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.

It's much simpler than weak bits, but I remember one of the Ultima games for DOS had a track with a small sector embedded in the middle of a long sector. So from the point of view of the controller after decoding, there were more sectors than could be written onto a track.

All the copy protection did was decrypt itself, read the starting address of the executable from a sector on that track, and jump to it. My fix was to just stuff that address back into the executable header. (I had actually purchased the game, I was just tired of having to insert the floppy every time I ran it off of the hard drive.)

This reminds me of "weak sectors" as used in the infamous SafeDisc and related CD protections, which exploit a digital aspect of the media (EFM encoding) to create an optical equivalent of a https://en.wikipedia.org/wiki/Lace_card :

https://web.archive.org/web/20090603002402/http://sirdavidgu...

I remember Dungeon Master on the Atari ST having fuzzy bits. There was also another game, I can't remember which that instead of locking up or not loading on detecting that the fuzzy bits were not fuzzy (and hence it was a copy) changed the gameplay so that the end boss was unbeatable.

Fabulous explanation of how they did copy protection in the old days. I remember the 8271 well. I'm not sure why they used such an old chip with the BBC B, as it was quite expensive. The WD1770 was better and cheaper.