Free42 : An HP-42S Calculator Simulator

What it is

Free42 is a re-implementation of the HP-42S Scientific Programmable Calculator and HP-82240 Printer.
It is a complete rewrite, not using any HP code. It does not require an HP-42S ROM image.
Free42 is an Open Source project. The executables and source code are released under the terms of the GNU General Public License, version 2.
All third-party code used in Free42 is either in the public domain, or licensed under terms compatible with GPLv2, or used with the authors' permission.

Project Status

The latest release is 2.2.

If you're interested in what's been going on with Free42 so far, see the project history.
If you have questions or comments about Free42, you can contact me, Thomas Okken, via email at thomasokken@gmail.com.
You can find answers to some frequently asked questions in the Free42 FAQ.

Donations

If you like Free42 and use it regularly, or if you simply want to sponsor the Free42 project, please make a donation.
You may donate any amount you wish, large or small.
Donations are processed by PayPal.

Page Contents

Downloads
Documentation
Loading and Saving Programs
Binary and Decimal Floating-Point
Extensions: Accelerometer, GPS, and Compass Support; HP-41 Time Module Emulation; Enhanced debugging: Step Into, Step Over, Step Out; Local Variables
Frequently Asked Questions

Downloads

Android
Get it at Google Play, or if you prefer the manual approach, download Free42Android.apk for side-loading.
Requires Android 2.2 or later.

iOS (iPhone, iPad, iPod touch)
Get it from the iTunes App Store.
Requires iOS 8.0 or later.
NOTE: This is an "iPhone only" app, and as such, if you're using an iPad, the App Store app won't show it to you by default. You must select the "iPhone only" option from the drop-down menu in the top-left corner first. The app runs fine on iPads; Apple just makes it hard to find.

Windows
Download: Free42Windows.zip
Requires Windows 2000 SP4 or later.
If, when you try to run Free42, Windows says "This application has failed to start because the application configuration was incorrect. Reinstalling the application may fix this problem," then you also need to download and install the Microsoft Visual C++ 2008 Redistributable Package.

MacOS
Download App: Free42Mac.zip
Download Dashboard Widget: Free42MacDashboard.zip
Both require MacOS 10.7 or later.

Linux
Download: Free42Linux.tgz
Built in Ubuntu 12.04, with glibc 3.2, libstdc++ 4.6.3, and GTK+ 3.4.2. It should work on any Linux (x86_64) with those or similar libraries.

Additional Downloads

Skins
Some skins to use instead of the faceplates built into Free42.

HP-42S/Free42 programs
A small collection of HP-42S/Free42 programs.

rom2raw
A C program that converts HP-41 ROM images to Free42/Emu42 compatible "raw" program files.
Download rom2raw.zip; source code and Win32 console executable included.

Free42 Source Code
Download: free42.tgz
Download for GTK only, no HP logo, for building Linux packages: upstream.

Building the Android version requires the Android SDK 2.2 and NDK r4b or later.

For the iOS version, I always use the latest Xcode and iOS SDKs. You may be able to build it with earlier releases, too, but I can offer no guarantees on that.

For the Windows version, you need Microsoft Visual C++ 2008 or later.

For the MacOS Application and for the Dashboard widget, you need the MacOS SDK 10.7 or later.

The GTK version requires Linux, or any reasonably Unix-like environment, with X11, GTK+, and the usual development tools and libraries.

Documentation

There is no manual for Free42 per se, but since it is an accurate simulation of the HP-42S, the original HP-42S manual should be adequate for most purposes. I also recommend the Alternative HP-42S/Free42 Manual, written by José Lauro Strapasson and Russ Jones; you can get it here, in PDF and Word formats.

The User Interface functionality that is specific to Free42, e.g. printer emulation, skin switching, etc., is fairly simple and should be self-explanatory, but Free42 also has extended functionality in the calculator simulation, namely, program import/export, access to the accelerometer / GPS / compass on Android and iOS devices, time and date functions, enhanced debugging functions, and local variables, which are all documented below.

Loading and Saving Programs

Loading and saving programs is possible in all Free42 versions except MacOS Dashboard. The exact process is pretty straightforward in the desktop versions (Windows, MacOS, Linux), but it is a bit more complicated on mobile devices.
For instructions for all versions, see here.

