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The HP-42S
Type Programmable scientific
Manufacturer Hewlett-Packard
Introduced 1987
Discontinued 1995
Entry mode RPN
Precision 12 display digits (15 digits internally),[1] exponent ±499
Display type LCD dot-matrix
Display size 2 lines, 22 characters, 131×16 pixels
Processor Saturn
Programming language(s) RPN key stroke (fully merged)
Firmware memory 64 KB of ROM
Program steps 7200
Ports IR (Infrared) printing
Power supply 3×1.5V button cell batteries (Panasonic LR44, Duracell PX76A/675A or Energizer 357/303)
Weight 6 oz (170 g)
Dimensions 148×80×15mm

The HP-42S RPN Scientific is a programmable RPN Scientific hand held calculator introduced by Hewlett Packard in 1988. It has advanced functions suitable for applications in mathematics, linear algebra, statistical analysis, computer science and others.

HP-41 replacement[edit]

Perhaps the HP-42S was to be released as a replacement for the aging HP-41 series as it is designed to be compatible with all programs written for the HP-41. Since it lacked expandability, and lacked any real I/O ability, both key features of the HP-41 series, it was marketed as an HP-15C replacement.

The 42S, however, has a much smaller form factor than the 41, and features many more built-in functions, such as a matrix editor, complex number support, an equation solver, user-defined menus, and basic graphing capabilities. Additionally, it features a two-line dot matrix display, which made stack manipulation easier to understand.

Production of the 42S ended in 1995.[2] In the HP calculator community, the 42S is famous for its high prices in online auctions, with good condition calculators typically selling at around US$400 (2016)[3] This is nearly a four-fold increase in price over its introduction cost and has created a scarcity for utility end users. Yet, this calculator is regarded amongst the best ever made in terms of quality, key stroke feel, ease of programming, and daily usability for engineers.[4]

HP-42S specifications[edit]

HP-42S battery compartment and the IR diode
  • Series: Pioneer
  • Code Name: Davinci
  • Introduction: 1988-10-31
  • 64 KB of ROM
  • 8 KB of RAM
  • Functions: Over 600
  • Expandability: Officially no other than IR printing (32 KB memory upgrade and over-clocking hardware hacks are possible)
  • Peripherals: HP 82240A infrared printer

HP-42S features[edit]


The HP-42S is keystroke-programmable, meaning that it can remember and later execute sequences of keystrokes to solve particular problems of interest to the user. The HP-42S uses a superset of the HP-41CX FOCAL language.

The HP-42S supports indirect addressing with which it is possible to implement a Universal Turing machine and therefore the programming model of the HP-42S can be considered Turing-complete.

Sample program[edit]

This is a sample program which computes the factorial of an input integer number (ignoring the calculator's built-in factorial function). The program consumes 18 bytes. No memory registers are used.

Step Instruction Comment
01 LBLFAC Start of program "FAC"
02 1 1 is put into X, hence the value to be calculated upon (which was initially in X) is lifted (pushed) into stack register Y
03 LBL00 Define label 00
04 RCL×STY Recall stack register Y and multiply with X
05 DSESTY Decrement stack register Y and if not zero ...
06 GTO00 ... go back to label 00
07 END or RTN Returns control (and result in X) to either the user or to a calling program.


In May 2017, SwissMicros released pre-production samples of an RPN calculator closely resembling the HP-42S, the DM42. The final product was released on the 2017-12-09. Even though slightly smaller (144×77×13 mm, 170 g) than the original HP-42S (148×80×15 mm, 170 g), the calculator comes with an additional top row of keys for soft menus, a keyboard layout supporting direct alpha character input, a much larger high-contrast display (Sharp low power transflective memory LCD with a resolution of 400×240, protected by Gorilla Glass) showing all four stack levels at once, ca. 75 KB usable RAM, a beeper, a callable real-time clock as well as an infrared port for HP 82240A/HP 82240B printer support and an USB interface (with Micro-USB plug) emulating a FAT16-formatted USB mass storage device for easy program transfer and state backup / transfer as well as for firmware updates. The calculator, which comes in a stainless steel case with black matted PVD (physical vapor deposition) coating, supports keyboard overlays and is based on a modified version of Thomas Okken's GPLed Free42 simulator with Intel's decimal floating-point math library for higher precision (decimal128) running on an STM32L476RG processor (ARM Cortex-M4 core, 128 KB RAM, 1 MB internal flash) with another 8 MB of external QSPI flash (of which ca. 6 MB are available to users). It is powered by a CR2032 coin cell or via USB and clocked dynamically at 25-80 MHz.


Further reading[edit]

External links[edit]