The HP-18C was a Hewlett-Packard business calculator which was quickly followed by the very similar but greatly improved HP-19B. The HP-18C was HP's first RPL-based calculator internally, even though this was not visible on user-level in this non user-programmable model. The user did have a solver (another HP first) available, but only had about 1.5 KB of continuous memory available to store equations.
The calculator had many functions buried in a menu structure. The clamshell design was fairly robust, but the battery door is the shortcoming of this whole line; 18C, 19B, and 28C/S.
The HP-18C was introduced in June 1986.
- Wickes, William C. (1988). RPL: A Mathematical Control Language. Proceedings Rochester Forth Conference on Programming Environments. Rochester, New York, USA: Institute for Applied Forth Research, Inc. pp. 27–32.
Several existing operating systems and languages were considered, but none could meet all of the design objectives. A new system was therefore developed, which merges the threaded interpretation of Forth with the functional approach of Lisp. The resulting operating system, known unofficially as RPL (for Reverse-Polish Lisp), made its first public appearance in June of 1986 in the HP-18C Business Consultant calculator.