Let me start with: I know that LZCNT is not supported on the Core i7.
However when I run theinstruction on my Core i7 I do not as I expect get an illegal instruction exception. Instead it performs a BSR instruction. This is while working in 64 bit mode and using 64 bit registers.
Is this a bug, expected behaior or is running unsupported instructions undefined (and this is therefore OK)?
For reference the opcodes are: BSR: 0xBD LZCNT: 0xBD (Same as BSR but has a prefix of 0xF3)