Author here. Another tag that could be applied to this item is "z80".
The TI-83+ and TI-84+ may be the cheapest way to get into Z80 assembly language programming these days. They go for about $20-30 on eBay, sometime cheaper locally. The programming environment has some non-trivial TI-OS layer that you have to punch through, and the dev tools are a bit clunky, but once you get past that, your inner Z80 programming can take over: read the keyboard, write characters to the LCD screen, write pixels to the screen, and so on. The TI 83 Plus SDK provides access to various low-level OS functions.
The TI-83+ and TI-84+ may be the cheapest way to get into Z80 assembly language programming these days. They go for about $20-30 on eBay, sometime cheaper locally. The programming environment has some non-trivial TI-OS layer that you have to punch through, and the dev tools are a bit clunky, but once you get past that, your inner Z80 programming can take over: read the keyboard, write characters to the LCD screen, write pixels to the screen, and so on. The TI 83 Plus SDK provides access to various low-level OS functions.
The main discussion about the calculator features of the RPN83P app is happening on the MoHPC site: https://www.hpmuseum.org/forum/thread-20867.html