Appendix A

Format of floppy and hard disk image files

The floppy disk format assumes a standard 1.44 Mb floppy disk, consisting of 80 cylinders, 2 heads, and 18 512 byte sectors per head (36 sectors per cylinder). The image file consists simply of each sector, starting with the first sector of the first cylinder on the first head, and proceeding sequentially through the last sector of the last cylinder on the second head. The total size of the image file is identical to the total capacity of a 1.44Mb floppy disk, or 1,474,560 bytes.

The hard disk image is formatted in a similar fashion, with the exception that the total number of cylinders, heads, and sectors per head varies. Because of this, the hard disk image file contains a 512 byte header before the raw data. This 512 byte header is identical to the information provided by the drive in response to the ATA command "IDENTIFY". Following the 512 byte header is the data for each sector from the device. As with the floppy, the data starts with the first sector of the first cylinder on the first head. Unlike floppies, however, the image file may end before the last sector of the last cylinder on the last head. If an attempt is made by SimNow to access data on the drive image that is beyond the end of the available data (but still within the bounds defined by the geometry of the device), SimNow will extend the image file dynamically.