Starting with release 2.0, you can use Copy and Paste to get programs into and out of Free42. In PRGM mode, Copy puts a text representation of the current program onto the system clipboard, and Paste takes a text representation of a program from the clipboard and loads it as the last program. This works in all Free42 versions, including mobile and MacOS Dashboard.

Binary and Decimal Floating-Point

While Free42 originally used Binary math exclusively, all releases starting with 1.4 have come in two versions, Binary and Decimal. The two look and behave identically; the only difference is the way they represent numbers internally.
All the Free42 versions on this site include both the Binary and Decimal versions, except for the iOS and Android versions, which are Decimal only.

Free42 Decimal uses the Intel Decimal Floating-Point Math Library; it uses IEEE 754-2008 quadruple precision decimal floating-point, which consumes 16 bytes per number, and gives 34 decimal digits of precision, with exponents ranging from -6143 to +6144.

Free42 Binary uses the PC's FPU; it represents numbers as IEEE 754 compatible double precision binary floating-point, which consumes 8 bytes per number, and gives an effective precision of nearly 16 decimal digits, with exponents ranging from -308 to +308.

The binary version has the advantage of being much faster than the decimal version; also, it uses less memory. However, numbers such as 0.1 (one-tenth) cannot be represented exactly in binary, since they are repeating fractions then. This inexactness can cause some HP-42S programs to fail.

If you understand the issues surrounding binary floating-point, and you do not rely on legacy software that may depend on the exactness of decimal fractions, you may use Free42 Binary and enjoy its speed advantage. If, on the other hand, you need full HP-42S compatibility, you should use Free42 Decimal.

If you do not fully understand the above, it is best to play safe and use Free42 Decimal.

Free42 Extensions to the HP-42S Instruction Set

Accelerometer, GPS, and Compass Support

Starting with release 1.4.66, Free42 for Android and iOS have three new functions to query the device's hardware. They are:

ACCEL Query the accelerometer. The three components of the acceleration vector are returned in the X, Y, and Z registers. Holding the device facing you, in portrait orientation, and the dock connector pointing towards the floor, positive X is acceleration toward the left, positive Y is acceleration downwards, and positive Z is acceleration away from you -- or, in terms of gravity, positive X is gravity pulling to the right, positive Y is gravity pulling upwards, and positive Z is gravity pulling toward you.
Note: Accelerations are returned in units of Earth gravities, not m/s² as you might expect. The iOS API documentation does not mention the specific conversion factor used by the device.

LOCAT Query GPS. The location is returned as follows: latitude in X, longitude in Y, elevation in Z, and horizontal and vertical accuracy in a two-element vector in T. The latitude and longitude are given in decimal degrees, with North and East being positive; the elevation and accuracies are given in meters. If an accuracy is -1, that means that the corresponding measurement is not valid.

HEADING Query the compass. This returns the following data: magnetic heading in X, true heading in Y, heading accuracy in Z, and the raw magnetic vector in a three-element vector in T. The headings and accuracy are given in degrees, where North is 0, East is 90, and so on; the components of the magnetic vector are given in microteslas. The components are oriented with respect to the device along the same axes as the accelerometer readouts.

HP-41 Time Module Emulation

I have implemented the following HP-41 Time Module functions: ADATE ATIME ATIME24 CLK12 CLK24 DATE DATE+ DDAYS DMY DOW MDY TIME, and one additional function, YMD.
I have not implemented these HP-41 Time Module functions:
Live Clock Display: CLKT CLKTD CLOCK
Clock Control: CORRECT RCLAF SETAF SETDATE SETIME T+X
Stopwatch: RCLSW RUNSW SETSW STOPSW SW
Alarms: ALMCAT ALMNOW XYZALM

These functions are documented in detail in the HP-82182A Time Module Owner's Manual and the HP-41CX Owner's Manual, both of which can be viewed on-line here, here, and here, or you can get them in PDF on CD-ROM or DVD-ROM here.

Note: the date functions handle dates between October 15, 1582, and September 10, 4320. The former is the first day of the Gregorian Calendar, and the latter is 999,999 days later.
The format in which the date functions accept and return dates is MM.DDYYYY when in MDY mode, DD.MMYYYY when in DMY mode, and YYYY.MMDD in YMD mode.

