https://pygmy.utoh.org/3ins4th.html
If you have new hardware with zero tools, I think it is still possibly the quickest path to more sophisticated options.
i haven't tried the 'three-instruction forth' approach myself (which i had misremembered as being due to brad rodriguez rather than frank sergeant) but that paper was definitely a major factor in forming my viewpoint that forth is not really a programming language but rather an interactive environment that happens to have a programming language (a language the 'three-instruction forth' omits!)
you probably do need a fourth 'instruction' in practice: reset, or at least some kind of interrupt that returns you to the interpreter when you're in an infinite loop. usually there's a separate reset pin, though, which leaves memory intact so you can debug what went wrong — or, at worst, set up the thing so you can power-cycle it. usually that goes without saying, but sometimes hardware is physically far away these days...
it's the minimal amount of extra stuff you need to add on top of assembly language to give you recursive subroutines with parameters; structured control flow constructs like if-then-else (ahem, if-else-then), for-next (uh, do loop), and while; user-defined data types; closures (does>); turing-complete compile-time metaprogramming; embedded domain-specific languages; multitasking; and paged virtual memory. all this costs you about 4k of ram and (usually) a 5× slowdown compared to assembly. nowadays some of those are commonly provided by hardware, but forth has them even on hardware and operating systems that don't
in forth you can very easily invoke your assembly code interactively, look at the results, hex-dump parts of memory, script your assembly code, script your hex dumping, script your scripts, and so on. and it makes a fantastic macro assembler if you can get used to the syntax. the built-in repl gives you a built-in user interface, and it also serves as a sort of interactive debugger; you can't step through your code line by line, but you can invoke it word by word, which is pretty close to the same thing
i did this screencast of debugging an integer square root routine in gforth three months ago https://asciinema.org/a/621404 but i should ofer two cautions:
- i'm no forth expert
- if you actually need square roots in gforth you should probably just use fsqrt. 1024 0 d>f fsqrt f>d d. gives you 32 as ahura mazda intended