XB and XH – Undefined behavior?

Nintendo added a handful of instructions to the stock V810 CPU when they made the NVC for Virtual Boy. Among them are [font=Courier New]XB[/font] (Exchange Byte) and [font=Courier New]XH[/font] (Exchange Halfword), which swap the lowest two bytes or both halfwords of a register, respectively.

These instructions are stored in instruction format II, which specifies two registers, but they only operate on one register, reg2. According to the development manual, if any register for reg1 other than [font=Courier New]r0[/font] is used, the operation is undefined.

But for the life of me, I haven’t been able to get any unusual results out of these instructions no matter which registers or input values I use. Does anyone have any information regarding cases where these instructions will misbehave?

For the time being, it looks like the register specified in the reg1 field is irrelevant.