I still have notebooks full of programs I wrote in 6502 machine language. I would write out the mnemonics then go through and hand-assemble, figuring branch offsets, etc. To make changes I'd scratch out, insert, refigure the branches, until it was too messy then I'd rewrite it fresh. The KIM was my first true love - ah the heady days of discovery! My second computer was an AIM-65. While I was waiting for it to arrive I wrote an assembler. It worked the first time.