 SP3.
https://retrocomputing.stackexchange.com/questions/19655/why-does-trying-to-break-into-the-nt-3-1-kernel-reboot-my-486dx4-machine