Free42 was written from scratch, without using any HP code. You do not need any ROM images in order to use it, yet it is fully HP-42S compatible.

You can use some skins instead of the faceplates built into Free42. Download them here, contains 20 skins, last updated March 7, 2010.

If you like this project, please consider donate to the developer.

Update Description:
v1.4.66 (08-04-2010):
- Removed the "Not Yet Implemented" Time functions; I decided not to implement them. They're also gone from the FCN catalog. The ADATE, ATIME, ATIME24, CLK12, CLK24, DATE, DATE+, DDAYS, DMY, DOW, MDY, and TIME functions remain; those are the time/date-related functions from the HP-41 Time Module that are actually useful in Free42. The functions that I decided not to implement after all are the following:
    o Live clock display: CLKT CLKTD CLOCK
    o Stopwatch: RCLSW RUNSW SETSW STOPSW SW
    o Alarms: ALMCAT ALMNOW XYZALM
    o Clock control: CORRECT RCLAF SETAF SETDATE SETIME T+X
- In DMY mode, DATE would display dates as DD:MM:YYYY, but that should be DD.MM.YYYY. Fixed.

v1.4.65 (03-04-2010):
- Implemented some more Time Module functions: ADATE, ATIME, ATIME24, CORRECT, RCLAF, SETAF, SETDATE, SETIME, T+X.

v1.4.64 (30-03-2010):
- Implemented some more Time Module functions: DATE+, DDAYS, and DOW.

v1.4.63 (28-03-2010):
- iPhone version: implemented ACCEL, LOCAT, and HEADING functions, for querying the device's accelerometer, location services (GPS on the 3G and 3GS; WiFi- based on all other models), and compass (3GS only).
- All versions except Mac Dashboard: for skins, increased the maximum macro length from 31 to 63. The old limit was too small to allow macros that step forward all the way to the end of the FCN catalog, and supporting such macros is necessary now that the length of the FCN catalog is no longer fixed at 42 rows.
- Also, changed several macros in the HP-41 and HP42CY skins where the FCN catalog was being traversed backwards, replacing N repetitions of "up" with 42-N repetitions of "down", so they will work correctly in the presence of an extended FCN catalog. (None of the other skins currently in the Free42 repository traverse the FCN catalog backwards, so they don't need updating.)
- PalmOS version: turning off printing did not flush the GIF file, so you'd end up with a truncated image. Fixed.
- When COMPLEX is executed with a complex number or complex matrix in X, and POLAR mode is active, it is possible for range errors to occur. Free42 would simply substitute POS_HUGE_VAL in such cases, but that was incorrect; it should report Out of Range or substitute POS_HUGE_VAL depending on the setting of flag 24. Fixed.
- R/S can now be pressed using the PC keyboard even when ALPHA mode is active.

v1.4.62 (07-03-2010):
- Windows version: now has an option in the Preferences screen to map the Calculator key, found on many newer keyboards, to launch Free42.
- iPhone version: the hostname lookup for the HTTP Server window is now done in a background thread, so that a slow or malfunctioning DNS server no longer causes the app to freeze on startup.
- When EDITN was active, you could overwrite the edited matrix with a scalar using STO, or delete it using CLV -- and then you would be stuck in the Matrix Editor, and even CLALL couldn't get you out. Fixed; you will now get the Restricted Operation message when you try to change or delete the active matrix behind EDITN's back.

- Added HP-41 skin to the Free42Skins.zip package. This skin mimics the HP-41 layout, and includes many macros to map additional functions to the keyboard. Contributed by Guenny.

v1.4.61 (25-11-2009):
- Decimal version: ->OCT (decimal-to-octal conversion) was broken; it actually converted octal to decimal. Fixed.

