Four types of RPN

[hansklav]

(09-03-2014 04:45 PM)Marcus von Cube Wrote:  

(09-02-2014 10:52 PM)hansklav Wrote:  (…) "top copy on pop".

I don't like this wording because it doesn't describe exactly what happens:

In the RPL machines and the Prime, the stack is of variable size. If a value is popped off the stack (consumed or dropped) the length of the stack decreases. Thus, the stack may well contain too little objects to perform a given operation. The prime differs just in the maximum size of this stack from the RPL machines.

In the other machines the stack has a fixed size. The question is what happens if a value is dropped or consumed: The remaining elements drop one level, except the top of the stack which is left unchanged. The duplication of the value is just a side effect of the drop, not an extra operation.

The term is not mine. ‘Top copy on pop’ for the replication of the contents of the T register on stack drop is used already in 1978 by John A. Ball in his book ‘Algorithms for RPN calculators’. The term is short, sounds nice, and describes exactly what the user needs to know. How this is done programmatically is another matter and of no direct concern to the average user (imho).

Hans