Commodore 1541 disk

The Commodore 1541 disk was a popular disk format used on Commodore 64 computers. It was a 5 1/4" single-sided disk with a capacity of 170 kilobytes. Users sometimes flipped the disks over to use the other side too. CBM DOS 2.6 was used.

The disks (in their standard formatting) had 35 tracks with from 17 to 21 sectors in each (outer tracks had more sectors), for a total of 683 sectors. Each sector held 256 bytes, but two bytes were reserved for a pointer to the next physical track and sector, leaving 254 bytes for data storage. Track 18 was reserved for directory and file allocation information including a Block Allocation Map (BAM). Custom disk formats could use as many as 40 tracks (the higher tracks were skipped in the standard format due to lower reliability of those portions of the disk).

Copy-protected disks introduced intentional read errors which interfered with copying programs, but such schemes were easily defeated with special copying programs that ignored such errors.

The later Commodore 1571 disk format was double-sided, but not compatible with 1541 disks that had been flipped over to write both sides, since in this case the flip side is reversed in orientation compared to a disk being read in double-sided mode.