Four types of RPN

[htom trites]

There are some more obscure variations, especially of the top of the stack on a drop:

• top copy
• top is zeroed (HP-35 does this until T has a non-zero value)
• top drop to empty stack
• top lifts past a height, drops to a height, then copies
• top lifts past a height, drops to a height, then zeroed
  

"Infinite", unbounded, expanding stacks can be handled in different ways. Usually there -is- a bound, just huge. Sometimes a chunk of the top of the stack is removed with a marker placed to recover it as the stack lowers (these are not infinite (there's a limit to the size of the address of the storage place) but can be much larger than the immediate register space.) Some of these variations have not been implemented in hardware calculators that I know of.

I personally refer to the four discussed types as HP-35 RPN, HP-48 RPL, HP-20b RPL, and Mac RPN.