A consequence of this is the so-called "undocumented" opcodes of the 6502. Extra instructions such as LAX (load A and X from the same value) "work" because the

A consequence of this is the so-called "undocumented" opcodes of the 6502. Extra instructions such as LAX (load A and X from the same value) "work" because the instruction activates two different blobs of logic (the one for LDA plus the one for LDX). From my experience, it's less clear that microcode would function this way when confronted with an illegal opcode.