ADATE Appends the contents of the X register to the ALPHA register, formatted as a date value: in DMY mode, DD.MMYYYY is displayed as DD.MM.YYYY; in MDY mode, MM.DDYYYY is displayed as MM/DD/YYYY; and in YMD mode, YYYY.MMDD is displayed as YYYY-MM-DD. In MDY and DMY modes, if the number of digits of the display mode is 4 or less, the century part is omitted; if it is 2 or less, the entire year part is omitted; and if it is 0, the months or days part is omitted (in DMY or MDY mode, respectively). In YMD mode, if the number of digits in the display mode is 2 or less, the day part is omitted, and if it is 0, the month and day parts are omitted.

ATIME Appends the contents of the X register to the ALPHA register, formatted as a time value: HH.MMSSss is displayed as HH:MM:SS.ss. In CLK12 mode, times with HH between -23 and 23 are displayed in 12-hour format: -23 to -13 as -HH-12 PM; -12 to -1 as -HH PM; 0 as 12 AM; 1 to 11 as HH AM; 12 as 12 PM; 13 to 23 as HH-12 PM. When |HH| ≥ 24, the sign is ignored, and HH is shown as is, with no AM or PM appended. In CLK24 mode, HH between -1 and -11 are displayed as -HH+12; all other values are shown as |HH|. No AM or PM are displayed.
If the number of digits of the display mode is 4 or less, the centiseconds part is omitted; if it is 2 or less, the seconds part is omitted; and if it is 0, the minutes part is omitted.

ATIME24 Like ATIME (see above), except it always formats the time in 24-hour format, regardless of the CLK12/CLK24 setting.

CLK12 Display the time in 12-hour format with AM/PM.

CLK24 Display the time in 24-hour format.

DATE Returns the current date to the X register. In MDY mode, the date is returned as MM.DDYYYY; in DMY mode, it is returned as DD.MMYYYY; and in YMD mode, it is returned as YYYY.MMDD. In addition, when executed from the keyboard, this also displays the date formatted as "MM/DD/YYYY DOW", "DD.MM.YYYY DOW", or "YYYY-MM-DD DOW", respectively.

DATE+ Takes a date from the Y register, and adds a number of days from the X register.

DDAYS Calculates the number of days between the date in the Y register and the date in the X register. If the date in the Y register is earlier, then the result will be positive.

DMY Display, return, and accept dates in day/month/year format.

DOW Calculates the day of the week for the date in the X register. Returns a number from 0 to 6, where 0 is Sunday, 1 is Monday, and so on. When this function is executed from the keyboard, the day is also displayed in a human-friendly format, i.e. MON for Monday, TUE for Tuesday, etc.

MDY Display, return, and accept dates in month/day/year format.

TIME Returns the current time in the X register. The time is returned in HH.MMSSss format, with HH from 0 to 23, regardless of the CLK12/CLK24 setting. In addition, when executed from the keyboard, it displays the time in "HH:MM:SS AM" or "HH:MM:SS" format, when the display format is CLK12 or CLK24, respectively.

YMD Display, return, and accept dates in year/month/day format.

Enhanced debugging: Step Into, Step Over, Step Out

Introduced in release 2.1, Free42 offers Step Over and Step Out. People familiar with Integrated Development Environments (IDEs) will probably know the three common types of stepping:

Step Into, or SST↓, executes one line of code, and if the line in question is a function call, the next line to be executed will be the first line of the function being called. That is, you are Stepping Into the function;
Step Over, or SST→, executes one line of code, and if that line is a function call, it is executed in that one step. You are not Stepping Into the function; rather, you are Stepping Over it.
Step Out, or SST↑, starts program execution, continuing until just after the end of the current function. You are Stepping Out of the function.

Step Into is the same as the original SST function in the HP-42S. Free42 2.1 adds the alias SST↓ for this function, but its behavior is the same, because it's just another name for the same function. Step Over and Step Out are new functions in 2.1. The three together can be assigned to a row in the CUSTOM menu, for example:

A note about Step Over: for most instructions, Step Over behaves the same as Step Into. In high-level language debuggers, the only difference between the two is with function or method calls. In Free42, Step Over steps over XEQ instructions, those being the keystroke-programming equivalent of function calls, but it also steps over SOLVE and INTEG, while Step Into will step into the function being solved or integrated.

