HP 39gs SysRPL

SysRPL or System RPL is only found on HP calculators. SysRPL programs have several advantages. They can be tiny (as the code for say DISPROW1 is stored in ROM). The can also be written and compiled on the calculator itself. Also there is a good chance there is a command that will do exactly what you want.

They also have disadvantages as Stack-Based programs hard to follow. Generally there is no type checking at either compile or run time. SysRPL is slow as most SysRPL commands are as they are only emulated.

HP 39gs Official Supported Entry Points List

SysRPL Programming Example