On Fri, 11 Jun 2004, Jules Richardson wrote: your description (quoted only in part) left out the address marks. Hopefully those ARE included. The simple answer to your question is sort of YES, they are intended to be compatible. But,... You are looking at a format structure that was developed by IBM (LONG before the PC). "FM" and "MFM" are encoding names, but don't include the track layout of the format. The FDC chips support those IBM "3740" formats, with some quirks. But there do exist FM and MFM formats, not supported by those chips, that have "standard" FM or MFM encoding, but do NOT have the same track structure. For example Amiga is MFM, but has almost no track layout structure, since the Amiga reads and writes an entire track, and then parses sectors from that in memory. Amongst the quirks are differences in gap lengths, and different address marks. Because of that, for example, NEC FDC chips (765) sometimes have trouble reading some formats, such as some TRS80 model III, because the WD 179x chip that wrote them used an index gap that was shorter than the NEC chip can handle (can sometimes be solved by not letting the NEC chip see the index pulse). And, the TRS80 (1771) TRS-DOS 2.x uses some data address marks that the 179x can not create. Thus, the model 3 can not create a disk that model 1 TRS-DOS can read. (that was the seminal factor that led to the creation of LDOS) And, the WD chips are capable of IGNORING certain fields in the sector header, but the NEC chip can not ignore them. Because of that, Kaypro DS (among others) diskettes can not be read through any manipulation of the BIOS (INT13h), and require some direct programming of NEC FDC chip to tell it what INCORRECT value to be looking for in the head number field of the sector headers of the second side. So, YES, they are standardized in their structures, making it possible to write programs such as XenoCopy. But, NO, they do not always adhere well to the "standard" -- Fred Cisin cisin(a)xenosoft.com XenoSoft http://www.xenosoft.com