Conversely, if you interrupt program execution while SOLVE or INTEG are active, Step Out will cause execution to continue until SOLVE or INTEG are done, as if SOLVE or INTEG are a kind of XEQ.

Local Variables

The LSTO function, introduced in Free42 2.2, can be used to create local variables. These are variables that exist only for the duration of the current subroutine, and are automatically deleted when it returns. For example, the following is a recursive implementation of the factorial, using the local variable T to preserve the stack register T, and using the local variable N to keep track of the parameter across the recursive call to compute FAC(N-1):

00 { 37-Byte Prgm }
01▸LBL "FAC"
02 X≠0?
03 GTO 00
04 SIGN
05 RTN
06▸LBL 00
07 R↑
08 LSTO "T"
09 R↓
10 LSTO "N"
11 1
12 -
13 XEQ "FAC"
14 RCL "N"
15 ×
16 RCL "T"
17 R↓
18 END

Since local variables make it possible to implement recursive algorithms that the real HP-42S can't handle, it also becomes much more useful to have a larger return stack. While the real HP-42S, and older versions of Free42, have an 8-level RTN stack, Free42 2.2 has 1024 RTN levels.

Frequently Asked Questions

This FAQ was last updated on March 31, 2019.

Why does my HP-42S program not work properly in Free42?
Do the Android and iPhone versions have a menu?
Why does Haptic Feedback not work on my iPhone / iPad / iPod touch?
Why does OFF not work in the iOS version?
Why does Free42 not show up in the App Store on my iPad?
Why am I getting results that are rounded to integers?
Why am I getting Size Error when I try to access any of the numbered storage registers, or try to use any of the statistics functions?
How do I assign SST↓ (Step Into) to the CUSTOM menu?

Why does my HP-42S program not work properly in Free42?

There are several potential reasons why an HP-42S program might not work correctly in Free42. To get the most obvious out of the way first: make sure to check that you entered the program correctly, and that the relevant settings, such as register size, angular and display modes, etc., are all correct.

Having ruled out human error, the reasons an HP-42S program may not work correctly in Free42 are:

Different floating-point format
The HP-42S represents numbers with a 12-digit decimal mantissa, and an exponent from 10^(-499) to 10^499; Free42 Decimal uses a 34-digit decimal mantissa, and an exponent from 10^(-6143) to 10^6144; and Free42 Binary uses native binary floating-point, which on all currently supported platforms is IEEE-754 double precision, with a 53-bit binary mantissa which is roughly equivalent to 16 decimal digits, with an exponent from 10^(-308) to 10^308.
These differences can cause Free42 to calculate different results than the HP-42S. For example, 1 ENTER 3 1/x - LASTX - LASTX - returns 1e-12 on the HP-42S but 1e-34 on Free42 Decimal, because of the wider mantissa; 1 ENTER .2 - .2 - .2 - .2 - .2 - returns 0 on the HP-42S but 5.55111512313e-17 on Free42 Binary, because 0.2 is a repeating fraction in binary.
Most of these kinds of compatibility problems can be avoided by using Free42 Decimal, but, although they are rare, there are programs that don't work correctly even in Free42 Decimal. Mostly these are programs that specifically rely on the size of the mantissa and/or the range of the exponent, but the extra precision of Free42 Decimal can also potentially throw off straightforward numerical algorithms, for example, if they use termination conditions that aren't robust.
Free42 is in Printer On mode by default
The HP-42S is set to Printer Off mode on hard reset, but because Free42 has a built-in printer simulator, I thought it made more sense to have it be set to Printer On mode on hard reset. This has the unfortunate side effect that programs that print but not display their results in Printer On mode may appear to malfunction in Free42. To rule out this possibility, check the Print-Out window for the results you are missing, or switch to Printer Off mode (POFF in the PRINT menu, third row) and try again.
N.B. Since release 2.0.3, hard reset clears flags 21 and 55, bringing its behavior in line with the real HP-42S. However, if such a version is installed as an upgrade, with an older state file still in place, the printer flags will retain their previous value, along with all other state. In other words, the change in 2.0.3 or later only takes effect in new installs, or when the old state file is deleted or corrupted, as indicated by the Memory Clear or State File Corrupt messages.
Free42 defaults to decimal-point or decimal-comma mode depending on locale
If your computer / smartphone / tablet is set to a locale where the decimal is a comma, Free42 is set to RDX, (radix comma) mode on hard reset; in all other locales, it is set to RDX. (radix dot) mode on hard reset. If you're seeing incorrectly formatted data in your program's output, you may have to switch to RDX. mode (RDX. in the MODES menu, or SF 28).
In programs, numbers are normalized
This is not actually a difference between Free42 and the HP-42S that causes HP-42S programs to fail in Free42, but it is something that can cause confusion. What's going on?
In programs, the HP-42S stores numbers basically as you enter them. If you want to enter the number 1000 in a program, you can enter 1000, or 1E3; the latter saves one byte, but apart from that, there is no difference.
Free42, on the other hand, stores numbers in programs in floating-point format, which means that the distinction between 1000 and 1E3 is not preserved. When it displays a program line containing a number, it formats the number in whichever way would be the most compact on the HP-42S, so, if you enter 1000, it is displayed as 1E3, but if you enter 10, it is displayed as 10. (When scientific and fixed-point representations are the same length, it chooses the latter, so 100 is displayed as 100, not as 1E2.)
Again, none of this has any effect on how Free42 performs calculations. I am mentioning this here because even though it may appear that Free42 does something differently than the HP-42S, this is a difference you should ignore.

