Changes
/* FDC Track Format */
The 765 FDC only supports the single-density IBM 3740 track format and the double-density IBM System/34 track format, which are the defacto standards for floppy disks in most computer systems. These formats have been created by an IBM engineer named Alan Shugart (not to be confused with Amstrad's CEO, Alan Sugar).
On Amstrad CPC, an MFM track contains about 6250 raw bytes: 200 ms per track (at 300rpm) / 32 µs per byte (with a bit cell of 4µs). Fun fact: You can squeeze a few more bytes on disk by using a floppy drive that spins a little slower, and counting on the tolerance of the FDC to make that disk readable on unmodified floppy drives.
Notes:
* The Index Address Mark (IAM) signifies the beginning of a track.* The ID Address Mark (IDAM) marks the beginning of a sector's header.* The Data Address Mark DAM (DATA AMor DDAM) marks the beginning of the actual data in a sector.* There are 2 types of sectors based on their DATA AM value. Data Deleted data sectors are marked by an FB F8 byte, while deleted sectors are marked by instead of an F8 FB byte.
* Gaps are necessary to accommodate variations in rotation speed between different drives and avoid overlapping.
In MFM encoding, the IDAM and DATA AM DAM are always preceded by three A1 bytes to help the FDC lock onto the data stream after a gap and accurately read the following datas. This is needed because MFM is more compact and harder to read than earlier encoding methods.
The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. The primary difference between 765 and 179x controllers is that the 765 only does standard track formats (preamble, marks and data fields), while the 179x will write anything you tell it in the write track (formatting a track) mode.
