The original 5¼-inch “minifloppy” from 1976 2 only supported 35 tracks, the 40 track disks came a little later. Some formatting programs add extra gaps between the sectors to minimize the tail gap: On some disks, the empty area is about the same size for all tracks. There are formatting programs that fill it with all SYNC marks instead: As long as there are no valid sector headers in the unused space, it can contain anything. When a disk is formatted, the pattern of 0x55 (alternating 0 and 1 bits) is written into this area. Speed zone 2 though looks like it could fit another sector, and in fact, DOS version 1 of the 2040 formatted tracks 18-24 with 20 sectors instead of 19, but this was too tight: If the motor speed was a little off, the last sector would overwrite sector 0. You can see this in the darker and more consistent shading in the image:Ībout 5 to 10 degrees towards the end of each track are unused, because the space would not fit another sector. Towards the end of the tracks, there is some unused space. Higher tracks are shorter, and they contain fewer sectors. Yet the linear length of the sectors is about the same – that’s the idea of the different speeds. But on the last 5 tracks (zone 0), a sector occupies an angle of about 21 degrees. The first 17 tracks (zone 3) are written at the highest speed: A sector occupies an angle of about 17 degrees. The first SYNC marks the sector header, the second one the sector data. Every sector is prefixed by two small red areas: These are the SYNC marks. Blue represents a longer sequence of 0-bits.Red represents a longer sequence of 1-bits.Every track starts at the bottom and is read counter-clockwise 1.A disk contains 35 tracks, counting from the outside to the inside.This animation shows 28 regular Commodore 1541 disks. G64 files are C64/1541 disk images that contain all bits as they are physically laid out on the 5¼-inch floppy disk.
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