nop notation nop-register notation A nop-register notation is a two-letter notation (AX, BX, or CX) surrounded by question marks that refers to that register being used by default when the instruction in question is being executed. But if the instruction code is followed by a no-operation ('nop') instruction code, the 'nop' instruction code will alter the register used. A 'nop-A' instruction indicates the AX register; a 'nop-B' instruction code indicates the BX register; and a 'nop-C' instruction code indicates the CX register. It does not apply to the transsmt virtual CPU because the Transsmt hardware does not use registers.