The structure of the data on the disc which defines the sectors is [[IBM System 34 double density format]].
Some FDCs are quite relaxed in the data that they accept. The following applies to the NEC D765AC-2 and was found through testing:
* The only address mark which identifies the ID field is 0x0fe. Attempting to use any other mark for an ID field will result in the ID field being ignored.
* The only data marks which identify the data field are 0x0fb (Data) and 0x0f8 (Deleted Data). Attempting to use any other mark fo for a data field will result in the field being ignored.
* Only 3 sync marks (4489) can proceed an ID field or data field. Fewer or more are ignored.
* Following the data separator synchronization the first sync mark must be seen within 3 MFM words.* When a valid ID field has been read, the data field must be found within a certain number of bytes from the end of the ID field. The minimum is 32 bytes. The maximum is around 44 but needs to be determined.
* Sector within sector is possible.
* If an ID field with a CRC error is found then the ID will not be reported by READ ID and the data field will not be read when using Read Data READ DATA or similar commands.
* Any data can exist in Gap 2 and will not affect reading of the ID fields or sectors.
* Any data can exist in Gap 3 and will not affect reading of the ID fields or sectors.
* The first sector on the track will not be seen when using READ DATA if the ID field is less than 896 microseconds from the index pulse. (TODO: Find exact value because 896 is within 4 MFM words).