The C64 came with a manual including the full memory map, what addresses to PEEK and POKE to control the sound chip, etc. "Proprietary blob" may be technically

The C64 came with a manual including the full memory map, what addresses to PEEK and POKE to control the sound chip, etc. "Proprietary blob" may be technically true in that the ROMs weren't open source, but it was simple, accessible, and documented enough that that doesn't well describe my experience with it. (Though it's not a machine I ever got deep into, myself.)