Commodore DOS

Commodore DOS, aka CBM DOS, is the disk operating system used with Commodore's 8-bit computers. Unlike most other DOS systems, which are loaded from disk into the computer's own RAM and executed there, CBM DOS is executed internally in the drive: the DOS resides in ROM chips inside the drive, and is run there by one or more dedicated MOS 6502 family CPUs. Thus, data transfer between Commodore 8-bit computers and their disk drives more closely resembles a local area network connection than typical disk/host transfers.

At least seven distinctly numbered versions of Commodore DOS are known to exist; the following list gives the version numbers and related disk drives. Unless otherwise noted, drives are 5¼-inch format. The "lp" code designates "low profile" drives. Drives whose model number starts with 15 connect via Commodore's unique CBM-488 serial (TALK/LISTEN) protocols, all others use the parallel IEEE-488.

Version 2.6 was by far the most commonly used and known DOS version, due to its use in the 1541 as part of C64 systems.

The revised firmware for the 1571 which fixed the relative file bug was also identified as V3.0. Thus it is not possible to tell the two versions apart by version number alone.

The 1541 Commodore floppy disk can contain up to 144 files in a flat namespace (no subdirectories); the directory is stored on reserved track 18, which is the center track of a 35-track single-sided disk. A file name may be up to 16 bytes in length and theoretically will be unique; by using direct access methods on the directory structure, it is possible to rename a file to that of another—although accessing such files may be difficult or impossible. Files with identical names usually serve no purpose except to inform or visually manage files. One popular trick, used, for example, by The Final Cartridge III, was to add files named "----------------" of type DEL< to the directory, and files could then be rearranged around those lines to form groups. Many game developers, warez group members and demoscene hackers used some more clever custom directory entries as well.

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