Note that INT1 was originally called ICEBP before Intel finally documented it publicly (very recently).
https://www.rcollins.org/secrets/opcodes/ICEBP.html
IIRC, older versions of the Linux kernel had a security bug because they didn't expect this to happen.
ICE mode was sort of a precursor to SMM, but both also coexisted for a time with slightly different behaviour. It was introduced in the 286, where instead of ICEBP there was "STOREALL" (opcode 0F04). F1 on that processor was a prefix instead, with the same function as UMOV on 386+. If you use them together - something Intel probably didn't intend - you can dump the internal CPU state to memory on a regular non-bond-out chip.
https://rep-lodsb.mataroa.blog/blog/intel-286-secrets-ice-mo...