The size ID byte tells the system how large the follwing Sector Data field is and is interpreted as follows : Unlike everything else, sector numbers use a convention starting with 01, not 00. Then it continues with track number (00-27), the side number (00-01), the sector number (01-09) and finally the size ID byte (00-06). The Sector ID begins with an FE byte as an Address mark. Gap bytes and Sync bytes precede both the Sector ID field and the Sector Data field. This area is missing the normal clocks and does not follow proper MFM encoding rules so the drive controller can figure out that this is not real data and the real data is coming up. Sync bytes follow a pattern of 00s followed by three A1s. After the Gap bytes come the Sync bytes, which help tell the drive that there will be a Sector ID field or a Sector Data fields coming up next. Without some tolerance, the Sector fields may overwrite each other. These Gap bytes, typically hex 4E, are used to compensate between slight differences between the drive that originally wrote the disk and the drive that is currently writing the disk. Before the Sector ID field there will be Gap bytes. Between the Sector ID field and the Sector Data field are two kinds of bytes. In between the data there is extra data allowing the disk drive to find the right data.Įach sector has a Sector ID field and Sector Data field. However, that is the space available for data bytes and a standard disk dump just dumps the data sectors. Thus you get a total disk size of 368,640 bytes. I distinguish between cylinders, which use both sides of the disk, from tracks, which only use one side of the disk. A standard MS-DOS format command formats a 360KB floppy disk with 40 cylinders, 9 sectors per cylinder and 512 bytes per sector. ![]() I am going to be using a standard 360KB floppy disk as an example here. Overview of Low-Level Floppy Disk Structure ![]() This information was written by The Great Hierophant.
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