[tptacek]

Strong agree on emulator and particularly stack vm. Would add: TCP/IP stack.

[vram22]

I once corresponded with someone who wrote his own working TCP/IP stack for his own home PC, an IBM PC Jr.

See Mike's PCJr page near end of this post:

http://jugad2.blogspot.in/2012/09/lissajous-hippo.html

[vram22]

Update: Visited his page again. He has now even created a web server for the PCJr and ran his site on it for a while.

[nialo]

mostly curiosity: why a stack vm in particular? (is it because you then need to write the compiler for it?)

[tptacek]

Because if you're familiar with conventional real register architectures, the stack VM forces you to rethink it, in a much simpler way. It's a useful simplifying abstraction.

[vram22]

There used to be a lot of interest in, and articles about, stack machines (real ones, not VMs) back in the days of mags like BYTE and PC Mag. Good reading.