The Catalogs of Functions. The HP-41C has three separate catalogs of functions. You can list the programs you have written; more than 130 resident HP-41C functions; all functions contained in plug-in modules (more about modules in a minute). There is never any doubt as to what is resident in the calculator—all you have to do is list the catalogs.
Key Reassignments. Nearly any function in the HP-41C (functions you have written, standard HP-41C functions, application module functions) can be assigned or reassigned to most key or shifted key locations on the keyboard. This allows you to ‘‘personalize’’ your calculator, positioning functions on the keyboard where you want them.
HP-41C Extensions. The basic HP-41C comes with 63 data storage registers or 63 registers of program memory (that’s 200-400 lines)—and you can define the combination of data storage registers and registers of program memory that you desire (for example, the HP-41C begins with a combination of 17 data storage registers and 46 register of program memory). But you are not limited to the basic machine capacity! You have the option to increase the capacity and capability of your HP-41C by adding up to four additional ‘‘plug-in’’ modules. Each module contains 64 date storage registers or 64 registers of program memory. You can increase the HP-41C capacity to a maximum of 319 registers of program memory or 319 data storage registers, in any combination!
But That’s Not All! The HP-41C has four input/output receptacles (ports). You will be able to plug in additional program memory/storage modules as well as complete technical application modules (application ‘‘pacs’’)—even a HP-67/HP-97-compatible card reader and thermal printer.
  Always turn the HP-41C off before inserting or removing any plug-in extensions or accessories. Failure to do so could damage both the calculator and the accessory.