The Android and iPhone versions do have a menu, which you will need to select skins, change settings in the Preferences, etc. To activate this menu, tap on the screen, anywhere below the status bar (or the top of the screen if there is no status bar) down to the top half of the display.

Why does Haptic Feedback not work on my iPhone / iPad / iPod touch?

The hardware and OS support needed for Haptic Feedback is currently only available on the iPhone 7, 7 Plus, 8, 8 Plus, X, XR, XS, and XS Max. Older iPhone models don't have it, and neither do any iPad or iPod touch models.

Unfortunately, iOS does not provide apps with a way to detect whether the devices they're running on have Haptic Feedback support or not, and so, in order to make sure that the option is available on all devices that do have that support, I have no choice but to put the Haptic Feedback switch in the Preferences screen on all devices.

If Apple ever adds a way to detect Haptic Feedback support, I'll change the Preferences screen so the Haptic Feedback switch is hidden or disabled on devices where it's of no use, but until then, the current confusing situation will continue. My apologies!

Why does OFF not work in the iOS version?

Apple's User Interface guidelines for iOS specify that apps should not have commands for exiting the app; exiting should only take place at the operating system's request, which in turn will only happen when the user presses the home button or uses the task switcher. Thus, the Free42 OFF command causing the app to exit was a violation of those guidelines.

For the longest time, Apple didn't seem to mind, but when I submitted 1.4.75a, it was rejected because of this issue. I had no choice but to comply, and so now, OFF just beeps, and OFF in a program causes the program to stop.

For people who liked and used the original OFF functionality, I added a back door to re-enable that behavior. Follow these steps:

Enter ALPHA mode: [Shift] [ENTER]
Type YESOFF using the ALPHA menu: [XEQ] [√x] [Σ+] [LN] [LN] [1/x] [LOG] [1/x] [1/x] [Σ+] [1/x] [Σ+]
Store the text in the X register using ASTO ST X: [STO] [.] [√x]
Perform OFF: [Shift] [EXIT]

With the string YESOFF in the X register, OFF works again. You only have to do this once; executing OFF this way sets a hidden flag telling Free42 to always perform the old-style OFF behavior from then on, regardless of the contents of the X register.

If you ever uninstall and reinstall Free42, you will have to repeat the procedure. You will not have to repeat the procedure when you get updates.

Why does Free42 not show up in the App Store on my iPad?

You must select "iPhone Only" in the selector in the top left corner.

