I'm pretty sure that the extra bit will get ignored. There is internal circuitry that expands the register number in the instruction to a 5-bit register number.

I'm pretty sure that the extra bit will get ignored. There is internal circuitry that expands the register number in the instruction to a 5-bit register number. (You need 5 bits to handle 8-bit registers, 16-bit registers, segment registers, and internal registers.) For segment registers, it drops the third bit. But I haven't tested this case, so I can't guarantee there isn't some weird side-effect.