Editing Commodore 1541 (section)

== Media ==
The 1541 drive uses standard 5¼-inch double-density floppy media; high-density media will not work due to its different magnetic coating requiring a higher [[coercivity|magnetic coercivity]]. As the GCR encoding scheme does not use the index hole, the drive was also compatible with [[Hard sectoring|hard-sectored]] disks. The standard CBM DOS format is 170 KB with 35 tracks and 256-byte sectors. It is similar to the format used on the PET 2031, 2040 & 4040 drives, but a minor difference in the number of header bytes makes these drives and the 1541 only read-compatible; disks formatted with one drive cannot be written to by the other. The drives will allow writes to occur, but the inconsistent header size will damage the data in the data portions of each track.

The 4040 drives use Shugart SA-400s, which were 35-track units, thus the format there is due to physical limitations of the drive mechanism. The 1541 uses 40 track mechanisms, but Commodore intentionally limited the CBM DOS format to 35 tracks because of reliability issues with the early units. It is possible via [[low-level programming language|low-level programming]] to move the drive head to tracks 36–40 and write on them, this is sometimes done by commercial software for copy protection purposes and/or to get additional data on the disk.

However, one track is reserved by DOS for directory and file allocation information (the BAM, [[block availability map]]). And since for normal files, two bytes of each physical sector are 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 are used for file contents.

If the disk side is not otherwise prepared with a custom format, (e.g. for data disks), 664 blocks would be free after formatting, giving 664{{resx}}254 = {{val|168656|ul=bytes|fmt=commas}} (or almost {{val|165|u=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 can be used.

Owing to the drive's non-use of the index hole, it is also possible to make "[[Floppy disk variants#Flippy disks|flippy floppies]]" by inserting the diskette upside-down and formatting the other side, and it is commonplace and normal for commercial software to be distributed on such disks.

{| class="wikitable"
|-
!Tracks !! Sectors<br/>(256 bytes) !! bits/s
|-
|align=right |1–17
|align=center| 21 || 16M/4/(13+0) = 307,692
|-
|align=right | 18–24
|align=center| 19 || 16M/4/(13+1) = 285,714
|-
|align=right | 25–30
|align=center| 18 || 16M/4/(13+2) = 266,667
|-
|align=right | 31–35
|align=center| 17 || 16M/4/(13+3) = 250,000
|-
|align=right | 36–42
|align=center| 17 || 16M/4/(13+3) = 250,000<!--Unsure, but considering the number of sectors/track it should be ok.-->
|}

Tracks 36–42 are non-standard.  The bitrate is the raw one between the read/write head and signal circuitry so actual useful data rate is a factor 5/4 less due to [[Group coded recording|GCR encoding]].<ref>{{cite web |title=Power20 Documentation - File Formats, Appendix E: Emulator File Formats |url=http://www.infinite-loop.at/Power20/Documentation/Power20-ReadMe/AE-File_Formats.html |publisher=infinite-loop.at}}</ref>

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, 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 ({{mono|$04}}) and size, and the label+ID+type offset ({{mono|$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')