What makes "FOCAL" FOCAL?

[Sylvain Cote]

The HP-41C was an evolutionary in some aspects and also revolutionary in others.

On the RPN side, the HP-41C was mainly an evolution with some twist.

The user experience related to the language changed drastically.
Previous models stored key codes in RAM and the interpreter had to find the desired function through a lookup table using the key code.
On the HP-41C, the uses of alpha introduced the possibility to store the table entry directly in RAM and do a reverse lookup to display the function name using the table entry.

The simple fact of displaying the name and not specific key codes allowed the RPN language to finally exist by itself.
Also, for the first time the RPN language became extensible through XROM facility instead of pre-defined finite set of functions.

What put the HP-41 apart from all the other hand-held calculators before it was its I/O capability.
This allowed HP & third parties to create a whole set of modules, peripherals and interfaces who were integrated seamlessly in the calculator through its I/O ports and XROM facility.
It also changed the nature of the beast making it not just a calculator, but also one of the first truly low power battery operated personal computer.