I have a 12C as well. The OS X desktop calculator has an RPN mode, but the very best RPN desktop calculator is Emacs Calc. It has an infinite stack, is progra

I have a 12C as well. The OS X desktop calculator has an RPN mode, but the very best RPN desktop calculator is Emacs Calc. It has an infinite stack, is programmable with Lisp, and can even operate on formulas in other buffers.