NOP on HP-41C

[Thomas Okken]

You can find out which is faster by simply trying it. Try both options, have them run for a few seconds, and see how far they count when running the same amount of time.

If the instruction following the ISG is guaranteed to always be skipped, it doesn't matter what you put there or what side effects it has. You could use, say, DEG, because it takes only 1 byte. Or, my personal preference, use something that has no side effects at all, to make it clear what is intended; efficiency isn't the only concern when programming, making it easy to read and understand is also a good idea. A nice straightforward NOP for the 41C is STO X.