Notes for HP RPN
Starting with the HP-35 in 1972, Hewlett-Packard has developed a
series of handheld scientific and business calculators based upon the
RPN stack interface. Although there are many differences in the ca-
pabilities and applications of these various calculators, they all share a
common implementation of the basic stack interface, which makes it
easy for a user accustomed to one calculator to learn to use any of the
The HP-28S also uses a stack and RPN logic as the central themes of
its user interface. However, the four-level stack and fixed register
structure of the previous calculators is inadequate to support the mul-
tiple object types and symbolic mathematical capability of the HP-
28S. Thus while the HP-28S is a natural evolution of the "original"
RPN interface, there are sufficient differences between the HP-28S
and its predecessors to require a little "getting used to" if you are ac-
customed to other RPN calculators. In this appendix, we will highlight
the major differences.
The Dynamic Stack
The most dramatic difference in the basic interface of the HP-28S
compared with previous HP RPN calculators is the size of the stack.
The other calculators feature a fixed, four-level stack consisting of the
X-, Y -, Z- and T -registers, augmented by a single LAST X, or L-regis-
ter. This stack is always "full" -even when you "clear" the stack, all
you are doing is filling the stack with zeros.
B: Notes for HP RPN Calculator Users