Yes, this is highly misleading, because Free42 is not "iPhone only," at least not in the sense that it only runs on iPhones. What Apple means by "iPhone only" are apps that are not designed to take advantage of the iPad's larger screen. Free42 is "iPhone only" in that sense: it runs on iPads, but it acts just like it does on iPhones.

Why am I getting results that are rounded to integers?

You're doing, say 7 ENTER 2 ÷, in FIX 04 mode, and the result is 3.0000, while it should of course be 3.5000.

When this happens, it's because the calculator is in BASE mode. You can verify this by pressing and holding any of the arithmetic or +/- keys: in BASE mode, they perform BASE÷, BASE×, BASE-, BASE+, and BASE+/-, respectively, and these commands interpret their arguments as 36-bit signed binary integers, and return results within those same constraints.

To get back to normal behavior, leave the BASE application by pressing EXIT until no menu is shown in the display.

Why am I getting Size Error when I try to access any of the numbered storage registers, or try to use any of the statistics functions?

Usually, Size Error means you're trying to access a numbered register that doesn't exist, i.e. the SIZE setting (in the MODES menu) is set too low. By default, the setting is SIZE 0025, which means you have registers numbered from 00 through 24.

When even RCL 00 returns Size Error, however, that usually means that the REGS matrix was inadvertently deleted. This also causes all statistical functions to fail, since they all use numbered registers to access the summation data.

To fix this, and restore REGS to its default size, say SIZE 0025: [Shift] [+/-] [▼] [Σ+] [0] [0] [2] [5].

How do I assign SST↓ (Step Into) to the CUSTOM menu?

SST↓, also known as Step Into, introduced in Free42 2.1, is not a new function; rather, it is a new spelling of the existing SST function. Since functions are listed in the FCN catalog by their primary spelling only, this leads to the question of how to assign SST↓ to the CUSTOM menu, with the arrow.

The answer is to spell it out using the ALPHA menu: [Shift] [1] [ENTER] [LN] [1/x] [LN] [1/x] [LN] [√x] [▼] [1/x] [√x] [ENTER] [Σ+]

(Replace [Σ+] with whichever key in the CUSTOM menu you want to assign SST↓ to.)

	Android Get it at Google Play, or if you prefer the manual approach, download Free42Android.apk for side-loading. Requires Android 2.2 or later.
	iOS (iPhone, iPad, iPod touch) Get it from the iTunes App Store. Requires iOS 8.0 or later. NOTE: This is an "iPhone only" app, and as such, if you're using an iPad, the App Store app won't show it to you by default. You must select the "iPhone only" option from the drop-down menu in the top-left corner first. The app runs fine on iPads; Apple just makes it hard to find.
	Windows Download: Free42Windows.zip Requires Windows 2000 SP4 or later. If, when you try to run Free42, Windows says "This application has failed to start because the application configuration was incorrect. Reinstalling the application may fix this problem," then you also need to download and install the Microsoft Visual C++ 2008 Redistributable Package.
	MacOS Download App: Free42Mac.zip Download Dashboard Widget: Free42MacDashboard.zip Both require MacOS 10.7 or later.
	Linux Download: Free42Linux.tgz Built in Ubuntu 12.04, with glibc 3.2, libstdc++ 4.6.3, and GTK+ 3.4.2. It should work on any Linux (x86_64) with those or similar libraries.
Additional Downloads
	Skins Some skins to use instead of the faceplates built into Free42.
	HP-42S/Free42 programs A small collection of HP-42S/Free42 programs.
	rom2raw A C program that converts HP-41 ROM images to Free42/Emu42 compatible "raw" program files. Download rom2raw.zip; source code and Win32 console executable included.
	Free42 Source Code Download: free42.tgz Download for GTK only, no HP logo, for building Linux packages: upstream. Building the Android version requires the Android SDK 2.2 and NDK r4b or later. For the iOS version, I always use the latest Xcode and iOS SDKs. You may be able to build it with earlier releases, too, but I can offer no guarantees on that. For the Windows version, you need Microsoft Visual C++ 2008 or later. For the MacOS Application and for the Dashboard widget, you need the MacOS SDK 10.7 or later. The GTK version requires Linux, or any reasonably Unix-like environment, with X11, GTK+, and the usual development tools and libraries.