Are the HP9100 and HP9810 calcualtors really RPN machines?

Subjet: line says it really, but I think I should explain... Are the HP9100 and HP9810 calcualtors really RPN machines? I think not. You ahve 3 registers, called X(Keyboard), Y(Accumulator) and Z(Temporary) which appear to be a stack. But it;s not really as stack IMHO. Entereing a number form the keyboard puts it in X, leaving the other 2 registers unaltered. At no time is there an automatic stack lift (AFAIK). THere are functions to effectively push and drop the stack (if it is one:-)) and to roll it round. Arthmetic operations perfrome something like Y:=Y(OP)X. Again, there is no lift or drop, there is no change to any register other than Y. 1-number operations normally do X:=f(X), at least one does Y:=f(Y), though. Again nothing else is changed, no lift or drop. You can get the user manuals from the Austalian museum site if you want ot read up on this... My (considered?) opinion is that it's certainly a postfix machine in that you enter both numbers into Y and X before doing an arithmetic operation. But becuase of the lack of automatic stack operation, it's not an RPN machine? What about the other 'RPN' desktop machines (friden, etc) Any thoughts? -tony