Four types of RPN

[hansklav]

(08-28-2014 10:42 PM)In another thread Joe Horn Wrote:  (…) We really, really, really need permanent, unambiguous, universally adopted NAMES for all these various RPN's (…) which hopefully some day can be clearly represented by meaningful, unambiguous, non-judgmental names (…)

In my (still not entirely finished) RPN Tutorial in Appendix G on RPN Variants I propose the following scheme:

Purely looking at the workings of the stack of RPN calculators (and not at their programming capabilities) a few RPN variants can be discerned. Both the bottom and the top of the stack may vary.

A. Bottom of the stack:

1. Classical RPN
2. Entry RPN
   

B. Top of the stack:

1. Small stack (4-, 5- or 8-level) with ‘top copy on pop’
2. Very large stack (>100 levels), no ‘top copy on pop’
   

Four types of RPN

All four possible combinations of the 2 × 2 possibilities of RPN variants actually do exist, although not always in hardware:

α. (alpha type) The combination of Classical RPN with a small stack and ‘top copy on pop’ is well known. Besides the 4-level stack of all classical HP RPN calculators (all HP RPN calculators except those listed below after β. and γ.) and their emulations there is the 5-level stack of the Heathkit OC-1401, and the 8-level mode of the WP 31S and WP 34S.

β. (beta type) All HP RPN graphing calculators (series 28, 48 & 49, the HP 50g and Prime) have Entry RPN and an unlimited or very large stack (128 stack levels in the Prime).

γ. (gamma type) The HP 20b and HP 30b have Entry RPN and a 4-level stack with ‘top copy on pop’.

δ. (delta type) The Calculator application in Mac OS X in RPN-mode combines Classical RPN behaviour of the ENTER key with an unlimited stack.


Discussion

Following Richard Nelson in his article in HP Solve (#27 p. 42) ‘RPN Evolves’ the terms Classical RPN and Entry RPN only refer to the differences at the bottom of the stack. I think Classical RPN is better here than Classic RPN because the latter term denotes a judgement (‘of the highest quality and outstanding of its kind’) whereas Classical RPN is purely descriptive, as in ‘classical music’ (formal music adhering to certain stylistic principles of the late 18th century).

This scheme is unambiguous and non-judgemental, and can be used for future implementations with different programming languages because it does not say anything about programming capabilities. And it conveys some meaning because it indicates the chronology and link to HP of the RPN variants, α type being the oldest, β type younger and γ type the youngest RPN variant first implemented by HP; and δ type another, non-HP type. The 3-level RPN of the HP series 9100 en 9800 could be called ‘pre-α type’.

Just my two cents.

Hans