Commodore 2040 disk

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File Format
Name Commodore 2040 disk

The Commodore 2040 disk, and the later, compatible upgrade, the 4040, was a disk format used on Commodore PET/CBM computers. It was a 5 1/4" single-sided disk with a capacity of 170 kilobytes.

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).

A form of GCR encoding was used in the data storage.

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 1541 disk format used with the Commodore 64 was basically the same as this, enough so that these disks could generally be read with the later drives, but due to slight differences in alignment it was not possible to reliably write to disks that had been formatted on the earlier drive with new drives.

While the disks were single-sided, users often punched an extra notch on the opposite side to permit the disks to be flipped over for use of the other side (the notch was necessary to prevent the disk from being seen by the drive as write-protected). This was done despite being discouraged by disk manufacturers, since the disks weren't designed to work well this way (only one side was certified for data storage, and also running it in both directions could cause debris to build up in ways unanticipated by the designers). However, some disk manufacturers eventually did produce double-notched disks for this use.

The still 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.

The filesystem on it would be CBMFS.


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