== Secret Instruction ==
The secret instruction is SALC (Set AL Register register to Carry). Its opcode is 0xD6. Intel put this in all its x86 processors but didn't document it as a trap. If a manufacturer cloned an Intel processor, the presence of the SALC instruction would prove that the clone stole Intel's microcode. [https://x.com/kenshirriff/status/1680629357348376576 Source]
Intel sued NEC for making 8086 clones, claiming that NEC ripped off Intel's microcode. NEC claimed they wrote their own microcode. NEC's chip didn't have the secret SALC instruction and Intel lost the case.