I am around your age and started then as well, also Z80. In my area it was rather normal to just write and run hex in your head. I still know the whole instructionset in hex as I wrote so much code with it. I totally fail to see the reason why anyone did use an assembler for that; I can just read and write the hex codes and did. Besides the time it took to get back after a crash it did not take much more time to 'get shit done' then as it does now. There were just vastly less programmers and thus less code was written. Unfortunately I don't have my 80s code anymore but my output has not changed much; better code and the code does more because of the libraries we have now, but it's around the same amounts. Running code in my head is great and was easier with something limited as an MSX (or zx for that matter); but it gives me a lot of advantage I believe.