That's a new name I hadn't heard that fits well: unintended opcodes. I also like unofficial. Undocumented isn't correct because these are quite well documented.

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adrian_b 6 hours ago | parent [-]

They are well documented now, after reverse engineering.

The manufacturer did not document them, so they really were undocumented.

The same happened with many other CPUs, like Zilog Z80, Intel 8086 and the following x86 CPUs.

They all had undocumented instructions, which have been discovered by certain users through reverse engineering.

Some of the undocumented instructions were unintended, so they existed only due to cost-cutting techniques used in the design of the CPU, therefore the CPU manufacturer intended to remove them in future models and they had a valid reason to not document them.

However a few instructions that were undocumented for the public were documented for certain privileged customers, like Microsoft in the case of Intel CPUs, so they were retained in all future CPU models, for compatibility.

	▲	bonzini 4 hours ago \| parent [-]
		Not always. LOADALL was used heavily by Microsoft's HIMEM.SYS on the 286, but was not preserved on subsequent models.