NOP R_d,I

This opcode may trigger side-effects in implementations, depending on the value of I, particularly when simulating (see Section 2.3).

All implementations should use d = 0, I = 0 as the break instruction for debugging, which should halt the processor.

All implementations should use d = 0, I = 1 as a true no-operation instruction.

The rationale behind this decision is that in an erroneous program, the most likely value to be encountered as a random instruction is zero, which will stop the processor.