Back when I was writing 6502, you didn't keep it all in your head, you kept it in reams of tractor-fed dot matrix paper, usually with a lot of scribbled pencil notes and corrections.
I've done a bit of embedded development for 8-bit machines in the modern era, and I sincerely tip my hat to the engineers who were doing it in the 80s. Even with the best tooling available, you'd have needed an enormous amount of organisation and discipline to keep track of your code. I was doing some research about the assemblers available for the Hitachi 6303[0]/Motorola 6800[1] in the early 80s, and I'm amazed by what people accomplished with the limitations of these tools. Just to name one thing, the two assemblers I've linked to only supported SIX character labels, and no local-only labels within subroutines either. To compare, in my own work I was limiting myself to FORTY characters for the sake of being descriptive.
Having said all this, the relative simplicity of these architectures is really refreshing compared to embedded work on modern systems. There's something to be said for being able to roughly keep track of the whole system state in your head.
Well, it depends. On some environments such as Lucas Arts they used Unix based OSes to develop SCUMM. With grep and such, editing and porting the engine could have been a breeze.
Half joking, by the time of the NES port he would have certainly had many available options for all that , but even with access to a timesharing system with real storage and all that in the earlier 80s having at least sections on paper for easy reference against printed manuals was handy.
This code is extremely well organized and documented. I don’t know what the original code would have looked like in terms of documentation, but I imagine there was a lot of it printed on paper with handwritten notes and stuff.
Overall though, assembly programming is much like any high level language: you organize your code into functions and blocks etc. Just writing one super long wall of code with a rat’s nest of jumps throughout is asking for serious trouble.
