Posted By
someone
on 2002-10-26
|  Why do illegal opcodes exist on this old 8-bit processors?
Well, that's simple to explain:
Because of the optimization of the opcode decoding and sequencing logic contained in every processor. This logic is "hardwired" (opposite to the microcoded instructions of the newer processor generations) and to minimize the number of logical gates needed for the CPU the logical terms, which describe the functions of the hardware, were heavily optimized (also for cost reasons, of course: smaller chip=more chips on every wafer=higher yield rate=MORE PROFIT!).
The side effect of this internal optimization is that some opcode bitfields are not fully decoded and therefore some bit combinations do two or more (or maybe only a part) of the internal actions a CPU does while executing an opcode.
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