During school summer holidays back in the day at Richvale Telecommunications, I and a few of my friends there, including a guy named Troy Hacker (not making this up!) spent most of a summer reverse engineering the "OS" for the 4040 and 8050 disk drives.<p>The 6504/6502 pairing in the drive with the 6504 handling disk controller operations and the 6502 handling IEEE 488 bus communications was ... interesting. Especially later on when Commodore created the 1541, where they emulated the functionality of the original 4040 design by doing everything via software on a single 6502, including the GCR decoding step which was handled by a dedicated ROM in the 4040 - all presumably to drive down the cost of the drive.<p>Since we didn't have a memory map of the drive (and certainly no easy way to debug code running on it) we had to read the boot code to figure out what it did and construct our own memory map from that. I still remember the moment when my 13 year old self realized that the code I was reading was blinking the drive LED lights as part of the POST test and then figuring out what the different error codes that I had never seen before were. Good times.
Not to be confused with the Winchester "30 30" disk drive, which was a hard disk pack, or the Deskstar "50/50" drive, which refers to the likelihood of getting data back off the thing...