Are the HP9100 and HP9810 calcualtors really RPN machines?

That is exactly my point/question. My view is that while postfix operators are necessary condition for soemthing being RPN, they are not a sufficient one. Suppose we take the simple exression (12+15)*(13+234). In what I think of as 'pure RPN', as used in languages like Forth, Postscript RPL[1], etc, tjat becomes something of the form 12# 15# + 13# 234# + *, where '#' is a symbol to terminate number, often just a space. HP's 4-level stack RPN as used on the HP35,etc is almost 'pure', it has some oddities (regarding the limited stack size ans the 'stack lift'), but it still works genrally as you'd expect. [1] IMHO anyone who claims RPL is not RPN doesn't undertand what RPN actually is! But on the 3 level stack machines, the 9100 and 9810, that expressiob becomes something like : 12 <up> 15 + (The result of that is now in Y) 13 <up> 234 + (the first result is now in Z, the second result in Y) <down> * It's that exrra <down> that bothers me. I feel that true RPN should automaticallty drop, that results should go on top of stack, and so on. [..] THey are cerytainly postfix. As I said abovem, I don;t consider them to be totally 'pure' RPN, becuase of the limited stack size, and the fact that sometimes numebr entrie overwrite top-of-stack, sometimes it pushes the stack. I can understnad why this was done, BTW, and genrally, apart expect an RPN machine to behave (the RPL machines _always_ seem to :-)). But the 3-level machines have rather more deviations from what I consider to be RPN. -tony