Appendix F
HP-41C Extensions
The system capabilities of the HP-41C can be greatly expanded by connecting it to one or more peripheral devices. Available as system extensions, these devices enable you to customize your computational system to suit your particular requirements. You can supplement the standard features of the basic HP-41C with:
  Memory Modules for increased program and data storage capacity.
  Magnetic card input and output.
  Printer output.
  Extensive applications libraries.
  Input and output through other peripheral devices.
Four input/output (I/O) ports are provided on the top of the HP-41C for interfacing with these devices. A detailed description of capabilities and operation is provided with each device. But to give you a feel for the remarkable power you can achieve by adding to your basic HP-41C calculator, lets look briefly at some of the devices available.
  Always turn the HP-41C off before inserting or removing any plug-in extensions or accessories! Failure to turn the HP-41C off could damage both the extension and the calculator.  
HP 82106A Memory Module
With up to 63 registers of program memory or 63 registers of data storage, or any combination, the basic HP-41C can swallow a whale of a computational task. But suppose your application requires even more program or data storage capacity. To meet your needs, Hewlett-Packard developed the HP 82106 A Memory Module. Each module contains an additional 64 registers that can be allocated as program memory or storage registers, or any combination. You can add four memory modules to your HP-41C system, providing you with a whopping 319 registers. (That’s 1000 to 2000 lines or program memory.)
Just as the internal memory of the HP-41C, the additional memory contained in each memory module can be allocated in various combinations between program and data storage. And all of the additional memory, just like the internal memory of the HP-41C, is Continuous Memory. As long as the memory module is plugged into the HP-41C, its contents are preserved for your later use, even while the HP-41C is turned off.