[sowbug]

The Apple II's disk controller could easily write "flaky bits" in software. By telling the controller to write three or more consecutive zero bits, you'd produce a sector that read back unpredictably. So your copy protection scheme would be to turn off sector checksums and then read your test sector a few times in a row. If the sector came back identical each time, then you were likely a copy rather than an original.

After reverse-engineering the verification code in a few different games, I wondered how the publishers produced those weird sectors. I called them "weak bits," coincidentally, because my theory at the time was that they modded the disk head to write the bit weakly so that it couldn't distinguish a one from a zero during readback. A friend at school had a copy of Don Worth's Beneath Apple DOS, which absolutely blew my teenage mind. Until reading that book, I didn't think that any single human could understand and clearly explain a complex system so thoroughly.

[scarybeast]

Interesting, I didn't know the Apple II's controller could write the combined data and clock bits. That's unfair :) I was also pointed to this Twitter thread where it is explained that they wrote Mr. Do weak bits with a hacked up Commodore 64 drive controlled by an Apple II!

https://twitter.com/JBrooksBSI/status/936476972611334147

[scarybeast]

I've also been referred to this which definitely belongs here. It's a write up of fairly advanced weak bits usage on the 1984 Apple II educational title "Kingdom of Facts".

https://ia800600.us.archive.org/2/items/TheKingdomOfFacts4am...