Commodore DOS, also known as CBM DOS, is the disk operating system used with Commodore's 8-bit computers. Unlike most other DOSes, 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. CBM DOS versions 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 serial IEEE-488 bus (IEC Bus) serial (TALK/LISTEN) protocols; all others use the parallel IEEE-488. 1.0 – found in the 2040 and 3040 floppy drives 2.0 – found in the 4040 and 3040 floppy drives 2.5 – found in the 8050 floppy drives 2.6 – found in the 1540, 1541 including the one built into the SX-64, 1551, 2031 (+"lp"), and 4031 floppy drives 2.7 – found in the 8050, 8250 (+"lp"), and SFD-1001 floppy drives 3.0 – found in the 1570, external 1571, and 8280 floppy drives (8280: 8-inch), as well as the 9060 and 9090 hard drives 3.1 – found in the built-in 1571 drive of C128DCR computers 10.0 – found in the 1581 (3½-inch) floppy drive Version 2.6 was by far the most commonly used and widely known DOS version, due to its use in the 1541 as part of C64 systems. Note: The revised firmware for the 1571 which fixed the relative file bug was also identified as V3.0. Thus it is not possible to differentiate the two versions using the version number alone. Technical overview 1541 directory and file types 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 located halfway from the hub to the edge of a 35-track single-sided disk. A file name may be up to 16 bytes in length and is theoretically 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