Media
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, one track was reserved by DOS for directory and file allocation information (so-called BAM, Block Allocation Map). And since for normal files, two bytes of each physical sector were used by DOS as a pointer to the next physical track and sector of the file, only 254 out of the 256 bytes of a block were used for file contents.
If the disk was not otherwise prepared with a custom format, (e.g. for data disks), 664 blocks would be free after formatting, giving 664 × 254 = 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 1983 Apple FileWare minifloppy drives used double-sided media, higher track pitch, and variable motor speed to achieve a storage capacity of 871 kB, or 435 kB per side.
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 scissors, knife, hole punch, or "disk notcher" tool that was specifically designed for this task. Most soft sectored and all hard sectored drives would have also required an extra cut-out for the index hole — a harder modification.
| Track | Sectors (256 bytes) |
bits/s |
|---|---|---|
| 1 - 17 | 21 | 16M/4/(13+0) = 307 692 |
| 18 - 24 | 19 | 16M/4/(13+1) = 285 714 |
| 25 - 30 | 18 | 16M/4/(13+2) = 266 667 |
| 31 - 35 | 17 | 16M/4/(13+3) = 250 000 |
| 36 - 42 | 17 | 16M/4/(13+3) = 250 000 |
Tracks 36-42 are non standard. The bitrate is after GCR encoding, so actual data is a factor 5/4 less.
The 1541 disk typically has 35 tracks. Track 18 is reserved; the remaining tracks are available for data storage. The header is on 18/0 (track 18, sector 0) along with the BAM (Block Allocation Map), and the directory starts on 18/1 (track 18, sector 1). The file interleave is 10 blocks, while the directory interleave is 3 blocks.
Header Contents. The header is similar to other Commodore disk headers, the structural differences being the BAM offset ($04) and size, and the label+ID+type offset ($90).
$00–01 T/S reference to first directory sector (18/1) 02 DOS version ('A') 04-8F BAM entries (4 bytes per track: Free Sector Count + 24 bits for sectors) 90-9F Disk Label, $A0 padded A2-A3 Disk ID A5-A6 DOS type ('2A')Read more about this topic: Commodore 1541
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