Can we have RPN back?

[Han]

They're all RPN, yes. But they can be distinguished by their programming "language."

Many of the calculators that have "immediate entry" (what a lot of folks call RPN -- see HP48 manual; I could not find a reference to RPN) rely on an RPL engine. I was specifically referring to the programming method. The older calculators had pgramming-by-keystroke only (what I am calling RPN machines) while the later models allowed the use of UserRPL (what I am calling RPL machines). They all, however, were running off of an RPL-based engine (running SysRPL, but only the later graphical models actually allowed users to type in commands as opposed to using keystrokes).

The "RPN" calculators were limited to keystrokes and 4-level stacks. The "RPL" machines had a stack limited only by memory, and could be programed using RPL commands. The "RPL" machines no longer behaved like the "RPN" machines even at the keystroke level. DUP-on-ENTER was omitted; Stack Level X -- the equivalent of Stack Level 1, was no longer tied to the input/command line. However, there were still a number of similarities.

That you may have never heard about the underlying layer is nothing out of the ordinary. It was never a regular term like RPN -- unless you did a lot of RPL programming.

From the HP48 FAQ:

Quote: RPL
RPL is the name of the language used to program the HP48 and
HP-28 series calculators. RPL stands for "Reverse Polish Lisp".
It's interesting to note that an HP Journal article incorrectly
described RPL as "ROM-based Procedural Language".

From Bill Wickes:

RPL stands for Reverse Polish Lisp. In the early days of RPL
development, we got tired of calling the unnamed system "the new
system," and one of the development team came up with "RPL,"
both as a play on "RPN" which has been the loved/hated hallmark
of HP calcs forever, and as an accurate indication of the
derivation of the language from Forth and Lisp.

RPL was never particularly intended to be a public term; at the
time of the HP Journal article (August 1987) on the HP 28C there
was an attempt to create a less whimsical name--hence "ROM-based
procedural language," which preserved the initials but had a
more dignified sound. The development team never calls it
anything but (the initials) RPL. You can choose either of the
two full-word versions that you prefer. Or how about 'Rich
People's Language?'

Even the HP42S has an RPL engine even though none of it was immediately available to the user. A quick disassembly will reveal a rudimentary version of RPL. I have not disassembled ALL Saturn-based calculators, but it would not surprise me the least if they all had some early form of RPL as the interface between user keystrokes and assembly code.