ENTER↑ is a misnomer, long live ENTER:

[Didier Lachieze]

(07-06-2014 04:30 PM)hansklav Wrote:  At the time when the first RPL calculator (the HP-18C) was developed in 1986 the use by many people of personal computers, with their Enter keys, gave most new users an intuitive postfix feeling for an ENTER key on a calculator, and HP made the logical switch by changing the function of ENTER such that 2 ENTER 3 ENTER + gives the same result als 2 ENTER 3 +. This use of the ENTER keys is also known as 'Entry RPN'.

If you're interested in HP calculator history, we can trace back the origin of the "Entry RPN" before the HP-28C to the HP-41 translator pac for the HP-71b. Bill Wickes who developed the HP-41 translator pac as an "after-hours" project was also the software project manager for the HP-28C/S [1].

In an article published in January 1985 in CHHU Chronicle ( THE HP-41 TRANSLATOR PAC FOR THE HP-71) Bill Wickes introduced the command line and the removal of the stack lift disable:

Quote:The HP-41 is strictly a key-per-function calculator. HP-71 FORTH and BASIC both use a command line approach instead […] The HP-41 translator allows you to use either method. The default mode is command lines. You can type in up to 96 characters of functions or numbers together, each entry separated by a space, then press [ENDLINE] to execute the whole sequence. […] But if you prefer key-per-function operation, you can make the built-in file KEYS41 the active keys file, so that in user mode, any HP-41 function can be immediate execute. [...]

The HP-41 Translator makes a bold break with tradition (here’s how you make a possible drawback into a feature) by not implementing any stack lift disable. Yes, that’s right, the ENTER^ key becomes a vestigial organ not worth including in the built-in keys file. [...]

Consider how a traditional HP RPN calculator handles number entry. [...] To terminate the number entry, you just press any non-numeric function key. The problem arises when you want to enter two consecutive numbers, with no operation in between. Hence the ENTER^ key. But here the HP-35 designers overshot the mark—instead of just having ENTER^ terminate digit entry, they (who knows why?) made it also carry out the unrelated task of duplicating the number into the Y-register, then disabling stack lift. [..]

The HP-41 translator does what the HP-35 and its descendants should have done: it has a special key (in this case the [SPC] key) that has no role in lift except to terminate digit entry (or to separate commands).

This article which is sub_tittled "Bridging the gap" is also interesting because it was published while the HP-28C was being finalized and it illustrates some of the intellectual steps on the road from the 41C to the 28C:

Quote: FORTH is a logical “next language” for HP-41 user language aficionados.

[1] From "HP28 Source, SYSEVAL's and Wickes"
Newsgroups: comp.sys.handhelds
Subject: HP28 Source, SYSEVAL's and Wickes
Message-ID: <17980006@hpcvra.CV.HP.COM>
Date: 4 Dec 89 18:13:54 GMT