Editing HP-41C (section)

== The alphanumeric "revolution" ==
[[File:LCD Display of HP41CX (cropped).jpg|thumb|The HP-41C's alphanumeric display allowed it to display numbers, characters, and symbols.]]
[[File:HP-41CV Calculator.jpg|thumb|upright=0.9|The HP-41CV had a memory five times larger than that of the first model HP-41C. On the lower, inclined flank of the keys, the alphanumeric characters are printed in blue, which are accessible after pressing the "ALPHA" key top right.]]

The [[alphanumeric]] [[liquid crystal display|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). By using an alphanumeric display, the calculator could tell the user what was going on: it could display error messages, such as showing ("<samp>DATA ERROR</samp>") upon attempting to divide by zero instead of simply displaying a blinking zero; it could also specifically prompt the user for arguments ("<samp>ENTER RADIUS</samp>") instead of just displaying a question mark.

Earlier calculators needed a key, or key combination, for every available function. The [[HP-67]] had three shift keys (gold "f", blue "g" and black "h" prefix keys); the competing [[Texas Instruments]] calculators had two ({{key top|2nd}} and {{key top|INV}}) and close to 50 keys (the [[TI-59]] had 45). Hewlett-Packard were constrained by their one byte only instruction format. The more flexible storage format for programs in the [[TI-59]] allowed combining more keys into one instruction. The longest instruction required eleven keypresses, re-using the shift keys four times. The TI-59 also made use of the {{key top|Op}} key followed by two digits to access another 40 different functions, but the user had to remember the codes for them. Clearly, a more convenient and flexible method of executing the calculator's instructions was needed. The HP-41C had a relatively small keyboard, and only one shift key, but provided hundreds of functions. Every function that was not assigned to a key could be invoked through the {{key top|XEQ}} key (pronounced ''E'''XEQ'''TE''&nbsp;&mdash; "execute") and spelled out in full, e.g. {{key top|XEQ}} FACT for the [[factorial]] function.

The calculator had a special user mode where the user could assign any function to any key if the default assignments provided by HP were not suited to a specific application. For this mode, the HP-41C came with blank keyboard templates; i.e. plastic covers with holes for the keys, so the user could annotate customized keys. Hewlett-Packard even sold a version of the calculator where hardly any keys had function names printed on them, meant for users who would be using the HP-41C for custom calculations only (thus not needing the standard key layout at all); this version of the calculator was colloquially known, within HP's Corvallis calculator team, as a "Blanknut" (because the development code name for the HP-41C's processor was known as the "coconut").<ref name="Coconut"/>

Alphanumeric display also greatly eased editing programs, as functions were spelled out in full. Numeric-only calculators displayed programming steps as a list of numbers, each number generally mapped to a key on the keyboard, often via row and column coordinates. Encoding functions to the corresponding numeric codes, and vice versa, was left to the user, having to look up the function&ndash;code combinations in a reference guide. The busy programmer quickly learned most of the codes, but having to learn the codes intimidated the beginners. In addition to this, the user had to mentally keep function codes separate from numeric constants in the program listing.

The HP-41C displayed each character in a block consisting of 14 segments that could be turned on or off; a so-called [[fourteen segment display]] (similar to the much more common [[seven segment display]]s, which can be used to display digits only). The HP-41C used a liquid-crystal display instead of the ubiquitous [[light-emitting diode|LED]] displays of the era, to reduce power consumption.

While this allowed the display of uppercase letters, digits, and a few punctuation characters (the [[FOCAL character set]]), some designs needed to be twisted arbitrarily (e.g. to distinguish S from 5) and lowercase letters were unreadable (HP only provided display of lowercase letters a through e). HP's competitor [[Sharp Electronics|Sharp]], when introducing the [[Sharp PC-1211|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 system]]s); this was later used by HP with the [[HP-71B]] handheld computer.