- Added HP42CY skin to the Free42Skins.zip package. This skin includes many macros to map additional functions to the keyboard. Contributed by Guenny.
* Added kacskin and kacskin_yellow skins to the Free42Skins.zip package. These are slightly modified versions of Michaels HP, and may be easier to read on some screens. Contributed by Keith Carangelo.

v1.4.60 (08-11-2009):
- Decimal version: SQRT could be off by 1 in the final digit. It now computes one additional digit and rounds it into the mantissa, which should make it accurate in all cases.
- Decimal version: The BCDFloat add and subtract functions could return denormalized results when one of the arguments was zero. Free42 would not display these denormalized numbers correctly, leading to strange results like 1.0001 LOG returning something that looked like zero but wasn't. Fixed.

v1.4.59 (31-10-2009):
- MacDashboard version: changed the background from green to gray, and changed the transparency to fully opaque.
- PalmOS Decimal version builds & works again; fixed the SQRT crash and the divide bug (caused by 16-bit integer overflow, oops).
- Decimal version: re-fixed the subtract bug which had originally been fixed in 1.4.53, but reappeared in the great BCDFloat/BCDFloat2 refactoring.
- Decimal version: 1e-10000 LOG and 1e-10000 LN returned <Not a Number>. Fixed.
- CLV and PRV didn't work from the CUSTOM menu, potentially even causing a crash. Fixed.

v1.4.58 (24-10-2009):
- Removed Byron's BIGSTACK and BIGLCD hacks; these will be maintained in a separate fork from now on. This protects the other Free42 versions from bugs in the 42S/iPhone code, and right now, it fixes some display glitches in the other versions.
v1.4.57 (19-10-2009):
- New LOG code (again), for exact results when the argument is a power of 10. The bug from version 1.4.55 is fixed.
- Subtract would lose one digit of precision for edge cases like 1 - 0.9999999999999999999999999; fixed.

v1.4.56 (12-10-2009):
- The new LOG implementation introduced in Decimal version 1.4.55 was broken; rolling back to 1.4.54 version. (Note that this means that powers of 10 do not yield exact results, but at least everything *else* is OK again now.)

v1.4.55 (10-10-2009):
- Some improvements in the new BCDFloat code:
    o LOG (common log) is now precise for powers of 10.
    o MOD function now works for large arguments.
    o rounding fix in multiply now gets 1/3*3-1 and 10/3*3-10 the same answer, 10/7*7-10 is not the same, but this is correct (it depends on where you cut off the number).
    o optimisations to subtract routine.
- Mac version: now uses $HOME/Library/Application Support/Free42 instead of $HOME/.free42 for persistent state, keymap, and skins. If this directory does not exist, but $HOME/.free42 does, the latter is renamed to the former.
- Switching from a version using the old BCDFloat code (1.4.51 or earlier) to a version using the new code (1.4.52 or later) wasn't handled properly; it should handle the change in how Infinities and NaNs are represented, but it didn't. This is related to the problem fixed in 1.4.54.

v1.4.54 (28-08-2009):
- The new BCDFloat code, introduced in version 1.4.52, uses a different convention for representing NaN and infinities: instead of using special values in the exponent, it now uses dedicated flags, and has a narrower exponent field. The code that converts between decimal and binary, when switching between Free42 Decimal and Free42 Binary while keeping the same state file, did not deal with this new convention, and, as a result, switching between those versions could cause numbers to become NaN or Infinity when they shouldn't. This release fixes this. It is only relevant to those who switch between Free42 Decimal and Free42 Binary.

v1.4.53 (17-08-2009):
- Decimal version: fixed a bug in the subtract routine that could cause incorrect, malformed results in some cases, e.g. 4 ENTER 3 / 1 - 3 * 1 - returned -:.E-25.

