Free42 (v.1.4) is an open source and skinnable emulator for the HP-42S calculator. Although, there are other emulators like Emu42, this one does not require the original ROM image and this emulator works on several platforms. Binaries are available for Microsoft Windows, Linux (x86), Pocket PC and PalmOS. There are also versions for MacOS X, Sharp Zaurus and Nokia 770. So if you have a PC and a PDA like me, this makes life a whole lot easier being able to switch from one to the other.

Besides being able to emulate the functionality of the HP-42S, a good emulator should also look and feel like the HP-42S. Thankfully, Free42 has several skins to suit your needs.

The default or “Standard” skin is quite plain:

But you do have the option of switching to a more “Realistic” skin:

Although this is definitely an improvement, I find it kind of large. So I’d recommend downloading the skin pack for your OS. The Windows version has 14 other skins, but I’m just going to highlight 3 of my favorites…

Here’s one in the “compact” style (42ck):

This “SemiReal42″ skin gives you the best of both worlds:

But here’s my favorite skin called “Ehrling42sm”:

As you can see, it’s all about choice.
But before you try out this emulator, let me warn you that this is an RPN calculator. If you take a closer look at any of the above screenshots, the first thing you’ll probably notice is it doesn’t have an “=” button! That’s because RPN stands for Reverse Polish Notation, which is a way of writing mathematical expressions without using parentheses and brackets. In RPN, every operator follows all of its operands so calculations occur as soon as an operator is specified. That’s why it doesn’t need an “=” button.

Here’s a quick lesson…

If you wanted to add 2 numbers on a “regular” calculator, you’d press:

1 + 2 =

On an RPN calculator, you’d press:

1 Enter 2 +

Yes, the number of key strokes (4) are the same, but RPN starts to shine when you have a more complex calculation like this:

(3+5) / (7+6)

On a “regular” calculator, you’d have to press 13 keys and you’d need to memorize or write down the intermediate answers:

3 + 5 =

7 + 6 =

8 / 13 =

On an RPN calculator, you’d just press 9 keys:

3 ENTER 5 + 7 ENTER 6 + /

Now, is that cool or what?!