floppy disk via: PC via one of: The HP-41C series are programmable, expandable, continuous memory handheld RPN calculators made by Hewlett-Packard from 1979 to 1990.

The alphanumeric LCD screen of the HP-41C revolutionized the way a pocket calculator could be used, providing user friendliness (for its time) and expandability (keyboard-unassigned functions could be spelled out alphabetically).

The longest instruction required eleven keypresses, re-using the shift keys four times.

The TI-59 also made use of the Op key followed by two digits to access another 40 different functions, but the user had to remember the codes for them.

[2] Alphanumeric display also greatly eased editing programs, as functions were spelled out in full.

Encoding functions to the corresponding numeric codes, and vice versa, was left to the user, having to look up the function–code combinations in a reference guide.

In addition to this, the user had to mentally keep function codes separate from numeric constants in the program listing.

HP's competitor Sharp, when introducing the PC-1211, used a dot matrix of 5×7 dots and displayed the characters in principle as we see them today on computer screens (and, in fact, many LCD screens on various embedded systems); this was later used by HP with the HP-71B handheld computer.

Enthusiasts around the world found new ways of programming, created their own software (such as a codebreaking game,[6] and a version of Hunt the Wumpus[7]) and expansion modules, and sped up the clock (see overclocking).

One of the discoveries of the community was that, because some FOCAL instructions were stored in memory as more than one byte, it is possible to exploit a bug in the program editor to assign strange functions to keys.

Through synthetic instructions, a user could access memory and special status flags reserved for the operating system, and do very strange things, including completely locking the machine.

Hewlett-Packard did not officially support synthetic programming, but neither did it do anything to prevent it, and eventually even provided internal documentation to the user groups.

The author praised HP's documentation as "among the best in the industry", and reported that the calculator was "much easier to program and debug" than the TI-59.

Stating "I found the HP-41C far more pleasurable to program and use than its predecessors" because of its alphanumeric display, the author concluded that it was "maybe not quite" a pocket computer.

It comes close enough for the jargon of computers to be useful in describing it", and praised the simplicity and reliability of the card reader and compatibility with HP-67 and HP-97 software.

[9] An HP-41C that flew on nine early Space Shuttle missions is on display in the National Air and Space Museum in Washington, D.C. HP-41Cs with some special hardware configurations (the addition of Velcro strips, pre-production time module, and louder beeper, as well as the removal of parts whose outgassing could cause contamination) were carried on early Shuttle missions for the purpose of performing mundane but necessary calculations, such as calculating the change to the center of gravity due to fuel consumption.

Four slots were available to add more memory, pre-programmed solution packs containing programs covering engineering, surveying, physics, math, finance, games, etc.

HP-41 extension (or expansion) modules allowed the user of an HP-41 programmable calculator to extend the functionality of the machine.

User groups found a way to merge two memory chips in a single module, thus freeing two expansion slots.

As the HP-41C had non-volatile memory (user programs and data were not wiped out when power was off) there was no absolute need for a permanent storage device, so the card reader was optional.

The printer had some graphical capabilities, to allow the user to design character shapes, or even turn on or off individual pixels.

The Extended Functions module added instructions to manage a bank of additional memory in the available addressing space of the machine.

Extended memory could also hold ASCII files, and a rudimentary text editor was added also.

Named DM41, it runs the original HP-41CX firmware with extended memory and realtime clock in an emulator on an ARM Cortex-M0-based NXP LPC1115 processor.

[12] Deviating from the original, it comes in a landscape form factor (as known from Hewlett-Packard's Voyager series) with rearranged keys, it features a dot-matrix display, switchable clock speeds, and, based on a Silicon Labs CP2102 converter chip,[13] it comes with a USB (Mini-B)[14] serial interface to exchange data with a PC etc.

for backup purposes, to possibly communicate with applications (like PC-based emulators),[13] or to update the firmware.

Since September 2020, its successor, the DM41X, has also been offered with an extended range of functions and a conventional design that is very close to the classic original version.

[15] In addition, a "turbo" mode is provided by the HP41CL upgrade board's "NEWT" CPU design, which allows the user to speed up their calculator by a factor of around 50 times.

[18] Furthermore, an optional RS-232 full-duplex serial connector (with a 2.5mm stereo jack physical interface) is available, if one has a free module slot.

[19] Unusually for pocket calculators, the devices still have a fan base more than 30 years after their production was discontinued in 1989.