RPN calculators were classic. No, although the register designations seem like assembly language it was not. Calculations were performed by translating from algebraic format onto the stack. The numbers used were pushed into the stack and operations were performed on last stacked against next to last stacked values. You could "rotate" the stack to move the values since the displayed value was a stack member the results could be swapped or stacked as necessary to perform nearly complex calculation. If you do understand stack hardware and assembly language this calculator was trivial to learn since it's paradigm was most similar.

Reverse Polish notation (RPN) calculators were classic. No, although the register designations seem like assembly language it was not. Calculations were performed by translating from algebraic format onto the stack. The numbers used were pushed into the stack and operations were performed on last stacked against next to last stacked values. 

You could "rotate" the stack to move the values since the displayed value was a stack member. The results could be swapped or stacked as necessary to perform nearly complex calculation. If you do understand stack hardware and assembly language this calculator was trivial to learn since its paradigm was most similar.