Editing Commodore 1571 (section)

== Disk format ==
Unlike the 1541, which was limited to [[Group Coded Recording|GCR]] formatting, the 1571 could read both GCR and [[Modified frequency modulation|MFM]] disk formats. The version of CP/M included with the C128 supported the following formats:

* [[IBM Personal Computer|IBM PC]] [[CP/M-86]]
* [[Osborne 1]] (double density upgrade)
* [[Epson]] QX10
* [[Kaypro]] II, IV
* CBM CP/M FORMAT SS 
* CBM CP/M FORMAT DS

The 1571 can read any of the many CP/M {{frac|5|1|4}}-disk formats.<ref name="128book">{{cite book | chapter-url=https://archive.org/details/The_Official_Book_for_the_Commodore_128/page/n109 | title=The Official Book for the Commodore 128 Personal Computer | publisher=Howard W. Sams & Co. | author1=Waite, Mitchell | author2=Lafore, Robert | author3=Volpe, Jerry | year=1985 | page=98 | isbn=0-672-22456-9 | chapter=The CP/M Mode}}</ref> If the CP/M [[BIOS]] is modified, it is possible to read any soft sector 40-track MFM format. Single density (FM) formats are not supported because the density selector pin on the MFM controller chip in the drive is disabled (wired to ground).

A 1571 cannot boot from MFM disks; the user must boot CP/M from a GCR disk and then switch to MFM disks.

With additional software, it was possible to read and write to [[MS-DOS]]-formatted floppies as well. Numerous commercial and public-domain programs for this purpose became available, the best-known being [[SOGWAP]]'s "Big Blue Reader". Although the C128 could not run any MS-DOS-based software, this capability allowed data files to be exchanged with PC users. Reading {{nowrap|[[Atari 8-bit computers|Atari 8-bit]]}} {{nowrap|130 kB}} or {{nowrap|180 kB}} disks was possible as well with special software, but the standard {{nowrap|Atari 8-bit}} {{nowrap|90 kB}} format, which used [[Frequency modulation|FM]] rather than [[Modified frequency modulation|MFM]] encoding, could not be handled by the 1571 hardware without modifying the drive circuitry as the control line that determines if FM or MFM encoding is used by the disc controller chip was permanently wired to ground (MFM mode) rather than being under software control.

In the 1541 format, while 40 tracks are possible for a {{nowrap|5.25" DD}} drive like the 154x/157x, only {{nowrap|35 tracks}} are used. Commodore chose not to use the upper five tracks by default (or at least to use more than 35) due to the bad quality of some of the drive mechanisms, which did not always work reliably on those tracks.

For compatibility and ease of implementation, the 1571's double-sided format of one logical disk side with {{nowrap|70 tracks}} was created by putting together the lower 35 physical tracks on each of the physical sides of the disk rather than using two times {{nowrap|40 tracks,}} even though there were no more quality problems with the mechanisms of the 1571 drives.