v1.4.52 (08-08-2009):
- New: Mac version. This is a native application, not a Dashboard widget; it is functionally identical to the PC versions (Windows, GTK, and Motif). The print-out window doesn't work yet, though printing to files does; everything else is working. The print-out window will be finished in a two or three weeks, barring the unforeseen. I'm releasing the app in this incomplete state because I feel it is stable and usable, and should be a big improvement for those currently using the Dashboard widget. This is a Universal application, so it should run on PowerPC and Intel-based Macs. It requires Mac OS X 10.5 or later.
- Decimal version: Merged Hugh Steers' latest BCD code. This provides more accurate transcendentals and some bug fixes.
- Also, Hugh improved the error estimate for the Romberg integration code; it now converges more quickly in many cases.
- MacDashboard version: now recognizes '*' and '+' on the numeric keypad.

v1.4.51 (10-07-2009):
- Changed the homepage URL to http://thomasokken.com/free42/.

v1.4.50 (29-06-2009):
- Adding or subtracting two complex matrices, and adding or subtracting a complex matrix in X to or from a complex scalar in Y, would cause memory corruption, resulting in a crash soon after. Fixed.

v1.4.49 (07-06-2009):
- BASE/ tried to catch division by zero by looking at Y instead of X. Oops! The result was that 0/1 would return "Divide by 0" and 1/0 would cause Free42 to crash. Fixed.
* PalmOS version: tall skins can now be selected on all devices, regardless of whether they have tall screens or not. On most devices, this isn't useful, but the tallness check had the effect of suppressing the 320x450 Ehrling42sm skin on the Tapwave Zodiac, which has a 480x320 screen; the result was that you couldn't use that skin even if you switched to portrait mode.

v1.4.48 (04-04-2009):
- Fixed PSE; it no longer freezes the UI for one second, but uses a proper timeout, so event processing can continue, and keystrokes during PSE have the proper effect (of stopping program execution and then being handled normally).

v1.4.47 (04-03-2009):
- Another UVEC bug: it should return Invalid Data when the magnitude of its argument (|z| for complex scalars, FNRM for real matrices) is zero. It returned its argument unchanged instead. Fixed.

v1.4.46 (22-02-2009):
- UVEC returned Invalid Type for complex scalar arguments. Fixed; it now returns z/|z| (same as the SIGN function for such arguments), like the real HP-42S.
- When functions with arguments, e.g. STO, FIX, etc., were entered into a program from the CUSTOM menu, they would get inserted before the current program line instead of after. Fixed.

v1.4.45 (28-01-2009):
- ASTO IND ST X was broken; instead of storing the first 6 characters of the ALPHA register into the register or variable pointed to by X, it stored them into the variable pointed to by the first six characters of ALPHA itself. Fixed.
- Added "Lienhard480x800" skin for 480x800 Pocket PC devices (Free42PocketPCSkins.zip package). Contributed by Thomas Lienhard. - Added "Sven320x240mini" skin for 320x240 Pocket PC devices (Free42PocketPCSkins.zip package). This skin only shows the keys for basic arithmetic and stack manipulation, making the keys large enough to use easily without a stylus. Contributed by Sven from Vienna.
- Added "Ramos480x640" skin for 480x640 Pocket PC devices (Free42PocketPCSkins.zip package). It has a distinctive, sparse look. Contributed by Noe Ramos.

v1.4.44 (16-08-2008):
- Made auto-repeat for number entry and ALPHA modes optional. The auto-repeat feature, introduced in release 1.4.11, is not found on the real HP-42S, and can cause problems if you don't expect this kind of behavior in a calculator. It can now be turned off in the Preferences dialog, though the default is still for it to be on. Note that this is unrelated to the auto-repeat for the 'up' and 'down' keys, which is authentic HP-42S behavior, and cannot be turned off.

v1.4.43 (12-04-2008):
- MacDashboard version: fixed event handling so that program execution now works. There's still plenty of room for improvement: key timeouts (so pressing and holding a key behaves like it does on the real HP-42S and on the other Free42 versions) and auto-repeat come to mind... But at least programs, the solver, the integrator, and interruptible matrix functions should all be usable now.

v1.4.42 (10-03-2008):
- Added a HI_RES_AWARE resource so that Free42 will now be able to take advantage of high-resolution displays without the help of Tweaks2k2 or similar utilities.