Commodore 1571 disk

The Commodore 1571 disk is the native format of the 1571 disk drive, released by Commodore along with the Commodore 128 computer. The drive could also be used with the Commodore 64 and VIC-20 computers, but on the 128 it had access to a new "burst mode" which greatly increased its speed. In addition, the 1571 drives could handle the Commodore 1541 disks from the older 1541 drive (in a special "compatibility mode" which emulated that drive well enough even to work with copy-protected disks), as well as some CP/M formats.

The main difference of the 1571 over the 1541 was that it was a double-sided drive, able to read both sides of the disk without flipping it, in contrast to the single-sided 1541 on which users often flipped disks manually to double their capacity. Such "flipped" disks would only work on the 1571 in 1541 emulation mode, manually flipped just like on the earlier drive, since attempting to read both sides at once in 1571 mode would fail due to the reverse side being recorded "backward" compared to the data orientation the new drive expects.

The 1571 disk has 35 tracks on each side (for a total of 70) with a variable number of sectors, totalling 1360 sectors, each of which held 256 bytes (though in the CBMFS file system two bytes are reserved for a pointer to the next logical block, leaving 254 actual data bytes per sector). This led to a total storage capacity of 329.4 kilobytes. Other filesystems might use the full 256 bytes per sector, and also might use the upper 5 tracks, making a total of 40 per side, which were skipped as in the 1541 (in that drive, the skipping was done because of unreliability of those tracks, and it was continued in the new drive for compatibility purposes even though they didn't have the same unreliability issue). The tracks on the double-sided disk were treated as 70 logical tracks, numbered sequentially.

A type of GCR encoding was used to encode the data.

Images of such disks stored in other media/filesystems can be stored in D71 or G64 format.

Links

 * Wikipedia article
 * DirMaster: reads C64 disk images / archives / files in Windows