The only possible "hidden" opcode is the "undefined instruction", 0xA5. Every other opcode is used and documented.

I once played with the 0xA5 on a genuine Intel 8052 and it seemed to me that it took one instruction cycle to execute, always set the carry bit, and seemed to have no other effect.

I was later told by an 8052.com reader that the 0xA5 instruction is actually a "MOV bit,bit" instruction where the second and third byte of the opcode indicates the source and destination bit. I have never gone back to check or verify this. This also might be dependent on the derivative chip you are using.

I've also heard that the "H-Card Access Cards" used by DirectTV have used 0xA5 as a new type of memory access that only applies in the same Access Card environment.

I suspect that if anyone has ideas for other uses of the undefined instruction, 0xA5, that it must be suggested not only in fact but also mentioning the specific derivative chip it applies to.

In any case, since the undefined instruction is actually undefined, using it is not advised. Any functionality it might have could be changed or eliminated in future productions of even the same chip you are using.

Craig Steiner