RPN83P: RPN calculator for TI-83+ TI-84+ inspired by HP-42S

[bxparks]

(11-11-2024 02:58 AM)spiff72 Wrote:  Is it correct that the "nSTK" mode that's available on free42 and DM42 is not available for RPN83P?

I did see the SSIZ option can expand the stack to 8, but I was curious if the "infinite" stack size was an option.

That is correct, NSTK is *not* available in RPN83P. A couple of reasons:

1) Usefulness. I'm not convinced that an "infinite" stack is particularly useful. Maybe if/when RPN83P becomes programmable, it could become more useful. But even then, the programming environment will be RPN, not RPL where the stack is far more critical. I think for RPN83P programs, a "local" stack feature would be more useful than an "infinite" stack.

2) Technical limitations. The RPN83P app is one of many other TI-OS apps that must coexist on the TI-83/84 calculator. All applications must share the ~24 kB of RAM provided by the OS. The memory management system provided by the TI-OS is limited and customized for the TI calculator environment: there is no general purpose "heap" memory for example. With my current knowledge of the TI-OS, I actually cannot think of a way to implement an "infinite" stack efficiently, both in terms of CPU cycles, and memory space. Every resizing of an OS appVariable causes defragmentation (because the OS is optimized to conserve memory), so it is a very expensive CPU operation. The alternative is to use an amortized O(1) allocation process, but that would waste significant amounts of precious memory.

With so many other features that could go into RPN83P, NSTK falls so low in the priority list that I doubt it will ever be implemented. Now, if an 8-level stack is insufficient, it is possible to increase the maximum size to something bigger without too much work. But I'd be curious to learn why 8 levels is not enough.