nop notation nop-head notation A nop-head notation is a two-letter notation (IP, RH, WH, or FH) surrounded by question marks that refers to that head being used as a default, when the instruction code in question is being executed. But if the instruction code is followed by a no-operation ('nop' or 'Nop') instruction code, the 'nop' or 'Nop' instruction code will alter the register used. A 'nop-A' or 'Nop-A' instruction code indicates the instruction pointer (IP); a 'nop-B' or 'Nop-B' instruction code indicates the read-head (RH); a 'nop-C' or 'Nop-C' instruction code indicates the write-head (WH); and a 'Nop-D' instruction code indicates the flow-head (FH).