Heh, yes those were the days. It was so nice that back then home computers came with pretty detailed low-level documentation on the inner workings and protocols plus assembly manuals.
Specifically because the internetz was not that ubiquous yet... (sure I was on Fidonet then but that was a bit different thing....)
That was back in -85 or something, give or take few years.

I had both C64 and my first love, MPF-II from Multitech, the company that was to became Acer later.
In my C64 I had nice peripherals like flippy disk driver and a matrix printer, which of course were not compatible with my MPF-II.
So I wrote emulation for the C64's synchronous serial connection in 6502 assembly (by hand in hex editor, of course, how else?)
and connected the devices to the memory-mapped expansion port of the MPF-II

It was quite feat for a teenager if I may say so... Figuring out the timings and everything without any measurement equipment...

To be fair, in those days without streaming and multi-threading, timing was a simple matter of adding up clock cycles for each instruction

Nokia made a cabled remote that you plugged between the phones and the headphone jack and had media control keys. I tested it and the Nokia music app on the N9 understood the media keys but gPodder did not so back it went to the drawer...