nop notation nop-stack notation A nop-stack notation is a single-letter noation (A, B, C, or D) surrounded by question marks that refers to that stack being used by default when the instruction code 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 code indicates the stack A; a 'Nop-B' instruction code indicates the stack B; a 'Nop-C' instruction code indicates the stack C; and a 'Nop-D' instruction code indicates the stack D. It only applies to the transsmt virtual CPU, because the TransSMT hardware does not use registers.