Good description of Apple ][ disk controller

Eric wrote: The patent does not describe the 13-sector format at all!!! It only describes an 11-sector non-GCR format that was never used by any software Apple actually publicly released. The patent is describing FM encoding, not GCR. The Disk II controller is capable of FM encoding, by explicity using only nibble patterns that have bits 7, 5, 3, and 1 set. In released versions of DOS, FM encoding is only used for the address field. There's no point in using GCR for the address field, as it only stores four bytes. So if the authors of Beneath Apple DOS based their description on the, patent, they clearly didn't actually understand it. For one thing, the hardware does not EVER automatically generate those clock bit, as Beneath Apple DOS claims. It doesn't do it if you write FM, and it doesn't do it if you write GCR. (The controller has no idea which you're using.) Nothing in the patent suggests that the hardware inserts clock bits. In fact, the patent pretty clearly explains that those clock bits are generated by software using the appropriate bit patterns. By the way, the reason that the 13-sector and 16-sector GCR data patterns, in which 8 channel bits are used to encode 5 or 6 data bits, are called nibbles (or nybbles) is a carryover from the FM format in which the 8 channel bits encode 4 data bits, a classical 4-bit nybble. It's an accurate description of the hardware, and of one way for software to use it. If you're writing a book about the product that actually shipped, you should: 1) Not assume that it works exactly the same way as a patent disclosure. 2) If you're going to use the patent as a source, take the time to actually understand what it says. Eric