This computers firmware is a variant of my Terminal-BASIC. It suffers from heavy weight, due to my limited abilities and C++ it's written in, but it is important for me as the window in the world of language translators and learning desk.
But I start to think about the alternative firmware, which is a compromise between the simplicity and flexibility. It will be based on Z80 CPU emulator.
On the host side there will be a piece of C++ code. First component is an interactive monitor, allowing to view/modify the Z80 emulator ram, load and store ram portions to or from SD card, may be some assemble/disassemble in the future. The second component is a Z80 emulator calls library, allowing to call the native C++ functions from the z80 code.
The main purpose of this firmware is the ability to write programs in C, using sdcc for z80, store multiple programs to SD card and run it without reprogramming the Arduino flash.
Unlike great projects, running CP/M emulator on Arduino DUE, this approach gives the ability to use the boards with a cople kilobytes of RAM to run simple programs, as long as CP/M will not run with less then 64k RAM, and more need to an emulator itself.
Compared to the Terminal-BASIC, which allows to run programs from SD card too, I expect, compiled C code, running in Z80 emulator to be much faster.