Now I have some time I can do some of those things I've been meaning to look at for a while, like reverse engineering the ROM on this thing which I think is even more of a marvel than the better known Sinclair. It's programmable and yet the ROM is 1.5K in size.
But it's hard - the instruction set is bizarre. For example ADX is a 4 bit add which behaves differently in the special case of adding 6. It's like someone printed out a program for a calculator in C, cut it up into random pieces, and made each piece a machine instruction whatever lines of code just happened to get grouped together. So someone saw an add followed by a comparison with 6 and thought "let's call that ADX".
https://www.seanriddle.com/nationalcopsi.pdf
But it's hard - the instruction set is bizarre. For example ADX is a 4 bit add which behaves differently in the special case of adding 6. It's like someone printed out a program for a calculator in C, cut it up into random pieces, and made each piece a machine instruction whatever lines of code just happened to get grouped together. So someone saw an add followed by a comparison with 6 and thought "let's call that ADX".
https://www.seanriddle.com/nationalcopsi.pdf