XEQ41

Not a lookalike: it runs the original 1983 microcode, so it answers the way the machine did quirks and all. Free, no ads, no in-app purchases.

XEQ41 is a hand-written emulator of a 1983 programmable RPN calculator, running the original manufacturer's microcode on a simulated NUT processor. It is not a re-implementation of the machine's behaviour — it is a simulation of its processor, so the calculator on screen executes the same instructions the hardware executed, and does what the hardware did for the same reasons.

THE DETAILS THAT GET TESTED

• Hold a key too long and the microcode itself cancels the function and prints NULL. That is 7,100 instructions — 1.23 seconds at original speed. Nobody wrote a timer; the ROM does it, and it stays 1.23 seconds when you run the machine faster. • BEEP is four tones, not three: 578, 361, 722 and 578 Hz, because the microcode falls through into a fourth. The test was written expecting three and the machine corrected it. • The display is modelled as a liquid crystal panel rather than sampled as a register, so the half-built strings the hardware never showed stay hidden and the 174 ms blink it does intend does not. • Continuous memory. Close it, force-quit it, come back next week — it is where you left it. • Shift-ON puts up a running clock, exactly as the manual says.

THE FACEPLATE IS DRAWN, NOT PHOTOGRAPHED

Thirty-nine moving key caps, the glass and the annunciator row are all vector, generated from a measured coordinate table — sharp at any size, with no image assets anywhere. A key going down travels, closes the shadow beneath it and darkens. The fourteen-segment display face is ours outright, drawn from our own segment table, so no third-party font ships.

Every key is an accessibility element whose label is composed from the printed legend, so VoiceOver says "x squared" where the plate prints x².

WHAT IS IN IT

• FOCAL programming, with program files in and out through Files • Extended Memory, with real file structure • Four ports for plug-in module images you supply • The 82143A thermal printer and its paper tape, once you supply its ROM image • Synthetic programming — the byte grabber runs • Saved machines that record what was plugged in and refuse to open as something else • Alarms, the clock, BEEP and TONE, LCD contrast, haptics • Fifty transcribed keyboard overlay cards • Three faceplate palettes, and an optional extra display for the stack or the machine's status • Seven FOCAL programs written for this app, free to use and modify

HOW IT IS CHECKED

Thirteen scripted scenarios run byte-identical against a second, independent implementation of the same processor. Six programs from printed sources reproduce their published answers, including a polynomial fit and a singular linear system whose correct answer is the degenerate one. All thirty coordinates of a published planetary ephemeris match to the seven digits the display carries. 673 tests.

PRIVACY

There is no networking code in the app at all. No account, no analytics, no advertising, no tracking, nothing collected. Everything stays in the app's own storage on your device.

WHAT IT IS NOT

It is not a general-purpose calculator — it is one specific machine, and if you have never wanted that machine it has nothing to offer over the one already on your phone. Third-party module images — the printer's ROM among them — are yours to import, not ours to distribute.

COMPATIBILITY

XEQ41 is functionally equivalent to the HP-41CX.

XEQ41 is not affiliated with, authorized by, endorsed by, or sponsored by Hewlett-Packard. HP, Hewlett-Packard and HP-41 are trademarks of their respective owners.