On 04/24/2017 06:51 AM, dwight via cctalk wrote: > Many format programs don't look at the index anymore as it isn't > important for reading the disk. It is only used to indicate that the > disk is turning.
That may be true for some systems, but for the NEC 765-based controllers found in PCs, the format operation always starts at the index (and ends there too). What isn't dependent on the index is the sector ordering; it's not at all uncommon to see a cylinder- and track- skew factor built in, such that cylinder 0, side 0 starts with sector numbering 1,2,3...18; cylinder 0, side 1 starts as 4,5,6...18,1,2,3 and so on. It avoids a lost rev when moving from track to track. Similarly, when reading, if the IDAM for a desired sector isn't seen after 2 index pulses, the 765 calls it a "not found". There are index-ignorant controllers out there, but the PC isn't one of those. My guess on the symptoms described is a scratch or spill. Sometimes these floppies can be brought back by a simple cleaning of the cookie. --Chuck