The original RPL (Reverse Polish Lisp) programming language was designed and implemented by Hewlett Packard for their calculators from the mid-1980s until 2015 (the year the HP50g was discontinued). It is based on older calculators that used RPN (Reverse Polish Notation). Whereas RPN had a limited stack size of 4, RPL has a stack size only limited by memory and also incorporates programmatic concepts from LISP. The first implementation of RPL accessible by the user was on the HP 28C (circa 1987) which had a Saturn processor. More recent implementations (e.g., HP 49, 50g) run through a Saturn emulation layer on an ARM based processor.