Here is the owner's manual for the HP-15c. This is all you need to learn to program it. It's what I used long, long ago. https://www.hp.com/ctg/Manual/c0303058

Here is the owner's manual for the HP-15c. This is all you need to learn to program it. It's what I used long, long ago. https://www.hp.com/ctg/Manual/c03030589.pdf https://www.hp.com/ctg/Manual/c03030589.pdf At the time it came out (pre-smart phone), programmability in your shirt pocket was whiz-bang stuff. Obviously not so any more. If you're seriously into programming, you'll probably be disappointed. I'd recommend Forth instead. Forth is a RPN calculator on steroids.