Earlier calculators needed a key, or key combination, for every available function. Hewlett-Packard were constrained by their canon ir error codes pdf byte only instruction format. The longest instruction required eleven keypresses, re-using the shift keys four times.
But the user had to remember the codes for them. Clearly, a more convenient and flexible method of executing the calculator’s instructions was urgently needed. The HP-41C had a relatively small keyboard, and only one shift key, but provided hundreds of functions. 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.
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. 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. Many users had used all four ports for memory expansion, leaving no room for other modules.