Its 49 step program memory and 8 data registers allowed a surprising amount of work to be done - for example a 42-step integration program left 7 steps to define a quite complicated equation to be integrated numerically.

The combination of small size and programmability made it an especially useful tool in cases where one-handed operation was needed. You could write a program to do a calculation, then hold the HP-25 in the palm of the hand, type in a number, and press the R/S key to run the program and get the answer, all with your thumb, leaving your other hand free. Only the Series 30 models will also let you work the same way - just - not even the later Voyager or current Pioneer models are small enough to allow this!

It became very popular - many people who could not afford an HP-65 or HP-67 used an HP-25 instead, and then moved to the HP-41 when that was introduced.

Its codename, "Squash", might be an allusion to the amount of functionality squashed into such a small package; in any case it naturally follows the HP-21 name "Pumpkin"

This article is part of the WMJARTS file. This file contains a series of articles written by Wlodek Mier-Jedrzejowicz and published in DATAFILE, the journal of the HPCC. The article was reproduced with permission of the author.