I have recently implemented the breadboard computer design on a PCB, calling it BeanZee, and which I plan to use as a development board for further learning and experimentation.

To make this usable I have finally got Marvin, my tiny monitor program, to the point where it is sufficiently complete to be usable / useful with BeanZee or the breadboard computer.

https://github.com/PainfulDiodes/marvin

With it you can now:

• Read from memory as hex data
• Write hex data to memory
• Load a program from an Intel HEX format file
• Execute a program from a specific address

This means you can build a program on your host computer using a cross-assembler or cross-compiler, and so long as you can produce an Intel HEX file you can load and run the program on the breadboad computer / BeanZee via a terminal emulator.

The repo includes a couple of really simple test programs.

I have been using SjASMPlus for assembling Marvin, but this does not include the means to generate Intel HEX output. For this I am using z88dk-appmake. 

I will be taking another look at z88dk, as this seems to have a comprehensive toolset for Z80 work.