The Commodore 1541 (aka CBM 1541, and originally called VIC-1541), made by Commodore International, was the best-known floppy disk drive for the Commodore 64 home computer. The 1541 was a single-sided 170 kilobyte drive for 5¼" disks. The 1541 followed the previous Commodore 1540 (meant for the VIC-20).
The disk drive used Group Code Recording (GCR) and contained a MOS 6502 microprocessor, doubling as a disk controller and on-board disk operating system processor. The number of sectors per track varied from 17 to 21 (an early implementation of Zone Bit Recording). The drive's built-in disk operating system was CBM DOS 2.6.
Each side of 170 kBs was split into 683 sectors on 35 tracks, each of the sectors holding 256 bytes; the file system made each sector individually rewritable.
However, since one track had to be used by the drive itself for directory and file allocation information (so-called BAM, Block Allocation Map) and two bytes of each physical sector were used as a "block" pointer to the next physical track and sector (as to implement interleave for faster access), a logical block would hold 254 out of 256 bytes.
If the disk was not otherwise prepared with a custom format, (e.g. for data disks), 664 blocks would be free after formatting, giving 168,656 bytes (or almost 165 kB) for user data.
By using custom formatting and load/save routines (sometimes included in third-party DOSes, see below), all of the mechanically possible 40 tracks could be used. The reason why Commodore decided not to use the upper five tracks by default (or at least more than 35) was the bad quality of some of the drive mechanisms which did not always work reliably at the highest tracks. So by reducing the number of tracks used and thus capacity, it was possible to further reduce cost - in contrast to Double Density drives used e.g. in IBM PC computers of the day which saved 180 kB on one side (by using a 40 tracks format).
The 1541 did not have an index hole sensor, making it straightforward to use the reverse side of a disk by flipping it. A disc could be converted to a "flippy disk" by simply cutting/punching a notch on the left-hand side, causing the drive to recognize both sides as writable. This would effectively double the storage capacity. The notch could be made with a knife, single hole paper punch, or "disk notcher" tool that was specifically designed for this task. To achieve the same effect on other drives would normally have also required an extra cut-out for the index hole — a harder modification.
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