The CPU was not prohibited from executing unsupported instructions
The legendary Intel 8086 processor appeared back in 1978. By modern standards, this is an incredibly primitive solution with 29,000 transistors and a frequency of up to 16 MHz. However, it turned out that something new can be found in such an old CPU in 2023. For example, secret instructions.
Undocumented secret instructions found in 45-year-old Intel 8086 processor
Hardware researcher Ken Shirriff (Ken Shirriff) decided to study the said processor and found a lot of interesting things there. For example, this CPU does not contain any prohibition on running unsupported instructions. That is, when receiving such instructions, the CPU still tried to execute them.
In total, the Intel 8086 supported 521 instructions stored in the Microcode ROM chip. Some of these 512 instructions were duplicated as backups, and some were never made public by Intel itself.
Of greatest interest is one of these instructions. It was created to protect intellectual property of Intel. If a company decided to illegally copy the Intel 8086 processor, it would also copy this feature of the CPU without knowing it existed. As a result, the CPU clone would perform a certain SALC (Set AL to Carry) operation when the appropriate bits of machine code were supplied. This would allow Intel to more effectively pursue any unscrupulous competitors.
Interestingly, a few years after the release of Intel 8086, the company had the opportunity to try out its trap in action, since the company considered that NEC implemented exactly a copy of Intel 8086/8088 in its V20 processor, but the court ruled in favor of NEC.
